SOIL TRANSITIONS

IN CENTRAL EAST BOTSWANA

(AFRICA)

W. SIDERIUS

This thesis will also be published as a publication of the International Institute forAerial Survey and Earth Sciences (ITC) — International Soil Museum.

Soil Transitions inCentral East Botswana

(Africa)

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Soil Transitions in

Central East Botswana

(Africa)

Proefschrift

ter verkrijging van de graad van doctor in de Wiskunde en

Natuurwetenschappen aan de Rijksuniversiteit te Utrecht,

op gezag van de Rector Magnificus Prof. Dr. Sj. Groenman,

volgens besluit van het College van Dekanen in het open-

baar te verdedigen op woensdag 2 mei 1973 des namiddags

te 2.30 uur

door

Woutherus Siderius

geboren op 13 augustus 1940 te Baarn

PROMOTOR: PROF. DR. IR. F. A. VAN BAREN

ACKNOWLEDGEMENTS

This study was conceived thanks to the co-operation of many whodirectly or indirectly were associated with its development.I am, Prof. Dr. F. A. van Baren, in particular indebted to you for yourscientific advice and organizational endowments that enabled me tofollow up my work in Botswana at the Soils Institute at Utrecht, and touse the facilities of your Department. Your concurrent effort and personalconcern made it possible to bring this study to its conclusion.

I owe sincere thanks to you, Mr. W. L. P. J. Mouthaan, B. Sc, not only foryour valuable suggestions in the field of the sand mineralogy, butmoreover, for your philosophical insight relative to soil genesis and yourguiding support.

It is a pleasure to thank J. G. Pike, M. Se, FAO project manager, for hisinterest and help towards the accomplishment of this investigation, andalso my colleagues of the FAO integrated survey team, of which I owespecial thanks to H. J. van Rensburg, M. Sc. for valuable suggestionsconcerning the description and recording of the vegetation.

I am grateful to the UNDP Office in Gaborone, Botswana, for theirassistance in the realization of this study and to the Department ofSurveys and Lands for permission to use cartographic material.My gratitude is further extended to Dr. R. Dudal and A. J. Smyth, Esq. ofthe FAO Land and Water Development Division, for their comprehensioncomparative to the subject, and to Dr. R. F. Loxton, consultant, for hisconstructive ideas made during his sojourns in Botswana.

Professor Dr. J. I. S. Zonneveld and the Staff of the Department ofPhysical Geography (Utrecht State University), Prof. Dr. D. J. Douglasand Prof. Dr. J. F. M. de Raaf, I acknowledge the profound scientificteaching received during my University education.

I express gratitude for the valuable critisism of Dr. L. Bal, D. Greutzberg,M. Sc, Dr. J. J. Reynders and N. M. de Rooij, M. Sc, on selective chaptersof the manuscript. The help of G. A. E. M. Hermans, M. Sc. and Dr.P. Maaskant in the determination of the rock specimen is appreciated, andthe latter is thanked for his part in the electron microprobe determina-tion.

Discussions with j . i>. Al, B. Se, J. H. V. van Baren, M. Se, J. A. .Boertna, M. Se. and S. P. Tjallingii, M. Se, prevented imperfections.I am grateful to the following persons for their expertise help, Mr.J. Drenth, Mr. H. E. Renaud, Mr. J. D. Schreiber, Mr. D. de Vries, Mr.J. van der Wal and Mrs. I. W. J. Wolters.

Thanks are also due to Mr. F. Henzen and Mr. I. Santoe for their assistancein drawing most of the diagrams and the reproduction of many illustra-tions.

I am grateful to A. Muller, M. Se , and his Staffat the Soils Laboratory ofthe Royal Tropical Institute, Amsterdam, for the able way in which anumber of analyses were carried out, and for the facilities made availableto me.The pleasant company of Mr. L. Lebodi on many field trips is memorizedwith great appreciation.

I thank Miss Weia S. Smid for her efficient typing of the manuscript.My grateful thanks go to Madame V. Balikci-Ossent and to my wife, forthe correction of the English.

This research was for the major part financed by the Netherlands Foun-dation for the Advancement of Tropical Research (WOTRO), The Hague.Its publication was supported by a grant from the Netherlands Ministry ofEducation and Sciences.

Sincere appreciation is bestowed to the Ministry of Agriculture of theGovernment of Boswana for their authorization to undertake this investi-gation and permission for its publication.

CONTENTS

INTRODUCTIONCHAPTER I ENVIRONMENTAL CHARACTERISTICS 15

1.1. Climate 151.1.1. Atmospheric climate 161.1.1.1. Precipitation 161.1.1.2. Temperature 171.1.1.3. Evaporation 181.1.2. Soil climate 191.1.2.1. Soil temperature ,201.1.2.2. Soil moisture 231.1.3. Paleoclimate 24

1.2. Geology 281.2.1. General 281.2.2. Parent material 30

1.3. Geomorphology 391.3.1. Topography 391.3.2. River systems 401.3.3. Landscapes 43

1.4. Vegetation 471.4.1. Natural vegetation , 4 71.4.2. Human induced vegetation 511.4.3. Vegetation patterns 52

CHAPTER II TYPICAL SOILS 55

11.1. Location of sample sites 55

11.2. Field observations 56

11.3. Soil properties 8411.3.1. Bulk density 8411.3.2. Soil reaction • 8411.3.3. Exchange capacity 85

CHAPTER III SOIL COMPOSITION 89

III. 1. Fragments coarser than 2 mm 89

111.2. Grain size distribution in the fine earth 89111.2.1. Sand 90111.2.2. Silt 91111.2.3. Clay 91

111.3. Elemental composition of the fine earth 95111.3.1. Major elements 95111.3.2. Trace elements 100

7

111.4. Mineralogy of special constituents 103111.4.1. Mineralogy of the sand fraction 103HI.4.1.1. Heavy minerals 104III.4.1.2. Light minerals 109111.4.2. Mineralogy of the clay fraction 112

111.5. Organic matter 116

III6. Salinity 118

CHAPTER IV ARRANGEMENT OF SOIL CONSTITUENTS 121

I V.l. Micromorphological observations 121

IV.2. Evaluation of the field data 140

CHAPTER V ROCK WEATHERING AND SOIL FORMATION 147

V.l. Rock weathering 147

V.2. Soil formation 152

CHAPTER VI SOIL CLASSIFICATION AND LAND USE 167

VI.1. Soil classification 167

VI.2. Land use 171

APPENDIX I METHODS 175

1.1. Survey methods 175

1.2. Laboratory methods 175

APPENDIX II SOIL PROFILE DESCRIPTIONS AND LABORATORY DATA 182

SUMMARY 245

SAMENVATTING 247

REFERENCES 249

CURRICULUM VIT AE 260

UPPER CATCHMENT OF THEMAHALAPSHWE AND BONWAPITSE

RIVERS* North

iliiilllip

ridge

i . J. Location map of the investigated area

INTRODUCTION

The importance of natural resources for Botswana, as expressed by thiscountry's export data, but even more as the basis for the life of more than95% of its population, is well understood by the present Government.Inventarization of these resources has proceeded rapidly since the coun-try's independence in October 1966. Within this programme, integratedsurveys were initiated in Northern and Eastern Botswana from 1968 to1972 as part of a FAO/UNDP-SF project. One aspect of these were anumber of soil surveys carried out by the author on different levels ofintensity. Although much data was gathered, advanced soil research ongenetic and related fields of interest could not be conducted within thecurrent project programme. This was felt to be necessary, however, inorder to come to a better understanding of the soil's properties andcharacteristics.For that purpose, an area was selected in Central East Botswana (fig. 1).The area located just north of the Tropic of Capricorn at 26° 18'—26° 50' Eand 22°45'—23° 18' S comprises the upper catchment area of two tribu-taries of the Limpopo river, namely the Mahalapshwe and the Bonwapitse.In these catchment areas a number of representative soils were selectedafter the area was surveyed on semi-detailed level. Differentiatingenvironmental characteristics played an important role in locating thesample sites.A total of 154 soil samples was collected and they form the raw materialfor this study, together with four samples from streambeds and ten rockspecimens.Under the present semi-arid climate, a number of soils or soil character-istics seem allochthone and questions about their present developmentand potential are better answered by looking into the past. The factorsinfluencing the pedosphere are dealt with in Chapter I. Decisive informa-tion concerns particularly the sections on climate and geology.The bulk of the soil data is presented in Chapter II, field descriptions andlaboratory data provide the necessary background for further detailedstudies. The results hereof, with special reference to the mineral constit-uents of the soils which account for more than 99% of their assemblage, isthe subject of Chapter III.As a logical development, the arrangement of these soil constituents arethen discussed and described in Chapter IV. Together with the materialfrom the foregoing chapters, main conclusions about rock weathering and

13

soil formation are presented in Chapter V.The last chapter deals with some aspects of the soil classification, also ashort assessment is given on the agricultural use of the investigated soils.Although a great amount of information was gathered and used, a substan-tial field of research of these or similar soils lies open. Present findings dohowever close a gap between much fieldwork and laboratory data andmay form the foundation for further research on Botswana soils, of whichpreviously little was known. The relevance of our results in stressed notonly to the area studied but also to similar areas in the world. Thisconcerns the soil's morphology in the field as well as the laboratoryapproach. New procedures for the computing of the data were applied andthese and other results are a novelty in the research on these soils.Apart from the scientific approach and its worth, may this publication bean aid to a better utilization of the land, creating thus a more prosperoussphere for the people living on it.

14

CHAPTER 1

ENVIRONMENTAL CHARACTERISTICS

I.I. Climate

1.1.1. Atmospheric ClimateThe central land-locked part of Southern Africa to which Botswanabelongs is affected by a wide range of climates. Greatest contrastingdifferences occur in the country from the extreme southwest to thenorthern part of the Chobe region, viz. from arid to sub-tropical condi-tions. The eastern part of the country in which the survey area is locatedhas a semi-arid climate, classified by Koppen (1931 ) as BSHw and charac-terized by Finch et al. (1942) as a dry climate of low latitudes-Bsh. Thisclimate is also known for the summer rainfall when unstable equatorial airmasses are farthest poleward.Recent work has revealed six characteristic synoptic situations which areresponsible for the weather conditions in this part of the SouthernHemisphere (Schulze, 1965; Pike, 1971).The main factors that control in first instance these climatic conditionsare as for any place on earth:1) its latitude, which determines the amount of solar radiation (just

north of the Tropic of Capricorn), while the amount of solar radia-tion expressed in cal/cm2 per year is about 281.000;

2) its position relative to the distribution of the sea and the land, most ofthe South African subcontinent is under the influence of the west-erly circulation;

3) its height above sea level (Schulze, 1 965).Grouping these situations, Pike (1971) recognizes the following successionof air masses and their result when they approach the fairly flat table landof the interior. First come tropical continental air and superior air whichhave subsided from high latitudes within the anticyclones of the subtropi-cal high pressure belt. In winter the superior air is brought to Botswana byNW winds and gives rise to warm and sunny days and cold clear nights.This weather prevails from April to September. The tropical air masscomes as NE and N winds in summer. The air is generaly warm and moistas the original subtropical high pressure belt becomes moistened over thehumid lowlands of the East of Southern Africa. Thunderstorms com-monly develop.The effect of the equatorial air mass is occasionally felt in the North ofthe country and in general tends not to pass south of 20 S latitude, if so,

15

the southernmost limit of the inter-tropical convergence zone brings morerain. At the beginning of the winter the superior air may be disturbed bycold polar and sub-polar air masses from the Antarctic, occasionallycausing frost.The rythmic change in the weather conditions gives rise to the recognitionof a number of seasons. Of these the summer and the winter are the mostoutspoken.Summer: December—January—FebruaryAutumn: March—April—MayWinter: June—July—AugustSpring: September—October—NovemberThe effect on the ground resulting from the circulating of the different airmasses is expressed in a number of climatological data such as precipita-tion, temperature and evaporation. As key metereological station for thestudy area Mahalapye with a thirty-year record was chosen, while forcomparison a second station was selected in the southeastern part of thecountry, Gaborone.

1.1.1.1. PrecipitationThe rainfall in the northern part of the country is received due to theattenuated effect of the inter-tropical convergence zone, in the easternpart it is dependent upon the positions of the tropical continental air.Thus rainfall in the eastern part of Botswana is brought about by aweakening of the anti-cyclone which promotes the flowing in of moist airfrom the Indian Ocean.

Table 1. Mean monthly and annual rainfall in mm over the period 1939/40—1968/69

MonthStation

MahalapyeGaborone

J

7381

.0

.7

F

9180

.8

.6

M

67.68.

56

A

33.751.4

M

12.15.

93

J

6.10.

35

J

04

.6

.4

A

1.43.1

S

712

.2

.9

O

29.543.8

N

70.158.7

D

82.87.

69

Year

476518.

.4

.7

The location of the stations is Mahalapye 23° 04' LAT. and 26° 48' LONG,and altitude 1001.0 m and Gaborone 24° 10' LAT. and 25° 55' LONG, atan altitude of 980.0 m.Although the yearly total precipitation (table 1) compares favourablywith other semi-arid areas, a high degree of variability and intensity isrecorded, which reduces the effect of the rainfall with regard to soil

16

moisture and groundwater conditions. The variability expressed as thecoefficient of variation was calculated on a decade basis as selected byPike (1971) for a number of key stations. This percentage is 33.5 forMahalapye and 28.6 for Gaborone.The coefficient of variation is generally inversely proportional to theannual rainfall. In the survey area the annual rainfall in the East exceeds450 mm with a 60% probability that the rainfall will exceed at least400 mm in any one year.Similar calculations were carried out by Jackson (1970). In extreme casesof limited rainfall, drought may occur. They are defined as such if theprobability of the annual rainfall is less than 60% of the mean annualrainfall. Drought periods in Mahalapye may occur in one out of everyeight years and in Gaborone in one out of every 15.Apart from the amount of rainfall its mode of precipitation is important.Unfortunately the rain falls often as heavy showers (up to 125 mm)causing rapid run-off and erosion, with little contribution to the storage ofwater in the soil. Continuous spells of soft penetrating rain are relativelyscarce. Over the year there are:40—60 days/annum with 0.25 mm or more rain, and10—20 days/annum with 10 mm or more rain.In addition, dry periods within the rainy season are common especially inlate December and early January. Long term prediction about the trend ofthe rainfall is difficult to make. However, there are indications that themean annual rainfall is decreasing; if this turns out to be exact it mighthave a disastrous effect on the current farming practices.

1.1.1.2. TemperatureThe air temperature, and to a large extent the soil temperature, is almostentirely determined by the sun energy transmitted as visible and heat rays.The incoming energy is governed by a number of environmental factorssuch as cloudiness, vegetation, slope and latitude (Russell, 1961). Theaverage solar radiation for the two stations is given in table 2.

Table 2.

MonthStation

dahalapyeGaborone

Average solar radiation in

J F

558.5 550.2594.5 470.8

M

476.8499.8

calclcni1

A

402.3417.1

day-monthly

M

372.5383.7

J

327.0341.8

J

350268

.6

.6

A

420.2433.5

S

454510

.3

.6

O

513.1554.0

N

529.2570.7

D

532.2588.8

The duration of bright sunshine falls between 70—90% of the totalpossible. The values recorded in the summer months are somewhat lower

17

due to cloudiness, the higher values of duration occur during the winterseason (Schulze, .1965). The resulting air temperatures are presented intable 3.

Table 3.

MonthStation

MahalapyeGaborone

Mean

J

25.25.

,2.5

monthly

F

24.825.1

M

and annual

A

i 23.4 20.423.3 19.4

M

1615

temperature

J

.5 13.2

.2 12.0

J

13.12.

\n

21

C.

A

16.115.3

S

20.520.3

O

23.523.3

N

2424

.6

.5

D

2424

.4

.7

Year

20.520.1

The temperature range reveals the distribution of the seasons, but hidesthe occasional extremes that may occur. An example is the occurrence ofgroundfrost, caused by active terrestrial radiation. Lowest temperaturesare recorded in June and July (13.2° C). The highest mean values areregistered in January for both stations. The maxima and minima tempera-tures are furthermore presented in table 5 (section 1.1.2).With reference to the main westerly circulation, it is noted that themaximum temperature over the oceans is retarded by approximately twomonths in relation to the maximum of the solar radiation at the summersolstice, which falls on 21 December (Pike, 1971).The combination of rainfall (P) and temperature (T) has lead certainauthors to define the dry season (Gaussen, 1963). By application of theformula P = 2 x T as the definition for the dry period, this period wouldoccur in East Botswana for six months, with emphasis on the period fromMay to August. Chemical weathering during this period will be considerablyreduced. Physical weathering, being the breakdown into smaller fragmentswhile the chemical composition remains unaltered, may be increased dueto sharper diurnal temperature contrast.

1.1.1.3. EvaporationThe resulting effect of rainfall and temperature calculated for the evapo-transporation shows a deficit in all months of the year for the Mahalapyestation. Data on the evaporation (Eo) and the potential evapotranspora-tion (Ept) are given in table 4.

As shown by Pike (1971), a factor of 0.68—0.75 must be applied to theevaporation figures from pans to obtain real natural open water rates.A composite diagram of the mean precipitation (P), air temperature (T)and potential evapotranspiration (Ept) (fig. 2) demonstrates clearly themarginal conditions for intensive agricultural use of the land with regardto cropping.

18

Table 4. Mean monthly and annual evaporation data in mm for Mahalapye

Month J M J J Year

open water (Eo) 212.8 181.6 165.3 123.9 91.4short green

68.0 77.6 109.8 154.9 197.5 198.5 203.8 1785.1

crop (Ept)wet baresoil (Ept)pan (Eo)

168.3 140.8 128.2 94.7 65.5 48.3 54.9 77.7 120.7 157.3 176.9 161.7 1395.0

157.2 131.8 119.6 87.4 59.5 42.6 49.6 73.9 112.6 146.9 149.6 151.1 1281.8247.5 207.8 210.9 157.1 130.0 102.5 119.8 154.0 214.2 264.1 247.3 242.0 2297.2

The occurrence of rainfall during the summer season promotes highevapotranspiration rates, which results in a limited improvement of themoisture conditions in general.

mean monthly rainfall in mm.,, „ Ept short green crop in mm.„ ,, „ wet bare Soil in mm.,, ,, air temperature in °C

STATION : MAHALAPYE

Fig. 2. Composite diagram of mean P, T and E„( values for Mahalapye

1.1.2. Soil ClimateThe climate of the pedosphere may be characterized by two factors,namely soil temperature and soil moisture. Both factors have considerableinfluence on a) the kind of soil processes that takes place and b) the rateat which these processes occur (Mohr and Van Baren, 1959).

19

1.1.2.1. Soil TemperatureSoil temperature data are available for the Mahalapye station and are usedas an indicator for the soil climatic conditions in the study area. Unfortu-nately, these data cover only one season. Comparative data are thereforealso given for Gaborone which has an eight-year record. These data arecomprised in table 5 while temperature graphs are given in fig. 3.The temperature readings were both taken in a reddish brown sandy clayloam comparable to profile 3, as described on page 47, with a baresurface.

Tin °C40 I

20-

J F M A M O J A S O N D

STATION: GABORONE years 1962-1970

T.in°C40 ï

20-

3 F M A M J J A S 0 N D

STATION: MAHALAPYE year 1968-1969

mean monthly air temp„ ,, soil at. 10 cm

,i ii 60 „

, „ 120 „

Fig. 3. Mean monthly air and soil temperatures for Mahalapye and Gaborone

20

Tabel 5. Soil temperature data in C

StationMahalapye

air temperature

StationGaborone

air temperature

depthcm

103060

120

10203060

120

yearlymean

25.125.025.024.920.5

25.325.025.024.924.820.1

monthlyhighest

34.232.632.229.925.2

35.535.434.431.730.025.5

meanlowest

14.718.317.319.113.2

10.813.315.416.618.912.0

absolutemax.

38.934.232.330.727.5

39.736.636.135.031.735.1

min.

10.416.717.218.610.4

5.611.113.613.816.71.8

difference

28.517.515.112.117.1

34.125.522.521.215.033.3

The fluctuations of the soil temperature generally follow those of the airtemperature. Maxima and minima are, however, delayed by about onemonth. The mean total soil temperature closely follows the values ob-tained at a depth of 60 cm (fig. 3). The amplitude of the variation of thesoil temperature decreases with depth.Lowest monthly soil temperature data are recorded at both stations inJuly while highest temperatures occur in January. Changes in soil tempera-ture, which become apparent upon comparison of summer and winterdata, are considered characteristic of the climatic conditions at thislatitude (fig. 4). The temperature decline in autumn and the temperaturerise in late winter and spring have implications for the behaviour of wateras vapour or liquid in the soil (see section 1.1.2.2).

The monthly fluctuations are of equal importance as the amplitude andregime of the soil temperature on a daily basis. The latter was taken at08.00, 14.00 and 20.00 GMT at five different depths, mean values arecalculated and presented in table 6.

Daily variations are greatest at or near the soil surface but decline beyond60 cm depth. This is the result of the conductivity of the soil and showshow soil temperatures are influenced by atmospheric conditions. Fromfigure 3 it is evident that soil temperature closely follows air temperaturedepending on cloudiness, amount of radiation, rainfall and cooling off in

21

T.,n°C4 0 T

30-5

120

60

—*~c^^

10 cm ^ .—

_.__ Tr.6Q.en)120 cm

JANUARY

^^ ^ — — - " T J I - —

DUNE

1 — . i

20,00hrs. G.M.T.

mean daily soil temperature at 10 cm depth

,. „ ,, „ 20 ,,,, 30 „

.. , 60 ,, „

.. .. .. „120 , ,

Fig. 4. Mean daily soil temperatures for the months January and June

the evening and night hours as a result of strong surface radiation.Taken over the year, the mean soil temperature is 4.9° C warmer than themean air temperature in Gaborone (25.0 versus 20.1° C), while the differ-ence in Mahalapye is 4.5° C (25.0° C for the soil and 20.5° C for the air).This is a departure from the usually accepted rule that the mean soiltemperature could be deducted from the mean annual air temperature byadding one degree Celsius to the latter value (Soil Survey Staff, USDA,1.970). A similar temperature course has been reported in other notablysemi-arid countries (Buursink, 1971).In terms of classifying the soil temperature regime the criteria of the 7thApproximation (1970) have been applied. As such the soils may bedescribed as having a hyperthermic temperature regime.The effect of the soil temperature on biological and mineral processes inthe soil has been and still is the subject of considerable research (Kai et al.,1969;Mueleer, 1970; Power et al., 1970).

22

Our readings of the soil temperature suggest favourable conditions forbiological life, germination and weathering. Summerizing, Russell (1961)states the optimum conditions for germination between 18—22° C. Therecorded temperatures in Botswana indicate that during summer this valuewill be exceeded at the topsoil. Optimum temperatures for the develop-ment of soil microbes are present (24—35° C), but higher values areprobably at the soil surface.

Table 6. Mean soil temperature in Cat Gaborone over the period 1962—1970

Month J F M A M J J A S O N Ddepthin cm

10203060

120

31.832.932.431.630.0

31.831.331.630.429.4

30.329.428.929.028.2

25.324.324.625.125.7

20.220.120.421.523.4

15.915.916.417.620.2

16.115.816.216.919.2

1918181919

.9

.8

.8

.1

.7

23.823.823.723.322.4

27.826.626.925.824.8

29.930.029.628.526.8

30.731.230.529.527.8

The high soil temperatures do however accelerate the decomposition ofplant material as was demonstrated by Mohr and Van Baren (1959).A complete and rapid decomposition of organic matter is recorded at32° C, a slower rate of mineralization at 24° C, and one third of the 32° Crate, when the temperature is 1 5.5° C. Subsequent low carbon and nitro-gen percentages in the soils encountered seem to support these findings.It is understood that many of these processes are considerably sloweddown when soil water is insufficient.

1.1.2.2. Soil MoistureSoil moisture conditions in the encountered soils do not only depend onthe amount of precipitation and its intensity and distribution, but evenmore so on chemical and physical soil characteristics. Of the latter the soiltexture is an important factor as it affects to a considerable degree theamount of water that will penetrate into and be held by the soil (Abrol eta l , 1968).Data on infiltration rates of topsoil in Botswana is scarce and are held torange between 8.89 and 12.19 cm/hr for a sandy loam texture (Siderius,1972).No moisture readings were carried out in the soils encountered; fieldobservations suggest, that at the end of the dry season (late September)the top 60 to 90 cm of moist soils are dry. In any case they are far belowfield capacity (1/3 atm.) and close to the wilting point (15 atm.).

23

Some moisture may be encountered below this depth, but it is question-able if this water can be consumed by plants. Observations in riverbeds ofa coarse sandy texture confirm the depth of 90 cm to which the soil maydry out due to evapotransporation.If soils are covered by vegetation their water balance may be even lessfavourable. Related to soil moisture obtained from rain water is the factthat in none of the soils groundwater was observed, so that the watersupply from a subsoil may be disregarded. Perched water tables may occurat low topographic positions during the rainy season but are not persis-tent.The water storage in the soil is therefore thought to be nil. This istheoretically exact if moisture values are obtained according to the proce-dure as outlined by Thornthwaite and Mather (1957) ranging from thesurface to a depth of 90 cm. These data also support the calculation byBlack et al. (1970).Rainfall is apparently sufficient to support the vegetation as described insection 1.4., meaning that the available water holding capacity of the soilsduring the growing season (from October to March) is adequate.During the rest of the year the vegetation is dormant and adapts itself tothe dry conditions. In terms of the USDA (1970) classification, the soilshave an ustic moisture regime. This is defined as a limited amount ofmoisture present in the season when the soil is warm.The amount of soil water, particularly water vapour, and its rate ofmovement are in turn influenced by the soil temperature. On an annualbasis, the soil temperatures have an upward gradient from June to Januaryand a downward gradient during the late summer and autumn. As soilwater vapour tends to move from warm to cold soil masses, the infiltra-tion rate of water may be opposed by the upward temperature gradient inthe spring and early summer. In the latter part of the summer and inautumn, water may be transmitted more easily through the soil (Rose,1969). Similar interpretations can be applied to the daily temperaturegradients. Little is known about the implications for the application ofwater, for example for irrigation.

1.1.3. PaleoclimateThe study of the climatic past of Africa has yielded a number of differentopinions and is still controversial in many ways. A point that led toconfusion is that the initial geological stratification for the Quaternarywas also used for climatic interpretations (Cooke, 1957).In addition, much of the salient discoveries were made in the SouthernHemisphere, notably in East Africa where a correlation could be estab-lished between geological formations and the dwellings and remains of theearly hominid.

24

The subdivisions of the Quaternary in Kageran, Kamasian (divided inKamasian sensustricto and Kanjeran) and Gamblian were consequentlyused to indicate pluvial periods.Temptation was great to link these pluvials with the glacial periods in theNorthern Hemisphere, especially with the well-known sequence in WesternEurope. Frenzel (1967) indeed suggests that climatic changes were syn-chronous in both hemispheres. Other workers are more cautious as to theexact dating of the periods in Africa, especially in the southern part of thecontinent where it remains difficult. Flint (1959) made an excellentcontribution in an attempt to unravel the confusion in chronology andcorrelation. Since then numerous factual studies have been published.Those by Van Zinderen Bakker (1963, 1964) contributed much to theexact dating of the stages of the Pleistocene by means of pollen analysesthrough which temperature changes could be better detected. Suggate(1968) and Morrison (1968) base their conclusion mainly on the weath-ering and state of weathering of the geosol, as does Kubiena (1963). Theopinion of the latter that the change in temperature foremost induced achange in precipitation is supported by many workers. An attempt toabsolute chronology was undertaken by De Heinzelin (1964). However,lack of data prevents a complete record of successions in Southern Africa,especially since it is unlikely to discover remains of humans in an environ-ment which was hostile to them at the beginning of their era due to verydry conditions (Korn et al., 1957).Thus precise equivalence of glacial and pluvial periods in both hemispheresis improbable, but may in some areas be closer than in others (Cooke,1957; Fairbridge, 1963).That changes in climate have taken place in Central Southern Africa wasproved in addition to paleo-botanical research by a number of landscapeand geological studies (Bond, 1957 and 1963; Korn et al, 1957; Mabbutt,1957; Bosazza, 1957; Flint et al., 1968; Netterberg, 1969).To summarize, Bond (1964) states that a wet (more humid) climatic phasemay show an increase in rainfall of 50%, while during a dry phaseminimum rainfall is less than 50% of the normal. Geological evidencesuggests a greater variability in rainfall than paleo-botanical data (VanZinderen Bakker, 1963). According to this author temperature fluctua-tions are believed to be more widely spread with a range of six degreesCelsius (-1 to+5°).The Quaternary history of Botswana has been the object of a limitedamount of studies. The main contributors are Wayland (1954), Bond(1964), and Groove (1969). The following table of Pleistocene successionsis modified after Bond (1964).

25

Table 7. The Pleistocene in Botswana

GeologicalSuccession

Relative Suggested Absolute CulturesRainfall Rainfall in mm

Period

blown sandsgravels of fossilrivers, silcrete

present rainfall about 375—500 mm/year J^resent

black soils andcalcrete nodules

blown sandsNata deltablown sandscalcrete

blown sandscalcrete

slightlywetter

drierwetterdriersi.wetter?

very drysi.wetter

450-500

less than 375? 500-625less than 375450-5007

125-150450-500

Late StoneAge

Magosian

Middle Stone

Fauresmith/Sangoan

Epi-Pl.

Age

UpperPI.

drierwetter

less than 375? plus 625

MiddleOld Stone Age

blown sands drier less than 375 Lower Pi.

(Period indication according to Harmse, 1967).

The succession of events is comparable to an outline as proposed byHarmse (1967) for the Highveld region in South Africa, but full correla-tion is not justified at this stage due to lack of data.From the combined data it is evident that in the past the climate diddiffer considerably in spells from the present situation. The rate of soilevolution was accelerated during the wetter periods (Harmse, 1967;Watson, 1969). Evidence thereof is also discussed in Chapter V. 1.1.It is difficult to assess to which extent the Tertiary did influence theformation of soils. Whereas landscapes of Tertiary age were detected inthe area, the correlation with the soils encountered on them at present isuncertain (Moss, 1968).Soils and landscape suggest that soil formation has taken place for a longtime and was intensified during the wetter periods of the Pleistocene(Frenzel, 1967).

26

Changes in climatic conditions are, however, not restricted to the past inSouthern Africa (Hofmeyer et al , 1963), but modulations have occurredin historic times. Verbal accounts from people in the study area revealmoister conditions than at present. This is in agreement with Pike, who ina recent account (1971) on the rainfall data, demonstrates the decrease inmean annual rainfall since 1958 for all stations in Botswana.

27

1.2. Geology

1.2.1. GeneralThe eastern part of Botswana including the study area is fairly wellexposed for geological survey work. The geology of the remaining part ofthe country is largely masked by a thick cover of Kalahari sand, whereexposures are rare. The stratigraphy of the country is varied and the age ofthe geological units range from Archaen rocks of the Basement Complex,dated as Precambrium, to the recent pan-sediments and sands of theKalahari Beds.The geological sequence originated as the depression in the ancient Afri-can shield, to which Botswana essentially belongs, and became filled atvarious stages with sediments and volcanic deposits (McConnell, 1959).The ancient shield has been detected only in the southeastern part of thecountry. The first stratigraphical table was prepared by Poldervaart andGreen (1954) and is mainly based on work by Du Toit (1954). Furtherrefinements and additions were made by McConnell (1959), Boockock andVan Straten (1962) and Frakes et al. (1970). Hyde (1972) summarizes theprevious work in a clear review. The stratigraphy may thus be described asfollows. The terminology is adapted partly from Krumbein et al. (1959).partly from Krumbein et al. (1959).The Archaeozic Era is strongly represented; the oldest rocks are connotedin the term Basement Complex. The complex is divided into: 1) undiffer-entiated gneisses with minor inclusions of intrusive granite (for examplethe Mahalapye Formation) and metamorphosed sedimentary and volcanicrock, 2) various schists and associated sedimentary rock types often meta-morphosed such as marbles, banded ironstones, quartzites and amphibo-lites.Rocks of both formations are exposed extensively in eastern Botswana.The succeeding rocks of the Kanye Volcanic Group and VentersdorpSystem occur mainly in the southeast of the country. They consist ofmassive felsitic lavas, dark slates, quartz-feldspar porphyries, grits, sand-stones and conglomerates.North and west of Gaborone an intrusive porphyritic granite underlies alarge area. The intrusion is tentatively dated as Post-Kanye VolcanicGroup and Pre-Ventersdorp System.The Proterozoic Era is represented by the Transvaal System, the Water-berg System, the "Ghanzi Group" and the Damara System; the Palaeozoicby rocks from the Karroo System (Ecca and Dwyka Series). The TransvaalSystem occurs mainly in the south and southeast of the country. Twoformations are recognized, the lower one consists of dolomites, bandedironstones, cherts, shales, quartzites and arkose. The upper one comprisesshales, quartzites, conglomerates, limestones, banded ironstones and

28

andesites. Certain sedimentary rocks in the Shoshong area are tentativelycorrelated with the Transvaal System.Dated as Post-Transvaal is a group of ultramafic intrusive rocks that occurin the southeast, south and southwest of the territory.The Waterberg System is extensive in southern and eastern Botswana;shales, siltstones, sandstones, limestones, grits and conglomerates are thedominant rocks. Dated as Post-Waterberg and Pre-Karroo are intrusives ofdolerites, syenites, diorites and mafic rocks. Apart from their occurrencein southeast Botswana, these intrusives also occur in a large area aroundShoshong, situated in the investigated region.The occurrence of the Ghanzi Group is restricted to northwestern Botswa-na. The Group is divided in two formations, the Ghanzi Beds Formationand the Kgwebe Porphyric Formation. The former consists of quartzites,shales, limestones and lavas, the latter of quartz feldspar porphyries andfelsites. Rocks of the Damara System are also restricted in their outcropto the northwest of the territory. Known rocks include quartz schists,quartzites, dolomites, limestones and marbles.The lower Paleozoic Era is mainly represented by the Karroo System. TheKarroo System is similar to the same formation in adjacent countries,notably South Africa, from which its name was derived. The System iswidespread in Botswana and underlies younger formations over most ofcentral and southwestern Botswana, including also a strip along the borderwith Rhodesia. The general trend of the Karroo basin is northeast tosouthwest. A number of series is recognized of which the most importantare the Ecca, Dwyka and Stormberg series. The grouped Ecca and Dwykaseries consist of shales, sandstones, grits, mudstones, limestones, tillitesand coal. The Stormberg series represent the Mesozoic Era. They arefurther devided into the Cave Sandstone and the Drakensburg Lava Stage.The main constituents of the former is a massive false-bedded eoliansandstone of various colours. It may also comprise some shales, mudstonesand marls. The Drakensburg Lava is locally an up to 400 m thick uniformflow of basaltic lava.The Tertiary in Botswana comprises the Kalahari System. Rocks belong-ing to this System may also be of recent age and therefore cover the wholeof the Cenozoic Era. Because of the uncertainty relative to the interrela-tionship of the various units of the system, the geological survey uses theterm Kalahari Beds. The oldest rocks consist of reddish, calcareous clayeymarls, usually encountered near ancient drainage lines. They are succeededby calcareous silicified sands and sandstones. Exposures are frequentlyencountered in ancient drainage lines and occasionally near pans. Therocks cover vast areas in the interior.The Upper Tertiary is represented by the eolian Kalahari sands, ranging incolour from reddish-brown to greyish-white. These sands are very exten-

29

sive over most of the Kalahari except where there are pans, drainage linesor rock outcrops. The sands may be consolidated to varying degrees,depending partly on the amount of silt and clay.During the Pleistocene, sands from older cycles of deposition have beenreworked (Poldervaart, 1957), while younger sands may have been derivedfrom disintegrated Cave Sandstone (Boockock et al., 1962).

The stratigraphy of the rocks is fairly well known, the structural geologyof the territory is however less clear. Folds have been detected in theoldest formations (Du Toit, 1954), while in a recent study Green (1967)indicates two sets of Post-Karroo faults in Botswana. One set is roughlydirected from the northeast to the southwest and the other set from thenorthwest to the southeast. The major fault trend in the investigated areahas the latter direction.The dating of the renewed folding is still a subject of argument.Mason (1969) postulates that the Post-Karroo movement of the majorityof faults in (the eastern part of) the country is merely rejuvenation onvery old fractures. It is therefore that suggestions by Vail (1968) andFairhead et al. (1969) as to the extension of the East African Rift Systeminto southern Africa should be considered with great care.

1.2.2. Parent MaterialThe soil parent material is derived from the following geological forma-tions:1) the Mahalapye Formation, described as an intrusive granite of the

Lower Precambrium, mainly consisting of migmatites and granites.The main outcrop in the survey area is formed by the Morale Hills.

2) Post-Basement Complex and Pre-Karroo intrusives, and therefore ofUpper Precambrian age, consisting mainly of dolerites, syenodioriteswith subordinate areas of syenites and ultrabasic rocks. The domi-nant outcrops form the Shoshong and the Marutlwe Hills.

3) The Ecca series of the Karroo System consisting of grits, shales,mudstones, sandstones, conglomerates and coal. They emerge mainlyin the northwest of the survey area and form partly the Kalahariescarpment.

4) Younger Kalahari sand.

1) The Mahalapye Formation in the area comprises the foliated roof zoneof a large Precambrian pluton and is correlated with the Messina series inSouth Africa (Poldervaart et a l , 1954; Grubb, 1961). The biotite granitewith parallel, tabular phenocrysts of microcline is intrusive in the Base-ment schists and gneisses in the Mahalapye area. Migmatisation of theBasement Complex by this younger granite is observed. Crockett (1965)

30

considers the Mahalapye Formation of palingenetic origin. The autoch-thone migmatites and gneisses are so advanced through granitization andmobilization that origin and affinity of much of the xenolithic materialis obscured. Where exposed, this material consists largely of biotite-richrocks as observed in the Morale Hills exposures.In the northern part of the catchment area of the Mahalapshwe river themobilized homogeneous granite is pale-pink to white with a medium tocoarse texture. The dominant feldspar is microcline. Of other mineralsgarnet occurs and biotite may be present if the granite is associated withxenolithic material. Migmatites are occasionally excellently exposed andconsist of dark schists intruded upon by granite (Jennings,,1963).Amidst the granite rocks the occurrence of a calcium-rich epidote-garnetrock sample (specimen 12 A) is to be noted. Neither the extension of thistype of rock nor its metamorphic facies are well known at present. Therock is classified tentatively as metadolerite.The elemental composition* of a number of rock samples of the MahalapyeFormation is given in table 8. They include samples from the Morale Hillsas well as from the granitic area around the Marutlwe Hill.

Me 8. Elemental composition of rocks from the Mahalapye Formation (% byweight)

impie

SA>A?B

'C'D

Rock type

syenodioriteleucogranitemetadoleriteleucogranitegranitegranodiorite

SiO2

61.081.157.676.478.681.6

Al2O3

11.47.65.7

10.18.37.5

Fe2O3

2.41.35.50.81.61.1

TiO2

0.800.320.690.280.460.29

CaO

5.622.53

15.111.912.822.20

M O

13.592.20

14.892.183.222.03

K2O

1.932.050.075.401.942.41

Na2O

3.102.900.082.712.962.76

P2O5

0.130.080.360.180.090.07

Loss onignition

1.00.81.60.81.51.2

The composition of the granites is similar to the one given by Crockett(1965) of granites a little south of the survey area and deviates onlyslightly from data given by Tyrell (1929), Dana and Ford (1932), Grout(1932), Wahlstrom (1948) and Mohr and Van Baren (1959).Microscopical qualitative analysis of thin sections made from the rockspecimen reveals the following.The granodiorite consists primarily of quartz, followed by plagioclase andsome orthoclase. Plagioclase in largely sericitised; biotite is partly trans-formed to chlorite, while epidote and colourless mica occur as secondary

* The element concentration is given as oxides.

31

products. Zircon is observed accessorily, pumpellyite occurs occasionally.Quartz is also the chief component of the leucogranite, followed byorthoclase. Plagioclase occurs but is also strongly sericitised. The smallamount of biotite is largely chloritised. Apatite and hornblende togetherwith epidote are present in subordinate amounts.The epidote-garnet rock consists mainly of epidote with pumpellyitesucceeded by chlorite, serpentine and garnet. Also present are titanite,quartz, and colourless amphibole. The rock composition and its occur-rence does suspect the metamorphic facies of an originally calcium-richrock, such as dolerite or gabbro, therefore the rock is named meta-dolerite.

2) The second source or parent material forms rather a contrast to thefirst one and consists of basic intrusive rock.The age of the intrusions is believed to be 590 ± 24 and 660 ± 26 m.y. asdetermined on two samples of the Shoshong sill (Jones et al., 1966). Thisdetermines their origin as Post-Waterberg and Pre-Karroo and as such ofUpper Precambrian to Cambrium. Of the same age, but of rather variablecomposition are several dykes, exposed particularly in the central catch-ment of the Mahalapshwe River (Grubb, 1961). The Shoshong Hills andthe Marutlwe Hill are the two most extensive sills in the survey area. Bothsills dip approximately 10° to the south, thus causing a drainage system todevelop into this direction. Much of the weathering products are thereforedeposited on the south side of these hills. Origin and composition of thesesills have been subject to a number of studies (Grubb, 1961;Cullen, 1961;Jones et al., 1966).Comparative research on similar intrusives in southern Africa was exe-cuted by Cox et al. (1967). These studies suggest that the Botswanadolerites belong to the southern province. The difference with the north-ern province is mainly based on the high values of K, Ti, P, Ba, Sr, Zr inthe northern province. The rocks are mainly dolerites and syenodiorites,with subsidary syenites and peridotites (Cullen, 1961).According to Grubb (1961) the Shoshong sill is a multiple intrusionformed by two successive influxes of basic magma. On the basis ofdifferentiation within the sill (increase of orthopyroxene phenocryststowards the base) five pétrographie zones can be recognized. Accompany-ing contact metamorphism was usually low-grade, although in theShoshong sill a roof zone of about three meters granophyric graniteoccurs, with a very coarse basal zone of red granite beneath the sill. Totalthickness of the sill is estimated at 400 m, with 150 m exposed. Theelemental composition of three random rock samples from the sills eluci-date the differences with the acid rock suite:

32

Table 9. Elemental composition of three rock samples from the Shoshong and theMarutlwe sill (% by weight)

No.

L2BS3BÎ7A

Rock type

DoleriteDoleriteDolerite

SiO2

50.852.951.9

Al2

15.15.14.

O3

310

Fe2O3

9.811.210.5

TiO2

0.630.800.59

CaO

11.269.829.84

MgO

9.277.32

10.38

KfO

0.861.080.75

Na2O

2.811.561.12

P2O5

0.100.150.07

Loss onignition

0.81.20.2

The above data are in general agreement with values given by Grubb(1961) and Cox et al. (1967). The trace element composition of the sills isnot known, but some data are available from similar sills in Swziland andfrom Karroo intrusives in the Tuli syncline (tables 10 and 11).

Table 10. Trace element composition of some dolerites in Swaziland (ppm)

Trace element

BaBeCoCrGaLiMoNb

mean

235< 331

1591812

424

range

85-390—

3 0 - 3595-2301 5 - 2010- 20

3 - 410- 30

Trace element

PbRbScSrVYZr

mean

<10<50

36300263

2679

range

_

2 0 - 65250-330200-350

10- 3535-100

Source: Cox et al., 1967.

In a recent account on the geology of the Karroo basalts in the Tulisyncline, which also touches Botswana in the northeast, Vail et al. (1969)report the presence of shoshonite. This alkali feldspar-bearing rock, pre-sumably named after the Shoshong dolerite sill, is described as a pheno-cryst basalt that contains abundant groundmass alkali feldspars as over-growth on plagioclase.Augite varies between trace to 3% and the percentage plagioclase rangesbetween trace and 10%. Although the majority of the basalt in the Tuliarea is classified as belonging to the northern province as defined by Cox(1967), the shoshonite clearly belongs to the southern province as de-scribed for the basalts in Swaziland. Trace element data from the Tulibasalts differ considerably from the Swaziland rocks. For the two rocksamples of the shoshonite the amount of trace elements is given in thefollowing table (Vail et al., 1969):

33

Table 11.

Element

Trace

Rb

1181183

element

Sr

13321273

composition of the

Ba

21602035

Zn

8682

Karroo

Zr

317331

basalt in

Nb

13314

the Tuli

Co

3630

syncline (ppm)

The above data may be more in agreement with the general compositionof the dolerite sills that form the Shoshong and Marutlwe Hills.Microscopic investigations of the dolerites disclose that the dominantmineral in sample 12B of the Shoshong sill is plagioclase, followed bypyroxene; a fair amount of epidote amd amphibole is observed. Augitedoes occur to a limited amount but is somewhat amphibolized locally;some biotite and quartz is observed. The second sample from the samearea (No. 83B) is dominated by pyroxene, that may be uralitised locally,followed by plagioclase. The amount of amphibole is extremely low, whilebiotite, chlorite and serpentine occur as accessory minerals.The rock sample from the Marutlwe Hill is similar to the latter describedsample. Pyroxenes are dominant except augite, while plagioclase occurs insubordinate amounts. Biotite, chlorite and serpentine occur infrequently.There has been little alteration while the optical structure is weaklyexpressed.

The process described as uralitization may explain the low percentage ofpyroxenes in the weathering products of the dolerite.Although no data are available on the content of rare elements in thegranites and in the basic rocks in the area, information herefrom is madeavailable by Edge et al. (1962) for comparable rock types in South Africa.The results are summarized in table 12 and supplement the knowledgeabout the parent material of the soils.

Note:The processes of sericitization and saussuritization that have effected the feldspars of the graniticand doleritic rock suites imply in petrographical sense the formation of sericite (fine scalycolourless mica) respectively of epidote (from calcium-rich plagioclase) during the late hydro-thermal phase. Uralization signifies the alteration of pyroxenes to amphiboles (fribrous horn-blende).

34

Table 12. Content of some rare elements in granitic and basic rocks in South Africa

(PPm)

Rock type ElementMolybdenum (Mo) Tin (Sn) Gallium (Ga) Scandium (Sc)

mean range mean range mean range mean rangegranitic 1.9 0.96-4.5 3.2 1.5-5.6 16.9 10.0-22.0 4.5 1.4-11.0

Yttrium (Y) Lathanum (La) Cerium (Ce) Niobium (Nb)

graniticbasic

mean3226

range5-55

13-33

mean8527

range29-17717- 36

mean525

47

range135-7903 5 - 67

mean7933

range17-1412 4 - 44

3) The sedimentary rock of the Ecca series are encountered in the north-west of the area; arenaceous and argillaceous types are dominant withinthe large variety of rocks that occur in this formation. Although nobedrock sample was collected from this rock type, the elemental composi-tion of the C material from a soil underlain by the Ecca series is given(table 13).

Table 13. Elemental composition of the fine earth fraction of a weathered rock fromthe Ecca series (% by weight)

Loss onElement SiO2 A12O3 Fe2O3 TiO2 Cao K2O Na2O P2O5 ignition

Rock typeS,chist 73.8 21.6 2.1 0.77 0.13 1.28 0.34 0.00 8.2

The trace element composition of the same sample in ppm indicates a lowamount of Mn (60), Cu (26), Zn (4), Co (18), Ni (30), and Sr (7), while Cr(146) and Ba (200) are slightly higher.The mineralogical analyses of the clay fraction indicate that kaolinitemakes up almost the entire composition. Heavy mineral determination onsimilar rocks in South Africa (Harmse, 1967) show low zircon andtourmaline percentages. The garnet percentage is high, epidote and horn-blende are rare, some rutile occurs.

4) The vast cover of Kalahari sand forms the parent material for a varietyof less developed soils in the interior. The soil under consideration islocated on the eastern edge of the Kalahari sandveld and is probably

35

developed in redeposited Kalahari sand (Wayland, 1953; and Poldervaart,vaart, 1957).The source of the sand has not been retraced sufficiently to make anybinding comments. However, it seems possible that especially the youngersands may have been derived from the disintegration of Cave Sandstone(Boockock et al., 1962). Elemental composition of a specimen from thissandstone is given in table 14.

Table 14.

Element

Elementalweight)

SiO2

Rock typeFine-grained 93.8sandstone

composition of a

A12O3

3.3

Fe2O3

0.3

rock specimen from the

TiO2 CaO MgO

0.08 0.51 0.84

Cave Sandstone (% by

K2O Na2O P2O5

0.87 0.17 0.10

Loss onignition

0.9

Microscopical evaluation of the sandstone confirms the very high amountof quartz, while it is also detected that SiO2 is the dominant cementingagent. Plagioclase is rare, the amount of zircon and tourmaline is very low.The sorting of the grains is moderate and the roundness is poor. Data onthe heavy mineral assemblage of the Cave Sandstone Stage as collected byHarmse (1967) indicate a fairly low percentage of zircon (15%) andgarnet (3%), but the percentage of tourmaline is high (41%). An almostopposite picture is obtained while looking at the data on the heavyminerals as available from a soil developed in the Kalahari sand. Zircon isdominant (43%), succeeded by tourmaline (28%); rutile is slightly less (7%versus 10%). The reason must be sought in the higher resistance toweathering of zircon causing it to occur frequently in the weatheringproduct of the Cave Sandstone.The source of the parent material for the majority of the soils is related tothe granitic rock suite. The dolerite and associated rocks follow, while therocks of the Ecca series and the younger Kalahari sands form the parentmaterial only in a few cases. To procure insight in the relation betweenthe bedrock material and the weathered rock as encountered in a numberof profiles, ratios as introduced by Jenny (1941) were applied. Tworeservations must be made. Firstly, the obtained picture is approximate asin no case was unweathered bedrock material (R) included in the soilsamples encountered, and secondly, alteration of the weathered parentmaterial (C) is possible because lateral or certical mineral transport is notaccounted for.The SiO2 /Al2 O3 and the K2 O+MgO+CaO/Al2 O3 indices indicate the

36

Fig. 5. Relation between rock and weathered rock based on leaching values

37

removal or accumulation of the cations of the numerator, if we assumethat the solubility of Al is negligible in the soil reactions encountered(Krauskopf, 1967).

As illustrated in figure 5, a number of distinctive groups occur. The firstgroup comprises the bedrock samples. The ratio SiO2 /Al2 O3 for thedoleritic rocks (Nos. 12B, 83B, 87A) varies less than for the granitic rocks(Nos. 1, 83A, 87B, C, D), the latter range between 3 and 6 and the formerbetween 3 and 5.A sharp distinction is present between C material from a number of Ulticand Oxic Haplustalfs; a Haplustalf (15), an Entisol (1.7) and an Inceptisol(5) on one side and two Oxic Haplustalfs (29, 32) and some other Alfisolson the other side. The former group shows a high SiO2/Al2O3 ratio of 7or more while the base metals are indicated between 0.30 and 0.80; thelast group has a ratio of 2—4 with base metals less than 0.30. The Cmaterial from most of the remaining granitic soils (Nos. 3, 4, 6, 8, 13, 14,18, 23, 33) not only has a far lower SiO2/Al2O3 ratio, in fact comparableto the one encountered in soils derived from the doleritic rock suite (Nos.22, 25, 26), but also has a lower base metal index. These data suggest thatthe development of soils derived from granitic material undergoes a stagein development in which an increasing amount of SiO2 is accompanied bya lower percentage of base metal ions. Further evolution indicates alowering of the silica content together with a lower amount of basiccations. Remarkable is also the correlation between the highly calcareousmaterial from three profiles (1.0, 24, 31.) and the epidote-garnet rock (No.12A), tentatively classified as metadolerite. Parent material from onesoils presumably derived from rocks of the Ecca series (No. 29) havean identical Sio2/Al2O3 ratio, with a slightly differing status in basemetal cations. Loughnan (1969) reports on a similar loss of cations fromthe environment. A comprehensive account on the mineralogy of the soilsis given in section 11.3.1. Remarkable is the presence of highly calcereousmaterial (calcrete) in the deep subsoil of profiles 10, 24 and 31. In thelatter, overlying soil material has developed as a Vertisol, while profile 10is classified as an Alfisol.

Although a relation seems to exist between the calcium-rich matter andthe metadolerite (see Fig. 5) their affinity can be doubted on basis of theoverlying soil material and keeping in mind the very old age of the graniticrock and "younger" basic formations. Micromorphological observationssuggest the displacement of the calcium-rich material in profile 25, but donot consider such action relative to profile 10.

38

1.3. Geomorphology

1.3.1. TopographyCompared to the remaining flat to almost flat profile of the Botswanacountryside, the Eastern part has a variable topography. This is particu-larly true for the study area where incised rivers, rock outcrops and hillsgive rise to undulating to rolling relief alternating with level stretches ofland. This is partly due to the different geological formations that occur inthe area, while river activity has been and still can be relatively active aspart of the Limpopo drainage system.As a result of the interaction between various agents a topography of thefollowing diversity is encountered. In the southeastern part the country-side is flat to almost flat with an altitude between 1050 and 1130 m. Themost outstanding topographical feature are the Morale Hills, a graniticrock outcrop that rises to about 1370 m above the surrounding country(Bawden et al., 1963). Therein shallow pans and drainage channels aredistinguished, the latter determined by the hills. The majority of the pansis observed in the Southeast of the area and the East. In addition to theorigin of the pans described by Harmse (1967) as eolian and byBoockock et al. (1962) as eolian and caused by animals and as part of anolder drainage system (Geyser, 1950), the solution of the underlyingcalcrete in the area under consideration could induce the formation ofpans. The Bonwapitse River is the most conspicuous drainage channel inthe area. In some places an incision of two to three meters in the floodplain is observed.In the immediate surrounding of the river-bed gully erosion occurs. Thistype of erosion is limited in the area and is often initiated by manthrough destruction of the vegetation, or along roads. Sheet erosion seemsmore common although its results are often masked.The percentage of rock outcrops and the difference between high and lowincrease towards the North and Northwest. This section of the area has inparts a rolling topography, although flat tracks of land do occur on thedivides.Local relief differences may be 50 m or more, the general altitude of thearea is between 1130 and 1310 m. Gully erosion is common along thedrainage channels from the Mahalapshwe River and its main tributary theMmitle, often resulting in truncation of soils. The topography becomesless differentiated in the headwaters of the Mahalpshwe River. Gentlyrolling topography prevails, although near the drainage lines 1—2 m deepgullies may be encountered. The most remarkable feature is the watershedbetween the Mahalapshwe River and the internal Kalahari drainage system.The divide has taken the form of an escarpment in some areas, while inothers a slow rise marks the transition towards the Kalahari sandveld. The

39

differences are mainly ascribed to changes in the geological formations.The scarp may stand 20—30 m above the surrounding areas. The westerlyand south-westerly part of the survey area is again characterized by asuccession of level to almost level topography of 1130 m altitude onlyinterrupted by the Shoshong and Marutlwe Hills. These may rise to about150 m above the surrounding land.Gully erosion is again confined to the area near the headwaters of theBonwapitse River and its tributaries. The river itself is incised 2—3 m inthe flood-plain. The hills are flat on top with raised edges. The debrisslope usually has a gradient of 25—30°.Granitic rock outcrops are rare in this area but some occur East ofMutane and Southwest of Shoshong.The inexperienced observer may find the local differences in topographyrather insignificant; however, together with the other soil forming factorsthey are the cause of variable soil profile development (Mabbutt, 1966).

1:3.2. River SystemsThe area is drained by two rivers, the Bonwapitse and the Mahalapshwe,which are classified as ephemeral streams because heavy precipitation isneeded to cause enough run-off for streamflow. These flows seldom lastlonger than a couple of days and often not longer than hours, after whichthe river is dry, except for its saturated bed.Depending upon rainfall, the magnitude of the flows measured for theMahalapshwe River at peak discharge is 752 m3/sec. (Hyde, 1972).The developed drainage pattern of this river is classified as dendritic(Lobeck, 1939; Finch et al., 1942). The stream is occasionally con-trolled by the structure of the geological formations and may then berectangular. The pattern as a whole reflects the situation such as candevelop on granitic rock. The Bonwapitse River is divided into twobranches. The northerly, originating near Kalamare, has a similar patternas the Mahalapshwe River. The southern one, which developes nearShoshong, is classified as rectangular. Weakened zones in the bedrockcaused by faults together with the slight dip of the hills to the south hasresulted in a number of streams running north to south. These flowtogether south of the hills and then form the southern branch of theBonwapitse River. The overall direction of the drainage is to the southeastfollowing the opposite direction of a broad, low westerly pitching anticline,which is the major structural unit of the area (Cullen, 1961) and embracesrocks of the Basement Complex, the intrusives and the Transvaal and KarrooSystems. There is no evidence that streamflow was once directed towardsthe west. In the northern part of the Limpopo catchment in Botswanathere is, on the contrary, topographical evidence that east flowing riversmay have been decapitated in the headwaters, approximately in the zone

40

Table 15. Data of stream deposits.

sample

sample Asample Bsample Csample Dsample E

sample Asample ßsample Csample Dsample E

sample Asample BSample Csample Dsample E

sample Asample Csample D

Granulometric analyses.

gravel

>2000/ i m

7.430.619.322.516.4

Chemica

org.C %

3.280.030.050.230.06

2000-1000fim

7.839.821.430.928.9

I data

N%

0.30tr

tr

0.02tr

sand

1 0 0 0 - 5 0 0 -500

fim

3.819.713.219.621.7

200/lm

5.215.714.419.227.2

C/N

10.9--11.5—

Heavy mineral composition of the

c

§ 17 tr

tr tr6 1

17 trtr tr

94187

4->

'c

1112

1

Elemental composition

SiO2

52.054.856.0

A12O3

25.617.924.9

Fe2O

11.313.812.1

— 0

tr 81 9

522916

of the clay

3 TiO2

1.381.811.11

200-100flm

2.424.013.212.015.5

pH-H

7.57.78.38.89.0

10050

flm

1.805.92.60.9

silt

5 0 -20txm

19.708.21.90.5

2O pH-KCl

6.7.6.7.8.

51161

2 0 -2

/ i m

24.80.8

11.43.12.2

sand fraction (50—200 micron)

c

221

112

o

51 359 228 -19 -16 1

fraction

CaO

0.240.650.23

MgO

4.974.982.55

oÖuuEJr-t

>-.

t r

tr

—

2

17

K2O

2.194.602.55

<D

raO.

-

—

6—

-

u

_O151

1—-

-

Na2O

0.150.770.00

clay

<2flm

34.50

12.310.73.1

OJ

b -

o

18214

1240

P2O;

0.830.591.38

0)

CUQ

2314376130

Loss on; ignition

12.59.65.4

41

where the Kalahari escarpment is featured nowadays. The dating ot theseevents is still tentative but is probably older than Miocene (Dixey, 1943).

The nature of precipitation together with soil and topographical factorshas resulted in a highly varying sediment load ot the rivers when in flow.Stream-bed samples taken from the two rivers at the outflow of the areashow a dominant very coarse textured composition. Alternating thereinare the finer sediments deposited in the tranquil environment at: the endot the main flows. High and low stream velocities and consequentlychanijinii carrying capacities arc the main cause of the textural differentia-tion. The overall character of the deposits are strongly influenced by thenature of the hinterland, such as geology, topography and vegetation.The locution of the samples is given on fig. 1. Details on sample sites are:Sample A : depth 0 — I).5 cm (fig. 6)Sample B : depth 5 - 1 5 cmSample C : depth approximately 400 cmSample D : depth 0 - 10 cmSample E : depth 0 - 1 0 cmSome chemical and physical data ot the deposits are given in tabel 15.

f:-

Fig. 6. Flood residue of a flow in the Malmlapslnrc river

42

The granitic component in the river samples is dominant in the sand frac-tion, particularly in the medium and coarse sizes. The finer componentsare chiefly provoked by the presence of basic rock. Although this may beevident for the southern branch of the Bonwapitse River that originates inthe doleritic sill, such is also the case for samples taken from the Maha-lapshwe River and is ascribed to the doleritic dykes that occur in thatcatchment area.

The interpretation of the Röntgen diffractograms yield the followingrelative to the composition of the clay fraction: sample A is mainlycomposed out of montmorillonite; in sample C kaolinite with montmoril-lonite, chlorite and quartz are present; sample D is almost entirely com-posed of kaolinite with some quartz, while sample E contains kaolinitewith montmorillonite, chlorite and quartz and is thus similar to sample C.The sandy loam deposit found 500 m upstream of the Mahalapshwe roadbridge at a depth of 4 m (sample E) underlies a mainly coarse sandy andgravelly bed. It presumably overlies bedrock, although in or just above thedeposit a coarse gravelly layer may be present. Similar successions havebeen reported from other rivers in East Botswana. The sedimentation ofthis kind of finer textured sediment, which is estimated at least 1 m thick,was most likely caused by other environmental conditions in the catch-ment area than prevail at present. They indicate a denser vegetative coverand a more evenly distributed rainfall regime (Watson, 1969). Furtherresearch on stream-bed deposits might contribute valuable knowledgeabout the history of this part of Botswana and possibly for the country onthe whole.

1.3.3. LandscapesThe division of the landscapes as given below resulted from an evaluationof the environmental factors that create these forms directed towardstheir usefulness in soil survey work. The use of land systems, land unitsand other variable scales of morphological entities in practical survey workis well recognized by a great number of workers and was successfullyapplied in Australia (Mabbutt et al., 1963) and was also adapted byBawden et al. (1963) in Botswana.

The observed morphology calls for the recognition of a number oflandscapes, which are indicated on figure 7.I) The landscape of the Morale Hills which is made up of the following

elements: the granitic rock outcrop, represented as a series of moreor less well developed domes that undergo strong exfoliation; theassociated debris slope, also called colluvium (Swan, 1970) or hillslope (Young, 1972), and the succeeding pediments. The latter are

43

Fig. 7. Landscapes and geology of the investigated area

44

well developed and of such an extent that their coalescence gives riseto a pediplain (King. 1967) of a slope between 0.5-3°. The slopeangle between the upper part of the pediment and the lower part ofthe hill slope is well marked. To the north and the south thetransition of the pediplain to an almost level erosion surface is notdistinct. The whole area was classified by Bawden and Stobbs (1963)as belonging to the land system of the Central Plains.In an account on the presence of peneplains in Botswana, Jennings(1962) states that the plains belong to the Late Tertiary erosionsurface, as defined by Dixey (1943) and King (1967) for SouthernAfrica. An altitude of 820 m to 1210 m is maintained for thissurface. These observations on the succession of landscape elementsis similar to those made by Fair (1949). Mabbutt (1955), Pallistcr(1956), Moss (1963) and Young (1972).

The landscape of the upper Mahalapshwe catchment and northernbranch of the Bonwapitse River is essentially an eroded peneplain ofan altitude between 1130 m and 1280 m, on the basis of itstopography the transition to the Kalahari sandveld is indicated differ-ently (lia). The main elements are rock outcrops (basic and acid)

Key to fig. 7.

Svmbal Geomorphology Geology

II..

IV

hills, colluvmm.pediment and pediplain

dissected peneplain

slightly eroded peneplain

tlat copped hills with asso-ciated scarp and colluvium

pediment

flood-plain

alluvial fan

plain (sandveld)

Archacn rocks of the Basement Complex, mainlygranites from the Mahalapye Formation

Arcliaen rocks of the Basement Complex un-differendated granites occurrence of dolcritic dikes

as II with some areas of Ecca Series

Post Waterberg-Pre Karroo basic intrusices, mainlydolcrite

Undifferentiated granitic rocks from the BasementComplex with subordinate occurrence of dolc-ritc

Pleistocene to recent calcareous clayey deposits

Holocene to recent undifferentiated deposits

Pleistocene to Holocene eolian sands

45

with their associated areas as described for the Morale Hills, but on asmaller scale. The granitic rock outcrops may have the '"kopje" tormor are classified as "whale backs" [Watson. 1964). The direction offractures and joints seems foremost responsible for the shape of hilldeveloped (Hurault, 1967). Here also the piedmont angle is welldefined.Depending on the density of the drainage the interfluvcs may carry avariable thickness of soil overlying weathered rock. Convex tostraight and then transitional to concave slopes are common, gra-dients of more than 3° occur. Alluvial deposits in the drainagechannels vary considerably, mainly depending on the rate of erosion.Overlooking this part of the investigated area, the imaginary surfaceformed by the summits of the rock outcrops seems to occur belowthe skyline, which is made up by the contours of die associatedhigher areas ^ landscape III). Similar observations were made by Moss(1968) in West Africa. Jennings (1962) dated the eroded plain asLate Tertiary, however it is not clear which surface is meant, thesummit level of the hills or the planation surface around the rockoutcrops.

HI) The landscape of the doleritic hills is characterized by the almost flatsummit level of the hills, followed by the "scarp" that is composedout of the outer upper rock boundary, the associated hillslope

; *&&?•-*£$*

l~ig. 8. I he eastern slope of The Mtirntlwe hill with associated pediment

formed by colluvium and pediments (Ilia), (fig. 8).The transition to the Bonwapitse flood-plain (Illb) is on a verygradual slope. Concave slope forms are dominant near the hillswhereas the gradient becomes flatter towards the stream. Graniticpediments are well developed on the northern side of the hills, whileon the southern side smaller pediments (sub-alluvial) and occasionallyalluvial fans ( 11 Lc) occur.The hill tops showing an altitude between 1300 — 3 350 m were corre-lated by Jennings (1962) with the Early Tertiary cycle of erosion asdescribed by King (1962). To what extent the summit of the MoraleHills (1370 m) belongs to this surface is still a matter of speculation.

IV) A similar postulation is made for the Kalahari sandveld, that risesabout 1350 m above sea-level. The plain, underlain by a Mesozoic orlate Tertiary erosion surface (Groove, 1969), is encountered in theextreme northwest corner of the study area. The transition to theLimpopo drainage system is variable, but often takes the form of apediment at an altitude between 1280 m — 1320 m.In the interior, rock outcrops in the form of sandstone mesas occur,while old drainage channels and pans are a common feature in someparts of the Kalahari (Boockock and Van Straten, 3962; Groove,1969). In the area concerned, such topography is absent and thelandscape elements limited to level or almost level "sandveld".

The genesis of the various landscapes has not been touched upon inten-sively as it falls outside the scope of the present study. The morphody-namical aspects are well described in more recent literature (Weise, 1970;Swan, 1970; Biidel, 1971). However, if applicable references to therelationships between the soils and the various landscape elements aregiven (see also Chapter II).

1.4. Vegetation

1.4.1. Natural vegetationThe term "natural vegetation" is not without double meaning and wasused to indicate a stable type of vegetation in complete equilibrium withclimatic and soil conditions (Eyre, 1968).Nowadays the term "climatic climax vegetation" is used in a situationimplying that among other conditions, no human activity has influencedthe vegetation. As such, the vegetation of the whole area under consider-ation does not satisfy the requirements of this definition. However, in anattempt to make a distinction between areas in which human influence isstrong and those where conditions have changed relatively little, the term"natural vegetation" has been maintained to indicate the latter.

47

This vegetation that developed under semi-arid conditions was describedby De Beer (1962) as a tree savanna, wherein Acacia species and Com-bretum varieties are dominant. On the revised physiographic vegetationmap of Botswana (Weare et al., 1971), the area is classified as an Acacianigrescens/Combretum apiculatum tree savanna (unit 21).The use of the terms "savanna" and "steppe" to indicate vegetation typesin Africa has led to confusion about the exact definition and meaning ofthese terms (Acocks, 1951; Aubreville, 1957; Richards, 1961; Cole, 1963;Walter, 1964). Much was clarified by Eyre (1968) who recorded threetypes of savanna in Africa, namely 1) high grass/low tree savanna, 2) acacia/tall grass savanna, and 3) acacia/desert grass savanna. The second type isrecognized in the area.Within a certain type of savanna, the grass cover and tne tree density mayvary considerably. It was found necessary therefore by Philips (1969,1970) to introduce a more detailed terminology with special regard toSouthern Africa. Consequently, terms as "woodland" and "grassland"with their variations came into use.In his study, Philips defines woodland as a community of trees and largewoody shrubs, occurring either singly or in a single or several storeyedclumps of varying size, separated by appreciable distances, but crowns notusually more than thrice the height of the trees, the intervening growthfrom stem to stem being grass with or without sub-shrub and forbs (Philips,1970).An important sub-division of the woodland is the shrub open woodlanddefined as "open woodland in which the widely scattered elements arelarge shrubs set in a matrix of grass." This woodland may occur naturallywhere edaphic control is marked, such as poor drainage, or may beinduced by man or animal, (see 1.4.2.)The overriding importance of the climate for the vegetation has beenrecognized for a long time in the world and in Africa in particular.Differentiation in composition and character of the vegetation within theclimatic regions is however caused mainly by edaphic factors such as soiland topography (Cole, 1963).In an attempt to clarify the long lists of vegetation species that seem toresult from ecological surveys within a vegetation type, Van Rensburg(1971) designated a number of trees, shrubs, herbs and grasses as indicatorspecies. Each species or combination thereof as characteristic for a certaincombination of factors can be applied distinctively to indicate differencesbetween for example, soil and topography. During soil surveys in the area,emphasis was placed on the recognition of the woody species and thegrasses with relation to the different soils encountered. As a result ofintensive ecological research in the terrain, the overall classification for thevegetation by Van Rensburg is named an "Acacia mixed woodland and

48

Fig. 9. ,'lfjcia mixed woodland dominated by Acacia nigrescens with some Comhretum apicu-laturn, site of profile So. 1 ; m the background one of the exfoliated domes of the Morale Hills

Combretum apiculatum woodland", (fig- 9).The Acacia mixed woodland consists of the following acacia species:Acacia nilotica, A. grandicornuta, A. gerrardii, A. tortilis, A. tenuespina,A. karroo and A, nigresceus. These species are often associated with Albi-zia harveyi. Combretum apiculatum occurs of ten together with Tenuinaliascricea (tig. 10).The correlation between the woody species and the soils is illustrated intable 16.The grasses may also be divided and correlated with different soil typesand vegetation communities. A very broad distinction is made betweenthe sweet and the sour veld. Whereas the former covers the grasslands ofthe warm areas, the latter comprises the cooler and wetter areas. Thesweet veld ramains palatable during the winter while the sour veid doesnot (Acocks. 1964). The 625—750 mm isohyet is taken as approximateboundary between these two types, a considerable transition zone doesexist. Due to human interference much of the indigenous perennial grasses

49

r/ij. /(). Omikrefitm iijiiculututn and 'Vcrmmalui sericea woodland, site of profile No. 2

Table 16. Relation between woody species, soil and topography

Vegetation ^woody species) Soil Topograph)

Acacia uiloticaA Ibizia harveyiAcacia nigrescens

Combretwn apicuhitttw

Terminalia sericea

A. grandieornutaA. tcnuispinaIndigo fera schim periLintonia nutansA. tort His

Kirkia accuminataSclerolcarya caffraCommiphora. spp.Albizia rhodesica

deep. dark, cracking claydeep. dark, cracking clayshallow to medium deep, blockedsandy loamas above and ;ilso on deep.sandy loamsshallow, blocked sandy loam.deep loamy sands and sandsdeep, dark, cracking andmulching clay

deep, reddish-brown sandy(clay) loamvery shallow and stonysoils

drainage linesdrainage linespediplain

pediplain

pediplain

pediment andflood-plain

pediplain

colluvium andhill summit

(modified after Van Rensburg, 1971).

50

that make up the sweet veld in the central pare of East Botswana, havebeen replaced by annuals. The original grass vegetation is visible only atplaces outside the reach of animal or man. The preference of the maingrass species for a certain soil is illustrated in the following table.

Table I 7. Relation between soils/woody vegetation and grasses

Soils/Woody Vegetation

Kalahari sandfTcrminaliasericea woodland

Sandy [oamiConibretumapiculatum woodland

Clay loams with or withoutcalcrete along drainagelities/.-lftiCJtJ mixed woodland

Clay loams and clays alongdrainage lines/.4c<Jt~/<j karroo.A. grandicoruuta and A. tonte spina

Clay soils of flood-plains-pediment/\ caria mixed woodland

(fig. 11)

tirasses

Anthephora pnbescensAristidü stipitataDigitaria milanjiatiaEragrostis pullensVrochloa brachyura

Digitaria )iii!anjianaEragrostis rigidior. E. supcrbaSclnuidtia pappophoinoides

Ccnchrus ciliurisDigitaria criant haPanicutn coloratura and P. )}ttixi)niwit'rochtoa tricJiopis

Diclianthimti papiltosuwUchaemutn brach y at her urnPanicum maximumSorghum verticilliflonim

Bothriochloa htsculptiaC\mbopogon exacavatusThemeda triandra

(modified after Van Rensburg, 1971

1.4.2. Human induced vegetationVegetation is not only modified by humans through the introduction ofcrops, fodder and legumes in a certain area, but even more evidently bythe effects of associated settlement activities, particularly in areas wherethe holding of livestock is a main source of income. These effects includedestruction of the vegetation by trampling, uncontrolled grazing, oftenresulting in overgrazed almost bare areas: uncontrolled cultivation includ-ing cutting and burning. As a result of the aforementioned activities theencroachment of bush is a serious problem in the Eastern part of Bots-wana. Woody species that readily encroach include Dichrostachys ccucrea,

Fig. 11. Acacia mixed woodland on clay soils dominated by Acacia grandicormita, site of profileNo. 25

Acacia fleckii, A. mellifera, A. nigrescens, A. temiespiua, A. tortilis andliuclea umiulata (Van Rensburg. 1971 ). Grazing potential is thus consider-ably lowered while the costs for the clearing of land for cultivation or otherpurposes rise accordingly. An additional factor is that original woodlandor grassland that has been disturbed is often permitted to rc-develop intoshrub open woodland or suchlike. Additional sub-divisions are then neces-sary to term the vegetation accordingly.

1.4.3. I'egetation patternsVegetation patterns have been identified and described in many areas, asreported by Wickens et al. (1971). These patterns seem to occur mainly insemi-arid and arid areas (Langdale-Brown, 1968). where soil moisture isthe most crucial factor for die development of vegetation (Wickens et al..1971: Worrall, 1960; Harrison et al., 1958). Most patterns appear toconsist basically of alternating areas with and without vegetation. Minordifferences in topography and textural composition of the soils in theareas seem to determine the variety in the vegetation.In the survey area three vegetation patterns can be detected on the aerialphotographs. The first pattern consists of alternating light and dark spots.

52

Ground examination revealed the light spots to be bare or almost bare ofvegetation while the darker spots correspond with patches of vegetation.This pattern seems largely confined to the Termiualia sericea woodland(fig. 12j.

Fig. 12, Aerial photograph of the vegetation pattern at profile No. It

The second pattern that evolves consists of alternating light and darkerstripes, which are disconnected and not continuous (vegetation aroundprofile 7). The explanation of the colour contrasts on the photos is similarto that for the first pattern. This second pattern is mainly encountered inCoinbretiim apiculatum woodland with Terminalia sericea.A third and less clearly defined type consists of alternating dark and lightlines, perpendicular to the slope (see fig. 23, profile 31).In these areas the controlling factor may be the run-off along the cattletracks, which follow an established pattern between the grazing groundsand the watering points. From our information may be deducted thatpatterns in vegetation in the area are more likely to originate on coarse-textured soils, that are sensitive to soil moisture for plant growth than onthe other, in general more loamy soils. In addition, soil depth can play animportant role in the suitability of a soil for vegetation. Depth consider-ably determines soil moisture properties, especially in the survey areawhere a rather undulating blockage in the subsoil is common. Woodyspecies as Tcrminalia sericea and Combretum apiculatum are readilycorrelated with coarse-textured soils (fig. 10).

53

Where the vegetation could follow up the pioneer species, the soil environ-ment becomes more suitable for continued plant growth, particularly asthe infiltration rate of the topsoil under the canopy increases (Lytord etal.. 1969). How far the human factor is responsible for the origin otvcgetational patterns could not be established. It appears, however, tliatthe patterns of the indigenous vegetation persist even in cleared fields.Vegetation patterns due to a particular soil-slope relationship or to slopeexposure giving rise to catenary vegetation successions are well observed inAfrica (Thomas. 1945; Harrison and Jackson, 195&). Their occurrence ishowever not marked well enough in the survey area to be applicable overlarge regions.With regard to the vegetation on hill slopes, it was observed that ou aslope exposed to the north it is more luxuriant than on a slope withexposure to the south. This variation is locally clearly visible in theShoshong Hills. Little is known about the existence of the vegetationpatterns in past or historic times, if edaphic factors are mainly responsiblefor their genesis, then it is not unlikely that the patterns may have existedfor a relatively long time. The composition and distribution, however, ofthe various vegetation types in Southern Africa in the Pleistocene diddiffer considerably from the present natural vegetation especially duringthe drier periods of that era. During those times the extent of the Kalaharigrassland increased notably with a decrease in the distribution of Brachy-stegia in particular. When wetter conditions prevailed Brachystegidj-.\cdcid/Cü)iiHÜphora woodlands tended to increase (Cooke. 1964).

54

CHAPTER II

TYPICAL SOILS

II.1, Location of sample sitesDuring and after a semi-derailed soil survey of the area a number of soilswas selected for this study. The sites chosen are as much as possiblerepresentative of the natural environment of the soils, although in somecases human influences could not be excluded. The essentials o^ the soildescriptions and some soil properties are given in the following para-graphs.The described and sampled soil pedons are located in a number of sampleareas, which coincide largely with the gcomorphological units as dealtwith in Chapter 1.3.3. The majority of soil profiles is situated on fourtraverses in general perpendicular to the slope, whereas the remainingprofiles are chosen individually within the units. The lines on the aerialphotographs are soil boundaries, often delineating vegetation boundariesand to a lesser degree gcomorphological and geological units.Sample area A exhibits the terrain of the Morale Hills (fig. 13j with theprofiles 1 to 4 on a traverse and the profiles 10 and 1 1 separately east andsouth of these hills on the pcdiplain.In sample area B the soils from the peneplain are located, the profiles 5 to9 (fig. 16) represent the different soils.Sample area C is part of the eroded peneplain where the profiles 1 2 to 18are located on a cross-section through the landscape (fig. 19).Sample area D represents the region of the doteritic sills and the associatedgranitic pediment with the flood-plain of the Bonwapitse River (Kg. 21 ).The soil profiles 19 to 28 are situated in this area on a traverse.Sample area E covers the extreme northwest of the survey area and is thetransition from the Mahalapshwe catchment area to the Kalahari sandveld.The soil profiles 29 to 33 represent the soils in this area; of these the sites31 to ?>3 are located on a cross-section (fig. 23).The sites arc also indicated on figure 1 and 7.The description of the soils in the field is in accordance with the guide-lines set by the FAO (1966). which are largely based on the Soil SurveyManual (1951 and 1961). For additional terminology the glossary of theSSSA (1964) is applied as well as the definitions suggested by the FAO/UNESCO (1968, 1970). The soil description was set to a depth of twometers or a lit hie contact, whichever shallover (Van Wambeke. 1962;Smith. 1965).The soil morphology controlled in first instance the frequence of sam-

55

plins;. In general a vertical distance up to 30- 40 cm was taken per sample,below a depth of 100 cm this thickness may be somewhat larger. It feltnecessary the sampling by soil auejcr was continued below a deptli of200 cm. Colour indication is for moist conditions unless stated otherwise.For complete soil profile descriptions and analytical data see Appendix 11.References to the soil genesis are resticted and fully discussed inChapter V.

11.2. Field observationsThe observations in the field arc çiven in the following sections accordingto the sample areas as outlined in 11.1. The environmental characteristicsof the soil profiles in an area are given in tables 18 to 22.

Sample area A: Soils of the Morale Hills toposequence and associatedpediplain (fig. I 3).

Fig. 13. Part of sample area A

56

Brief description oj profile So. 1 - Ultic HaplustalfThe dark brown A horizon of a slightly gravelly coarse sandy loam textureis underlain from 20-60 cm by a reddish brown slightly gravelly sandyclay loam (B21j. which gradually transists to a B22 of similar nature: ablockage of boulders is encountered at 110 cm. The soil is structurelessand with a neutral soil reaction, root development is confined to the top40 cm.Horizonation is weakly developed, smooth gradual boundaries prevail. Theanalytical data indicate a low CEC/soil that increases with depth from 6.1to 1 2.2. calcium is the dominant exchangeable cation followed by magne-sium.The base saturation is above 50% and increases regularly with depth to8y/(.- in the 1322. The percentage organic carbon is low and decreases withdepth.

I i riefdescription of profile So. 2 — Ultic HaplustalfThe dark brown loamy coarse sand that makes up the A horizon overlies adeep, brown B horizon of a slightly gravelly coarse sandy loam texture,the clay percentage and the amount of coarse fragments increase regularlywith depth; strong brown mottles are observed in the upper part of the Bhorizon (B21), while red mottles occur in increasing amount between88—120 cm depth: quartz gravel blocks further examination at 145 cm;structural development is confined to the top 23 cm. weak subangularblocky structures are common, while root development is at the maxi-mum in the top 60 cm: the soil reaction is slightly to medium acid-Chemical data of this soil reveal a very low CEC/soil between 3.50 and6.61, that increases regularly with depth, the dominant exchangeablecations are calcium and magnesium: the base saturation varies around 43%in the A and is at a minimum in the mottled B horizons (23'/f), butincreases in the B23 and B3 horizon to respectively 37% and 40%. Theorganic carbon percentage is highest in the topsoil and decreases withdepth.

Uric f description of profile No, 3 — Ultic Haplustalf (fig. 14)The dark reddish brown A of coarse sandy loam texture overlies ahomogeneous, dark red to reddish brown B horizon from 2] to 98 cm,in which a regular increase (if clay is encountered, giving rise to a coarsesandy loam texture in ehe Bl and a sandy clay loam texture in the lowerpart of the B, cutans are well expressed in the B2t (62—98 cm); redmottles in a brown matrix form the contrast in the B23 from 98 - 1 1 8 cm;structural development is weak to absent while root development ismainly confined to the top 40 cm; the soil reaction is slightly acid; a

57

lig. 14. Vltic Haplnsmlf, profile No. J Jïg. 75. "A^nic" ILiplitftiilf, profile -\'i.'. 4

blockage of coarse angular gravel occurs at 133 cm depth. Yellowish redmottlin" in a brown matrix différenciâtes the B3 horizon ( 11 H- 1 33 cm).According to the analytical data the dominant exchangeable cations areCa and Mg. rhe CEC/soil value is very low, but increases regularly withdepth from 3.95 to 9.09 in the B2t, followed by a decrease in the deepsubsoil. The base saturation percentage is above 50'.' for all horizons andincreases with depth to 73'.r in the B3. The organic carbon content is verylow and insignificant in the deeper soil horizons.

Brief description of profile Xo. 4 • ".\ijuic" Haplu^talf (fig. 1 5)The horizonation of the soil is fairly well developed and expressed asfollows. A dark brown AI of a coarse sandy loam texture is underlain from16-78 cm by a reddish brown B horizon of sandy clay (loam.i texture:from 78 cm onwards yellowish red mottles occur in the brown matrix,followed bv dark ^rey mottles in a brown matrix, B2g (7H—97 cm): below1 12 cm depth the matrix colour is a very dark grey in which brownishmottling appears, the transition to the C horizon is marked by thepresence of calcium carbonate accumulations of varying hardness; theamount of CaCOj increases considerably with depth. Ar a depth of 1 25 cm

extremely hard mottled granite is encountered. The structure is weaklydeveloped and mainly confined to the top 43 cm, as is the root development;cutans are clearly visible in the lower part of the B horizon, notably in theB2g; the soil reaction is neutral in the topsoil and the upper part of the Bhorizon and mildly to moderately alkaline in the B2ca and the C horizon.The supporting chemical data show a clear increase in the CEC/soil from4.39 in the Al to 26.7 in the lower subsoil. The dominant exchangeablecations are calcium and magnesium followed by potassium. The basesaturation is very high, 80% or more, and increases irregularly with depth.Clay mineral and salinity data suggest a deviation from the profiles 1 to 3most likely due to its physiographic position.

Brief description of profile No. 10 — Typic RhodustalfBelow the shallow A (0 — 10 cm) of a reddish brown colour and a coarsesandy loam texture, a deep, dark reddish brown to dusky red B horizon issituated till 91 cm. The amount of clay and coarse fragments increasesregularly with depth, resulting in slightly gravelly sandy clay loam tex-tures; the B is underlain by calcrete of varying hardness. The horizonboundaries are gradual but for those in the subsoil; subangular blockystructure is common in the topsoil, while root development is normal inthe top 50 cm. The soil reaction is neutral to moderately alkaline in the Chorizon.The analytical data of this profile demonstrate the low CEC/soil increasingwith depth from 7.96 to 17.8; calcium and magnesium are again thedominating exchangeable cations, the base saturation of the soil is above80 throughout and approaches 100% in the C. The percentage of organiccarbon is above the average in the topsoil (0.6%) but decreases consider-ably with depth.

Brief description of profile No. 11 — Oxic HaplustalfThe A horizon has a coarse sand texture and is from 16—108 cm underlainby a homogeneous, dark brown to yellowish red B horizon with a uniformslightly gravelly loamy coarse sand texture; below 108 cm the B3 horizonis characterized by a brown colour and a slightly gravelly coarse sandyloam texture, red mottles are clearly visible in the matrix colour, thefollowing C horizon has a brown colour in which the contrast is formedby yellowish brown mottling. At 173 cm depth the soil is underlain bycoarse and very coarse angular gravel. Horizonation is gradual, structuraldevelopment is weak, and roots are best distributed and dense in theupper 50 cm of the soil, the soil reaction is strongly acid to neutral.The accompanying chemical data show an extremely low CEC/soil (be-tween 1.75 and 2.63) in the A and B horizons; calcium and potassium arethe dominant exchangeable cations, the percentage organic matter is very

59

low indeed. The base saturation is variable, while high in the A horizon(52%), it decreases to 29% in the B21 and subsequently increases to 46%in the lower part of the B horizon.

Marked profile differentiation is observed in soils at low topographic sites(profile No. 4) and where the parent material deviates from the commonlypresent granitic rock or its weathering products (profile No. 10).The general decrease in coarse fragments in and on the soils away from theMorale Hills is further noted.The soils of the pediplain are also characterized by a considerable soildepth in comparison to soils on the associated (erosion) plain (see Samplearea B), while in a number of cases the profile is underlain by gravel andnot by weathering rock, although it is feasible that the latter was notreached because of extreme depth of the bedrock.The differences in the remaining soil profiles were more noticeable thanexpected, considering the common origin of the parent material and theprevailing climatic conditions. While vegetation marks the various soils inthe terrain, field observations suggest a stage difference in profile develop-ment mainly due to changing soil moisture conditions.

60

l'jblc 18. Sample area 1, information on the profiles 1 to -I. 10 und I i tit time of sampling

Profile 10

date examination July 1970 July 1970 July 1970elevation I 150 Tn 1140 rn 1123 ml.mdform concave hillslopc gently sloping almost flat(phys.pos.) y/r- pediment pediplainLmdform surroun- hilly to the soutli gently sloping flatding country gently sloping to

the north

July 19701107 malmost flatdrainage charmaialmost flat

July 19701082 mflat pediplain

July 19701041 mflat pediplain

flat

O\

micro-topography

vegetation

parent material

drainage

moistureconditionssurface stones

evidence oferosionhumaninfluence

few shallow gullies10-20 cm deep.30 cm wideAMW*A. nigrescensA. tortilis

granitic colluvial

well drained(cl. 4)slightly moist

throughoutvery stony(cl. 3)slight watererosionnil

occasionalshallow rills

AMW

A. nigrescensCombretum apicu-la tu m

granitic

well drained(cl-4)

dry throughout

nil

some watererosie; nnil

nil

AMW

A. erubescens, A.nigrescens, A.tortilis

granitic

well drained(cl.4)dry throughout

nil

nil

nil

tew gullies, deep50 cm, 1 m wide

AMWA. Tortilis,Dichrotachyscenerea, encroa-ching, A. erubescensgranitic

poorly drained(cl. 2)dry () 71) cmmoist belownil

moderate watererosionnil

nil

AMWA. nigrescens, A.tortilis

mcta-docriti-

well drained

(cl.4)0- 80 cm dry.moist belownil

nil

nil

nil

Terminalia sericeaopen woudl.

granitic

somewhat excessivelydrained (cl. 5)dry throughout

nil

nil

nil

AMW = Acacia Muted Woodland

Sample area ß: Soils ot the granitic plain associated with the pediplain ofthe Morale Hills (fig. 16).In table 19 information on the sites and the soils is tîiven.

•S«

62

The soil connotation reads as follows:

Brief description of profile No. 5 — Lithic VstropeptA dark brown coarse sandy loam (Al ) is underlain from 21 - 36 cm by adark reddish brown gravelly coarse sandy loam (15). which changes ab-ruptly to altered granitic rock with sharp distinct mottles at 36 cm:extremely hard rock is encountered at 55 cm. A weak subangular blockystructure is present in the A horizon, to which most of the roots are alsoconfined: the soil reaction is neutral.The chemical data belonging to this profile indicate a very low CEC/soilthat increases with depth from 3.50 to 4.83; the base saturation is high inthe A (79/?) but decreases in the B and C horizons to respectively 41%and 35'r; calcium and magnesium are the dominant exchangeable cations;the organic carbon content is normal (0.55f/v) and follows the usual trendwith depth.

Brief description of profile No. 6 — "Aqitic" HaphtstalfThe A horizon (0 — 23 cm) is dark brown and has a coarse sandy loamtexture, it overlies a (dark) reddish brown sandy clay loam that mergesinto a sandy clay in the B21 from 43 — 69 cm: prominent dark red mottlesare observed in the lower part of the B21 and increase in amount andprominence in the B2g (69—105 cm); this horizon is underlain by a grey,slisihtlv gravelly coarse sandy loam in which black mottles arc abundant;the boundary with the C at 150 cm is abrupt; structurai development isconfined to the topmost soil, the soil reaction is slightly acid.Accompanying analytical data indicate an increasing CEC/soil with depthfrom 5.27 to 9.77, the base saturation increases regularly with depth from40'v to 69'/. while calcium and magnesium arc the dominant exchange-able cations; the amount of organic carbon is similar to the one inprofile 5.

Brief description ofprofile No. 7 — Ult'tc Haplustalj (fig. 17)

The topsoil is a dark loamy coarse sand and underlain from 15 to 34 cmby a dark brown loamy coarse sand (B21); the B3 is encountered from55--Ó9 cm. characterized by a slightly gravelly coarse sandy loam textureand of a brown colour: this horizon abruptly overlies the C horizon.The soil is structureless, while the soil reaction is medium acid, horizona-tion is faint, the dry soil colours differ considerably from the moistcolours.Additional chemical information reveals a low fluctuating CEC/soil (be-tween 3.07 and 4.79). while the base saturation percentage varies between90'Y in the A to 55'/r in the B21. Calcium is the dominant exchangeablecation followed by magnesium and potassium, the percentage of organiccarbon is low and decreases regularly with depth.

63

K*.

Pig. 17. Detail of the transition from the lïg. IS. \quic Haphtstalf, profile No. 8B horizon to the C horizon in profile No. 7,Vltic Haplustalf

Brief description of profile No. 8 — Aquic Haplustalf {(is.. 1 8)The Ap horizon is composed of a coarse sandy loam dark brown in colour;it overlies a dark reddish brown sandy clay loam (B21. 21 —47 cm) whichin turn is underlain by a dark reddish brown and dark brown sandy clayfrom 47 — 104 cm, the B2g: grey mottles are encountered in the upper partot the B2g, while red mottles appear increasingly in the lower part; thetransition to the B3 is clear at 104 cm, the subsoil has a brown colour anda slightly gravelly coarse sandy loarn texture; C material occurs from1 22 - 1 44 cm, in which the amount of coarse sand is considerable. Struc-tural development is poor, the soil reaction is neutral to mildly alkaline inthe B3. Noteworthy is the animal activity in the soil.

Analyses performed on the samples indicate a CEC/soil from 7.14 to 16.2,while the base saturation increases regularly with depth from 6 5 A to 857r.calcium and sodium are the dominant exchangeable cations followed bvmagnesium and potassium, the amount of organic carbon is low.

64

Brief description of profile No. 9 — "Aquic" HaplustalfUnder a very dark greyish brown coarse sandy loam (Al from 0—16 cm) aB of dark brownish colours and of a sandy clay (loam) texture isencountered; below 56 cm depth the colour changes to very dark grey, inwhich strong brown mottles occur, the texture is a sandy clay; thishorizon is underlain by the B2g of a sandy clay texture in which distinctred mottling is visible in a dark grey matrix; the C horizon commences at112 cm, hard granitic rock is reached at 140 cm depth. The structure iswell developed, the soil reaction changed from neutral to mildly andmoderately alkaline in the B.From the additional chemical data it appears that the CEC increasesirregularly with depth from 5.27 to 14.2, the base saturation percentage isabove 50% throughout and at a maximum in the B2g horizon; calcium,magnesium and potassium are the dominant exchangeable cations; thepercentage organic carbon is low and diminishes in the subsoil.

The soil morphology indicates an increase in profile development in soilsencountered at slightly lower topographical sites under the same climaticconditions and the same parent material. In some areas, especially in thebottomlands, the presence of water together with poor drainage causedanaerobic soil conditions, whereas in other areas restricted soil moistureconditions are expressed partly in limited soil depth.

65

asTable 19. Sample area B, information on the profiles 5 to 9 at time of sampling

Profile

date examinationelevationlandform(phys.pos.)landformsurrounding countrymicro-topographyvegetation

parent materialdrainage

moisture conditions

surface stonesevidence of erosionhuman influence

5

July 19701115mflat plain

flat

nil

AMW withA. nigrescens,

6

August 19701107 mflat plain

flat

nilAMW withencroaching A. grandi-

Combretum apiculatum cornuta andA. tortilis andA. erubescens

graniteimperfectly drained(cl. 3)dry throughout

nilnilnot cultivatedat present

Dichrostachyscenerea

granitepoorly drained(cl. 2)dry throughout

nilnilcultivation

7

April 19701108 mflat plainslight elevationflat

nilAMW withCombretumapiculatum,encroachingA. tortilis

granitesomewhat excessivelydrained (cl. 5)0—15 moist, drybelownilnilnil

8

September 19701126mflat plainslight depressionflat

nilAMW withA. Nigrescens, A.robusta, Dichros-tachys cenerea,encroaching A.tortilisgranitemoderately welldrained (cl. 3)0-70 dry, slightlymoist belownilnilcultivation

9

June 19701140 mflat plain atbottom landflat

nilAMW withA. tortilis andDichrostachyscenerea encroaching

granitepoorly drained(cl. 2)0-30 dry, slightlymoist belownilnilnil

The soil profiles 1 2 to 18 are located in sample area C, representing theeroded plain (fig- 19).Descriptive information on the soil sample sites is given in table 20.The brief soil descriptions read as follows:

l'ig- 19. Sample area C

Brief description of prof île \!o. 12 — Ultic HaplustalfBelow a dark brown slightly gravelly coarse sandy loam topsoil (Ap),occurs a fairly homogeneous B21 (from 15—32 cm) and a B22 (from32 — 59 cm) of sandy clav (loam) texture and with a reddish brown colour.This upper B is underlain by a dark reddish brown mixed with yellowishred clay, the B2t from 59—87 cm, which overlies a gravelly sandy clay ofthe same colour; coarse fragments increase with depth; the B3 is underlain

67

from 102 cm onwards by decomposed granitic rock of a gravelly claytexture. Horizonation is well expressed, but structura! development ispoor: the soil reaction is neutral.The report on the chemical analyses indicates a regular increase in theCEC/soil (from 8.81 to 20.00) in the C. while the percentage basesaturation follows the same trend: an increase from 70% to 91% in thedeep subsoil. Calcium and magnesium are the dominant cations on theexchange complex, the organic carbon percentage is above average in thetopsoil (0.64 and 0.61%), but decreases considerably with depth.

Brief description of profile No. 13 — Ultic HaplustaljThe dark reddish brown A horizon of slightly gravelly coarse sandy loamtexture overlies from 21 cm onwards a dark reddish brown gravelly coarsesandy loam, the B21 ; the B22 is characterized by an increase in clay notmanifested as cutans, and in coarse fragments, while also distinct yel-lowish red mottles occur; the B3 commences at 76 cm depth and has agravelly sandy clay loam texture and a reddish brown colour, this horizonis underlain by the C from 118-175 cm. Horizonation is moderately welldefined but the structure is poorly developed, root distribution is con-fined to the top 50 cm, the pH water indicates a medium to slightly acidsoil reaction.

Supporting chemical data demonstrate a varying CEC/soil with values of1 5.7 in the Al and 26.6 in the C and a range from 6.70 to 9.38 in the Bhorizon; the percentage base saturation increases from 20 to 79%; cal-cium, magnesium and potassium are the dominant cations on the ex-change complex, the percentage of organic carbon is low.

Brief description of profile No. 14 — Ultic HaplustalfA dark brown gravelly loamy coarse sand forms the Al (0 — 18 cm), thefollowing horizon is a reddish brown slightly gravelly coarse sandy loam(B2. 18—56 cm) that overlies a yellowish red gravelly coarse sandy loamwith distinct red and black mottles, while the C horizon is reached at90 cm depth. Horizonation is clear, structural elements are absent toweakly developed, the soil reaction is slightly acid, root development islargely confined to the top 50 cm.Accompanying chemical data show a very low CEC that increases slightlywith depth from 3.59 to 5.81. the base saturation percentage is fairlyconstant between 55% and 59% but increases sharply in the C. Thedominant exchangeable cations are Ca. Mg and K, while also Na is presentin a small amount, the percentage of organic carbon is very low.

Brief description of profile No. 15 — Typic NatraqualfThe succession of soil horizons commences with a black to very dark grey

63

A horizon of coarse sandy loam texture followed, from 43—69 cm, by avery dark ^rey sandy clay loam with columnar structure in which dark redmottles occur: this horizon overlies a B2g. respectively from 69-91 andfrom 91-127 cm of a (dark) grey slightly gravelly sandy clay texture withcommon distinct yellowish and red mottles; the B3 commences at1 27 cm, is greyish brown and has a slightly gravelly sandy clay loamtexture; from 158-180 cm the C horizon occurs, consisting of decom-posed granitic rock. Structural development is well expressed in the. top60 cm of the soil, but is poor in the subsoil, the soil reaction changes fromslightly acid in the A to neutral in the B2 and strongly alkaline in the B2çand the C horizon.

The analyses report a slow increase in the CEC/soil with depth from 7.15to 18.8 in the C (apart from the value in the A l l ) ; the base saturationpercentage increases from 60% in the topsoil to fully saturated conditionsin the subsoil. Ca and Na are the dominant cations on the exchangecomplex followed by Mg and K; the organic carbon percentage is unusu-ally high (1.84) in the A l l .

Brief description of profile So. 16 — Typic Vstorteut (fig. 20)Dark brown slightly gravelly coarse sand (the AI ) is underlain from12—46 cm by a gravelly loamy coarse sand of a brown colour (B21):texture and colour change slightly in the B22 (46—67 cm) which overlies abrown gravelly loamy coarse sand, while decomposed granitic rock isencountered at 98 cm. Structural development is poor to absent, rootdevelopment is confined to the top 60 cm, the soil reaction is mediumacid throughout, dry colours differ considerably from the moist ones.Chemical analyses of the samples indicate an extremely low CEC/soil.varying between 1.75 and 1.31, while the base saturation percentage isalso low, increasing with depth from 1 1% to 22%; Mg and K are the majorcations on the exchange complex, the organic carbon content is extremelylow.

Brief description of profile So. 17 — Lithic UstipsammentThe A l l (0 — 15 cm) consists of loamy coarse sand and has a very darkgreyish brown colour; coarse fragments increase with depth in the A12(15 —34 cm) which is abruptly underlain by coarse rock fragments andquartz gravel at 34 cm; structural development is absent, the soil reactionis slightly acid, the moist colour values are two units lower than the dryones.The accompanying chemical data reveal a low CEC of 3.14, while the basesaturation percentage is 45% and 56% respectively in the Al l and A12:the dominant cations on the exchange complex are calcium and potas-sium, the percentage organic matter is low.

69

L. • , ' t

;. 20. Typic [Tstortent. profile So. (6

Brief description of profile Xo. 18 - "Aqiac" HaplustalfThe Ap (0-15 cm) consists of dark reddish brown coarse sandy loamoverlying; a dark brown coarse sandy loam to 36 cm, the B21. below thisdepth the B22 of brown sandy clay loam texture with distinct red mottlesoccurs, the boundary at 5 5 cm to the underlying B2g is clear. This horizonis marked by a greyish brown matrix in which red and yellowish red andstrong brown mottles occur: downwards trom 104 cm, the B3 of slightlygravelly sandy clay texture and grey colour marks the transition to thedecomposed granitic rock. Structural development is confined to the B2g(moderate to weak medium columnar structure), horizonation is clear,while the bulk of the roots is encountered in the top 36 cm; the soilreaction is slightly acid in the topsoil and neutral to mildly alkaline in thesubsoil.The CEC/soil increases regularly with depth from 5.26 to 16.70. while a

70

similar trend is observed in the percentage base saturation (increase from48% to 95%); Ca and Mg are the major exchangeable cations, followed byK and the Na; the latter cation is of importance in the B and the Chorizon, the percentage organic carbon in the topsoil is average (0.55%)and decreases with depth.

The variety in soils is well expressed when comparing soils at differentphysiographic positions. An additional factor in this area is soil erosionthat induces the shallow soils on the higher sites, while decomposition ofsoil material is encountered in the lower areas. The occurrence of rockoutcrops is common, while some soil properties such as salinity andstructure are well expressed in lowland soils. Soil differences also becomeapparent through changes in vegetation.

71

Table 20. Sample area C, information on the profiles 12 to 18 at time of sampling

Profile

Date examinationelevationlandform(phys.pos.)

landformsurr, countrymicro-topogr.

vegetaion

parent materialdrainage

moistureconditions

rock outcrops

evidence oferosionhuman influence

12

June 19701170 mflat divide

flat

plough ridges

Acacia MixedWoodl. withencroachmentof A. tortilis

granitemod. welldrained (cl. 3)

0—15 cm dry,si. moist below

nil

nil

dry landframing practices

13

February 19711167 mgently slopingconcave slope3%gently sloping

nil

AMW with A.nigrescens andA. tortilisencroaching

granitewell drained(cl. 4)

dry throughout

nil

nil

nil

14

February 19711180 mflat divide

flat

nil

Combretumapiculatum/A.nigrescens W.with encroach-ment of A.erusbescensgranitewell drained(cl. 4)

dry throughout

occasionalwhale backexposuresnil

nil

15

February 19711172mgently slopingvalley bottom

flat to almostflatfew shallowgullies, 50 cmdeep, 1 m wideAMW with A.karroo, A. nilo-tica and Dichro-stachys cenerea

granitepoorly drained(cl. 2)

16

February 19711170 mgently slopingconcave slope3%gently sloping

nil

Terminalia seri-cea Woodl. withOrozoa panicu-losa and A.nilotica

granitewell drained(cl. 4)

moist throughout 0—12 cm dry

nil

some watererosionnil

si. moist below

nil

nil

nil

17

September 19701185 mgently sloping

gently sloping

nil

Terminalia seri-cea with A. tor-tilis and Dios-pyros lycoides

granite gravelsomewhat ex-cess, drained(cl. 5)dry throughout

kopje and whaleback exposures

nil

nil

18

February 19711180 mgently slopingvalley bottom

gently sloping

nil

AMW with Dios-pyros lycioides

granitepoorly drained(cl. 2)

0—15 cm dry15-55 cm si.moist, 55 + cmmoistnil

nil

dry land farming.practices

Sample area D (fig. 21) comprises the soil profiles 19 to 28. comprehen-sive information on the soils and the sites are given in table 21. while briefdescriptions arc documented in the following paragraphs.

F-'ig. 21. Sample urea I)

73

îiriej description ot profile Xo. 19 - ÏAthic l'storthentA shallow 10-15 cm1 dark reddish brown clay loam rests directly on harddolcrite; the soil reaction is neutral.The chemical data indicate a CEC/soil ot 27.1 and a base saturation of69 ' ' ; calcium is the major cation on the exchange complex, the percentageorganic carbon is high ( 1.28%).

liricj description oj profile \'o. 20 - Vdic VstocreptThe AI 1 from 0 30 cm consists of a dark reddish brown gravelly coarsesandy loam and merges gradually to the A12 ot the same texture andcolour; at 65 cm coarse 'Travel occurs, the soil reaction is neutral.Analyses performed on the samples taken reveal a low CEC/soil value,respectively 9.64 and 8.10 in the A1 1 and the A12. while the percentagebase saturation increases with depth from 78''r to 8 7 ' ' ; the dominantexchangeable cations are Ca and Mçr, the percentage organic carbon isnormal.

liricj description of profile .\o. 21 - Vltic HaplustolfThe topsoil has a reddish brown colour and a coarse sand texture, itoverlies a yellowish red loamy coarse sand {B21 j from 33-80 cm; the 1322(80—129 cm ! has a coarse sandy loam texture and is ot a dark reddishbrown colour: the transition to the C (sec Chapter 1.2.2.) is similar to theB2. but coarse fragments increase with depth, the soil reaction is slightlyacid throughout, while structural development is weak to absent; majorroot development takes place in the top 80 cm.The analytical data show a slight increase of the CEC/soil with depth from3.91 to 4.78, the percentage base saturation varies irregularly between36'/' and 4 l'i- ; Ca is the main cation on the exchange complex, theamount of organic carbon is very low in the topsoil (mean 0.1 7?c).

Brief description of profile So. 22 — Typic PellustertThe topmost soil (0-2 cm) is formed by a very dark greyish brown sandyclay, which is the beginning of a deep A horizon of a very dark greycolour and a clay texture: from 67 cm onwards calcium carbonate accu-mulations occur, while the profile at 1 27 cm is underlain by dark reddishbrown clayey 1IC material: a gravel blockage is encountered at 170 cm.The structure is coarse prismatic from 2 -28 cm and determined byvertical cracks, the remaining part o^ the solum is massive, the soilreaction is moderately alkaline throughout and increases a little withdepth from pH water 8.2 to 8.5. Roots are confined to the top 60 cm.Results of analyses indicate fairly high CEC/soil values throughout,ranging between 32.9 to 39.5: the base sa uur at ion increases regularlydownwards, and is always above 80' 'c. the soil is fully saturated in the Aca:

74

calcium and magnesium arc the main cations on the exchange complex;the organic carbon percentage is fairly high throughout (0.5'/ or more).

Brief description of profile ,\'o. 23 — Udic Rli odi is talf (plate I)The dusty red sandy clay A horizon is succeeded below 1 2 cm depth by ahomogeneous deep, dark reddish brown to dusky red B horizon of clayeytexture; the B3 has a fine gravelly sandy clay loam texture and is dark redin colour; it commences at 122 cm, while a blockade of calcrete isencountered at 175 cm depth: weak medium subangular blocky structuresarc confined to tbc topsoil in which also the main root developmentoccurs; the soil reaction is slightly acid to neutral.According to laboratory data, tiie CEC varies with depth between 18.1and 21.2; a similar trend is observed for the percentage base saturation•: decrease from 7(Y/f to 71%) with exception of the B3 horizon: thedominant cations arc Ca and Mg followed by K and a trace of Na. Theamount of organic carbon is above average in the topsoil (1.26/r), butsharply decreases to normal values below 12 cm.

Brief description of profile i\ro- 24 — 1'ypic ChromustertThe A horizon is a dark brown colour and has a sandy clay loam texture;it overlies a dark brown calcareous sandy clay loam (the A from19—45 cm), the succeeding A horizon is a very dark greyish brown,slightly gravelly calcareous sandy clay loam which is underlain, from76 cm onwards, by a brown, highly calcareous slightly gravelly coarsesandy loam; the pcdon is blocked ut 170 cm by hard calcrete. Structure ismoderately developed in the top 76 cm. horizonation is well defined, thesoil reaction is moderately alkaline in the topsoil and strongly alkaline inthe subsoil.According to analyses, the CEC in the soil ranges between 32.9 to 35.9 inthe Al and AC horizons, but decreases to 25 in the C: the percentage basesaturation is HH'A in the Al and is at a maximum in the remaining part ofthe soil. Calcium and magnesium arc the dominant cations on the ex-change complex, the percentage organic carbon is 0.58' ' in the A anddecreases with depth.

Briej description of profile Ao. 25 — Typic Pellustcrt (fig. 22)The topmost surface horizon (Al l , 0 — 3 cm) is very dark grey and has aclayey texture: it overlies a deep, fairly homogeneous very dark grey clay(loam) Al 2 and Al 3: below 67 cm the soil colour changes to black, whilecalcium carbonate is encountered in increasing amounts; the AC from96 cm to 142 cm is a very dark grey silty clay loam; it overlies a red andvery dark grey, highly calcareous, very gravelly coarse sandy loam, the (Il)C:structural elements are observed in the top 67 cm. cracks and roots arc

75

Fig. 22. 'i'ypic Pellnstcrt, profile No. 25

also common in this part of the soil. The soil reaction is moderately tostrongly alkaline, horizonation is gradual.Chemical analyses demonstrate a high CEC/soil that increases regularlywith depth in the A from 52.3 to 77.6. but decreases sharplv in the IIC(45.4); the base saturation is fairly constant between 82r/c and 857c withexception of the AC horizon for which a saturation of ll'/r is obtained.The percentage of organic carbon is above average throughout the Ahorizon (0.83—0.44*7r). the percentage of CaCO3 increases irregularly withdepth.

Brief description of profile No- 26 — Typic HaplttstalfA dark reddish brown sandy loam (A, 0-24 cm) is underlain by a fairlyhomogeneous dark reddish brown B of a sandy clay loam texture, coarsefragments increase with depth, giving rise to gravelly sandy clay loam anda highly calcareous B2ca.

76

The solum is blocked at 120 cm depth by coarse gravel. The soil reactionis neutral in the Al, changes the mildly alkaline in the B21 (24—48 cm)and increases to moderately and strongly alkaline in the subsoil. Structuraldevelopment is weak, the bulk of the roots is confined to the top 40 cm.The chemical data indicate an increase of the CEC, with depth from 12.5to 20.9, and also show fully saturated conditions; calcium, magnesium andpotassium are the main cations on the exchange complex.

Brief description of profile No. 27 — Lithic UstorthentA dark reddish brown clay abruptly overlies dolerite at 20 cm depth; thesoil reaction is neutral.As shown by the chemical data the CEC is 33.1 and the base saturation79%, calcium and magnesium are the major cations on the exchangecomplex.

Brief description of profile No. 28 — Lithic UstorthentA black clay rests abruptly on dolerite, commencing at 38 cm depth. Thesoil reaction is strongly alkaline.The analytical data shown a CEC of 46.2 and fully saturated conditions,calcium and magnesium are the main exchangeable cations.Contrasting soils are mainly due to the different soil parent materials, thedoleritic versus the granitic rock suite. Shallow and stony soils are con-fined to the hills and the areas immediately surrounding them. Slightchanges in soil topography seem responsible for differences in soil moistureconditions and associated development of soils derived from granitic parentmaterial.

77

Table 2t. Sample area D, information on the profiles i9 to 28 at time of sampling

Date examination

Date examinationelevationlandform(phys.pos.)

landformsurroundingcountry

micro-topography

vegetation

19

July 19701320 mflat hilltop

flat topsteep

nil

open Wdld.Commi-phora afri-cana, Kir-kia accu-minata,Scleroca-rya caffra

20

July 19701148 mbase collu-vium,slopingsteep to Sgentlysloping tothe N.shallowgullies20-50 cmdeep, 50cm wideAMW, Aca-cia nigres-cens andCommi-phoraafricana

21

Febr. 19701131 mpedimentgentlyslopinggentlysloping

old ploughridges

Combre-apicula-tum/Aca-cia nigres-censWdld.

22

Febr. 19701123mflatflood-plain

flat

faintgilgai

AMW, Aca-cia grandi-cornuta, A,tenuspinaencroach-ing A.tortiles

23

Febr. 19701125mpedimentgentlyslopinggentlysloping

nil

AMW, Aca-cia tortilesDichrosta-chyscenerea

24

Febr. 19701126 mpedimentgentlyslopinggentlysloping

nil

AMW, A.Grandicor-nuta, A.karroo, A.tenuespina,Dichro-stachyscenerea,Bosciaalbitruca

25

Febr. 19701131 mpedimentgentlyslopinggentlysloping

faintgilgai

AMW, A.grandicor-nuta, A.karroo, A.tenuespina,Dichro-stachyscenerea

26

July 19701147mbasecolluvium,slopingsteep Ngentlysloping S

nil

AMW, Bos-cia albi-trunca, A.tortiles

27

July 19701280 mflat hilltop

flat tobroken

nil

open Wdld.Commi-phora afri-cana, Kir-kia accu-minata

28

July 19701 270 mflat hills

flat

faintgilgai

grassland

Table 21. Continued

Date examination

parentmaterial

drainage

moistureconditions

surfacestonesrock out crops

evidence oferosionhumaninfluence

19

dolerite

dry

exc. stony

extr.rockyslightwaternil

20

colluvialderivedfromdoleriticand grani-tic rockwelldrained(cl. 4)

0-20 cmdry,moistbelowvery stony

nil

moderatewaternil

21

granite

welldrained(cl.4)

dry

through-out

nil

nil

nil

once clear-ed for dryfarming

22

calcareousalluvia

poorlydrained(cl. 1)

0-50 cmdry si.moistbelownil

nil

nil

nil

23

granite

modera-tely welldrained(cl. 3)si.moist

nil

nil

slightsheetnil

24

dolerite

imper-fectlydrained(cl. 2)dry

nil

nil

nil

nil

25

dolerite

poorlydrained(cl. 1)

0-30 cmmoist, si.moistbelowvery stony

nil

nil

nil

26

colluvialderivedfromdoleriticrock

welldrained(cl.4) -

0-20 cmmoist, si.moistbelowexc. stony

nil

slightsheetnil

27

dolerite

drybelow

exc. stony

extr.rockynil

nil

28

dolerite

dry

exc. ston]

extr.rockynil

nil

The soil profiles 29 to 33 are located in sample area E. A part of this areais shown in figure 23. Comprised site and soil in formation is presented intable 22; brief soil descriptions are given below.

Fig. 23. Part of sample area E

Brief description of profile No. 29 — Oxic HaplustalfBelow a brown A horizon of loamy sand texture occurs a reddish brownsandy loam, the B21 from 22—36 cm; this horizon is underlain by a fairlyhomogeneous yellowish red sandy clay loam in which dark red mottlingand coarse fragments increase with depth, giving rise to a prominent redmottled, gravelly sandy clay loam, the B3 (80—95 cm); weathered schist isencountered from 95 —113 cm. The soil reaction is neutral in the A, butslightly acid in the B and medium acid in the B3 and C horizon.Horizonation is gradual, while the soil has no visible structure and thebulk of the roots are found in the upper 50 cm of the soil; cutans arerather clear from 50—80 cm depth (B2t).

The accompanying chemical data show a low CEC/soil slightly increasingfrom 3.23 to 5.23 in the B, but subsequently decreasing in the deepsubsoil and the parent material to respectively 4.34 and 4.78. The percent-age base saturation is between 43% and 53% in the topsoil and decreasesslightly with depth.Calcium and magnesium are the dominant cations on the exchange com-plex, the percentage of organic carbon is low: 0.29%.

Brief description of profile No. 30 — Typic UstipsammentA very dark greyish brown sand (All) is underlain from 23 cm downwardsby a homogeneous brown sand; the A3 is encountered at 120 cm

80

depth and has a strong brown colour and the same texture as the overlyingA horizon; the transition to the C is gradual, the latter consists of sand,strong brown with faint yellowish red mottles. Structure is absent (singlegrained) apart for a weak subangularblocky structure in the A l l , main rootdistribution is in the top 80 cm, the soil reaction is slightly acid andbecomes medium acid in the A3 and C horizons.Chemical analyses of the samples taken indicate an extremely low CEC/soil of 2.19 in the A l l , 1.31 in the A12 and 0.87 in the remaining part ofthe solum; the base saturation is 73% in the A l l , 31% in the A12 and variesbetween 14% and 17% in the remaining part of the A and C. Magnesiumand potassium are the main exchangeable cations, the percentage oforganic carbon is extremely low.

Brief description of profile No. 3i — Entic ChromustertA dark greyish brown sandy loam overlies a dark greyish brown sandyclay, the A l l , from 15—39 cm; the succeeding A horizons are greyishbrown and of a slightly gravelly sandy clay loam texture; below 116 cmdepth the AC is represented as a brown, slightly gravelly sandy loam, whilecalcrete forms a blockage at 176 cm; the soil is highly calcareous through-out with a moderate alkaline soil reaction, structure is moderately devel-oped; horizonation is gradual, the bulk of the roots occurs in the top40 cm, pressure faces are best observed in the A12 from 36—67 cm. 7Additional chemical data show a fairly uniform CEC/soil ranging between20.7 and 25.4; the soil is fully saturated, while calcium and magnesium arethe main exchangeable cations. The percentage of organic carbon is highin the A (0.85%) and remains at 0.5% till a depth of 67 cm; the percentageof calcium carbonate is above 10%.

Brief description of profile No. 32 — Oxic HaplustalfBelow a dark brown, coarse sandy loam a reddish brown and yellowish redB horizon occurs of a uniform clayey texture with dark red mottlingincreasing with depth, forming respectively the B21 from 30—61 cm andthe B22 from 61—88 cm; the lower part of the B is underlain by a strongbrown, slightly gravelly clay, the B3 (88 — 110 cm), which merges clearlyto the C horizon. Red and blackish mottling increases in amount andprominence with depth as does the amount of coarse fragments; the soilreaction is medium to slightly acid. Structural elements are weakly devel-oped to absent, major root development is restricted to the upper 60 cmof the soil.

Supporting data from the chemical analyses visualize a low CEC/soil varyingbetween 3.9 and 9.8, while the percentage base saturation is between50—60% in the B and 68% in the A. Calcium and magnesium are the bestrepresented cations on the exchange complex, the amount of organiccarbon is normal.

81

Brief description of profile No. 33 — Ultic HaplustalfThe soil has a dark brown, coarse sandy loam A horizon that merges at30 cm into a dark brown coarse sandy loam which is underlain by a darkbrown, slightly gravelly coarse sandy loam with reddish brown mottles,respectively the B21 (30-58 cm) and the B22 (58-99 cm); weatheredgranitic rock is encountered from 99 — ] 30 cm, horizonation is fairly welldeveloped, structural elements are weak to absent, the soil reaction isslightly acid.Data of the analyses illustrate a very low CEC/soil that increases slightlywith depth from 4.38 to 7.06, the percentage of base saturation increasesfrom 42% in the A to 73% in the lower B; calcium followed by magnesiumare the main exchangeable cations, the percentage of organic carbon isabout average (0.48%) and decreases sharply with depth.

Under a uniform climate the interaction between the parent material andthe topography is the most important factor of soil development.The soil differences are also well indicated by the changes in vegetationthat the soils carry. Different soil moisture conditions are thought to berelated in the first place to the time of sampling, summer with rain versusthe dry winter. However, moist soils sampled in the dry period arecommonly observed at depressional sites.

82

Table 22. Sample area E, information on the profiles 29 to 33 at time of sampling

CO

Date examinationElevationLandform (phys.pos.)Landform surr.countrymicro-topography

vegetation

parent materialdrainage

moisture conditions

evidence of erosionhuman influence

29

Sept. 19701320 mgently slopingpedimentgently sloping

nil

Combretum apicu-latum/Acacianigrescens MixedWoodland with A.fleckii, A. robustaCommiphora mollis,Dichrostachys cenereaschistwell drained (cl. 4)

dry throughout

nilmay have been oldcultivated land

30

Sept. 19701350 mflat plain

flat

nil

Terminalia seri-ricea Woodlandwith A. fleckii,Diospyros lyciodes.Grewia retinerois

eolian sandwell drained (cl. 4)

dry throughout

nilnil

31

Febr. 19701320 malmost flatpedimentflat

shallow gullies30—50 cm deep,1 m wideAcacia MixedWoodland withA. gerrardi andChretia caerulea

metadolerite (?)moderately welldrained (cl. 3)moist throughout

some gully erosionnil

32

Febr. 19701300 malmost flatplainflat

nil

Combretum apicu-latum Woodland,some A. robustaand Commiphoramollis

granitewell drained (cl. 4)

moist throughout

nilnil

33

Febr. 19701280 malmost flatplainflat

nil

Terminalia scri-cea Woodland,some A. robusta,Ehretia caeruleaand Euclea un-dulata

granitewell drained (cl. 4)

0—10 cm dry, moistbelownilnil

II.3. Soil properties

Subjects discussed in this section are: 1) bulk density, 2) soil reaction and3) exchange capacity.The relation between these properties, the soil constituents and theirarrangement, is a factor that controls soil characteristics to a great extent.

11.3.1. Bulk densityThe range of bulk density measurements expressed as g/cm3 for 43 sam-ples, is between 1.1 and 1.8.The lowest values are obtained from the very topsoil of a granitic bottomland soil (profile 15, 0—2 cm), as well as from the sandy soil of theKalahari (profile 30). In the latter, the bulk density ranges between1.2g/cc in the topsoil and increases slightly to 1.4 g/cc in the subsoil.Similar values are commonly observed in the topsoil of all soils encoun-tered.The deeper horizons of soils derived from granite, in which also anincrease of clay with depth is noticed, represent the highest bulk densitydata from 1.5 to 1.8 g/cc. The black clay soils derived from doleritic rockhave a bulk density of between 1.4 and 1.7; these data largely correspondwith other figures on similar soils (Dudal, 1965).A regular, gradual increase in bulk density with depth occurs mainly in thegranitic soils which show reasonable profile development; sharp changesare merely restricted when reaching the C material. The lower bulkdensity values in the topsoils are ascribed to the higher volume of soiltaken up by the root system, the amount of organic matter in thesehorizons and to intensified biological activities. Corresponding with higherbulk densities in the subsoils of most hydromorphic Alfisols are changes inclay mineralogy. These include an increase in the 2 : 1 lattice Al-silicates.In addition, the compaction of the subsoil horizons through the weight ofthe overlying soil must be taken into account while reviewing the higherbulk density values in deeper parts of the soil.

11.3.2. Soil reactionMeasurements of soil reaction expressed as pH-H2O and pH-KCl indicateacid to neutral conditions for most granite soils and neutral to alkalineenvironment for soils derived from the basic rock suite.Exceptions for the former include the subsoils of a number of bottom-land soils, such as profile 4, 15 and 18, as well as the subsoils which arecharacterized by a high percentage of CaCO3 (profile No. 10).The pH-H2O is 1.0 unit higher than the pH-KCl for all samples.The overall trend of the pH values with depth varies considerably andcannot be taken as a characteristic applicable to all the soils, if, however,

84

the profiles are grouped according to their profile development, it appearsthat in soils with a more pronounced morphology the pH regularlyincreases with depth, while in soils with faint horizonation the pH staysfairly constant. This feature is also best observed in soils derived fromgranitic parent material. In the sandy soil from the Kalahari the pHdecreases considerably in the subsoil, while also in less developed soils thepH is in general lower than in their better-developed counterparts. Thismay be illustrated while comparing the soil acidity data from the profiles11, 12 and 14 with those of profiles 4, 13 and 31.The correlation between the pH values, the C.E.C. and base saturation issummarized in table 23; see also section II.3.3.

Table 23. Correlation between pH values, C.E.C. and base saturation

X

CECCECSat.Sat.pH-H2O

y

pH-H2OpH-KClpH-H2OpH-KClpH-KCl

r

0.7710.6760.7590.7350.920

yyyyy

;ression

= 0.05= 0.04= 0.03= 0.03= 0.94

equation

x+ 6.16x+ 4.82x+ 4.95x+ 3.55x - 1.02

st. err. x

8.7710.1615.5716.200.35

st. err. y.

0.560.670.580.610.35

In a semi-arid environment acid to neutral conditions are encountered inthe majority of granitic soils that have been subjected to leaching. Thismeans that the bases in solution accumulated to lower sites and/or thatonly but a small amount of metal cations was made available through theweathering of the parent material.

11.3.3. Exchange capacityDirectly related to the pH is the percentage base saturation (Scheffer andSchachtschabel, 1970). The percentage indicates the relative proportion ofthe adsorbed bases and hydrogen on the exchange complex. The trend ofthe percentage base saturation is variable, in some soils a definite increaseof the saturation is observed, for example in profiles 1, 8, 10, 12 and 18,while in other soils a clear decrease of the value is apparent (profiles 30and 32).There are soils in which the base saturation percentage decreases at first,only to increase at a greater depth, or there are others where it remainsfairly constant throughout the solum.The mean values of the saturation percentage was taken to investigate thepossibility of grouping soils relative to their adsorbed cations. The criteria

85

for the groupings were partly derived from suggestions made by theFAO/UNÉSCO (1970).Soils with a high base saturation percentage (75 — 100%) are Vertisols,Entisols (basic) and most hydromorphic Alfisols. Those with a low basesaturation (0—25%) include the Entisols (acid). Most of the Haplustalfshave a base saturation between 50—75%, some Oxic, Ultic and TypicHaplustalfs have however a lower saturation of bases (between 25—50%).The fluctuations in base saturation percentage in the granitic soils iscommonly related to their stages of development. The relation betweenbase saturation and pH is shown in figure 24 and correspond with findingsby Scheffer and Schachtschabel (1970).

• •o o o o

• i

3.91

cf^iö 25 50 75 b a t a aaturatlon (l)

Fig. 24. Scatter diagram and regression line for pH-KCl and base saturation percentage

86

As the organic matter content in the soils is low, the cation exchangecapacity depends almost entirely on their mineral fraction. Of th'ese thecolloids with the fine silt, and to a lesser extent the coarse silt and thesand fractions, control the ion exchange properties (Wiklander, 1969).The dominant exchangeable cations are calcium and magnesium, followedby potassium and occasionally soidum. The C.E.C. expressed as the totalquantity of cations that a soil can adsorb shows a wide range (Buckman etal., 1967).This is not only due to the environmental factors, but also to the stage insoil development. In general the data indicate a slight increase of theG.E.C. with depth. On the basis of the values obtained and partly accord-ing to the USDA criteria (7th Approx. 1970) the following divisions canbe made.

Table 24. CEC$oii as related to different soils

GEC/meq/100 g soil

50+25-50

10-25

0-10

Soil Order

VertisolsVertisols, Entisols on basic materialand some AlfisolsSubsoils of hydromorphic Alfisols,Rhodustalf and some HaplustalfsInceptisols, Entisols on graniticmaterial, other Alfisols

Profile No.

2522, 23, 24, 27, 28

4,8, 9, 10, 12, 13,15, 18, 19,26,311,4, 13, 14, 16,18, 20, 21, 29, 30,32, 33

The correlation coefficient between the CEC ^ and the grain size frac-tion is variable. For example, this coefficient is 8.07 between the CEC ^and the fine silt + clay fraction except for the soils that have a coarsetexture (loamy sand or sand), while the correlation is 8.56 between theCECso i l and the silt fraction not taking into account the profiles 21, 30and 31. The correlation coefficient between the CEC u and the clayfraction is best expressed in medium-textured soils.Thus, generally speaking, the correlation between the CECso i l and theclay fraction is low in coarse-textured soils (respectively the profiles 3, 30and 33). Similar findings were reported by Syers et al. (1970) for sandysoils in New Zealand.The decisive factors are the type of clay and the amount of clay present.This is evident when the CEC .. is recalculated as CEC meq/100 g clay.In three profiles only the CECcl is below 24 meq/100 g clay in the•greater part of the B horizon.

87

Furthermore, in most soils where other than smectite clay minerals arepresent, the CECc|a varies between 25 — 50 meq/100 g clay, which corre-sponds well with data from Scheffer and Schachtschabel (1970).The data are summarized in table 25.

Table 25. Correlation and regression equation between CEC i and various particlesizes

x y r regression equation st. err. x st. err. y

7.8.6

325868

394

.81

.11

.67

CEC 2-50/im 0.856 y = 0.45 x + 5.99CEC <20 /im 0.807 y = 0.86 x + 22.38clay CEC 0.888 y = 0.62 x - 2.69

Similar high correlations, especially between the CEC u and the siltfraction, were reported by Buursink (1971) and were also quoted byRuellan et al. (1967). The above data stress once more the importance ofthe textural composition of the soils in relation to their exchange pheno-mena.The above mentioned soil descriptions and related soil properties allow aclassification of the soils according to the 7th Approximation (1970), inorder to facilitate their discussion in the following Chapters.Eight groups are recognized (nos. 1 to 8), while symbols indicate thesegroups on the graphs in the text.Further information on the soil classification is presented in Chapter VI.

Key to fig. 24, 28,Symbol

D

+

e

a

o•

A

A

and 29.Group

1

2

3

4

5

6

7

8

Order

Vertlsols

Inceptisols

Entisols(acid)

Entisols(basic)Alfisols

Alfisols

Alfisols

Alfisols

Subgroup

Typic PellustertTypic ChromustertEntic ChromustertLithic UstropeptUdic UstocreptTypic UstorthentLithic UstipsammentTypic UstipsammentLithic Ustorthent

Ultic Haplustalf

Oxic HaplustalfTypic Haplustalf

Aquic HaplustalfTypic HaplustalfTypic RhodustalfUdic Rhodustalf

Profile

22, 252431

52016173019, 27,

1,2,3,14, 21,11, 29,26

(4), (6)151023

28

7, 12, 13,3332

. 8 , (9), (18)

88

C H A P T E R III

SOIL COMPOSITION

Soil composition and mineralogy was assessed according to particle sizedistribution in the fine earth. The last two sections of this chapter dealwith organic matter and salinity.The judgement of the soil texture in the field embraces all these constit-uents and comparisons between field and laboratory data are thereforeinfluenced by the nature of analyses performed to determine the texturein the laboratory.As is common practice, the fine earth fraction of the soil was treated withdiluted HCl and with H 2 O 2 ; as such the laboratory data on the particlesize distribution reflect the textural composition of the mineral particlesonly.

111.1. Fragments coarser than 2 mmThe indication of this fraction is given according to FAO Guidelines forSoil Description (1966) and mainly refers to the fine, gravelly part of thesoil, which is also called grit.In a number of profiles, this gravel represents a considerable percentage ofthe total soil, which is for the greater part attributed to the nature of thesoil parent material. Thus soils developed from the granitic rock suite havea higher percentage of coarser material than soils derived from doleriticmaterial and from schist and sandstone. In most solums the amount ofcoarse fragments, if present, increases with depth, thus indicating theactivity of the soil forming processes. In the immediate surrounding ofrock outcrops the topmost soil contains a high amount of coarse material,mainly due to gravity transport from the hills nearby. The study of thecomposition of material coarser than 2 mm indicates that, apart fromwhole pieces of rock that resemble the rock composition, the smallerfractions between 2—5 mm are mainly composed of quartz, while in somecases feldspar contributes. To a certain extent, the occurrence of theseconstituents indicates their resistance to weathering processes (Chap-ter III.3).

111.2. Grain size distribution in the fine earth

The following texture classes are applied: sand (2.0—0.5 mm), silt(50 — 2 /im) and clay (smaller than 2 ßtn), (Soil Survey Manual, 1951).

89

III. 2.1. SandThe grain size limits assigned to facilitate a subdivision of the sandfraction are the following: coarser sand (2.0—0.5 mm), medium sand(0.5-0.02 mm), fine sand (0.02-0.01 mm) and very fine sand(0.01-0.005 mm).The sand fraction as a whole is an important constituent in the majorityof the soils, often representing more than 50% of the fine earth fraction.This feature is clearly detectable in a number of texture graphs, con-structed on the basis of the mean values of the different fractions for thewhole soil.In addition, the dominant sand grade is indicated in a sand triangle,constructed through the recalculation of the particle size distribution forthe total soil on a 100% sand basis (fig. 25).

100AO

% coarse sand

Fig. 25. Sand triangle

Summarizing, it is seen that soils derived from the granitic parent materialcontain a higher amount of sand than soils derived from the doleritic rocksuite. Within the granitic soils, the dominance of coarse sand is recognizedabove the other sand grades. In addition, a decrease in the amount ofcoarse and medium sand is accompanied by an increase in the clayfraction and occasionally in the silt fraction in the better developed soils.This is in agreement with findings from Thompson (1961).The soil derived from the Kalahari sand mainly consists of medium andfine sand, which is in accordance with the textural composition of the

90

parent rock (Chapter 1.2) and confirms earlier statements made by Polder-vaart (1955).

IU.2.2. SiltThe silt fraction is subdivided into coarse silt (50—20 /urn) and fine silt(20 — 2 pm). The total silt fraction plays a subsidiary part relative to thetexture composition of the soils. In soils derived from granitic parentrock, the combined percentage is about 1.0%; however, this percentage isconsiderably higher in soils developed from or on doleritic rock (25%).The lowest percentage of silt (0.5—2%) is encountered in the soil from theKalahari sandveld (profile 30) and the highest percentage (mean 33.1%) isfound in profile No. 25 (Vertisol).The ratio of fine silt and coarse silt is variable. In soils derived fromgranitic material, the amount of coarse silt is slightly higher than theamount of fine silt. Soils developed on more basic rock show a littlehigher percentage of fine silt, as is also the case for soils on Kalahari sandand schist.

III. 2.3. ClayThe clay fraction comprises particles smaller than 2 micron; no subdivi-sion within this fraction was made. The texture diagrams show an interest-ing trend of the clay fraction relative to the development of the soils andtheir origin. Greatest contrast between the total clay percentages is foundin profile 22, black, cracking, clay soil from the Bonwapitse floodplain,and the sandy soil from the Kalahari, profile No. 30 (45.8% clay versus3.7% clay).An increase in clay within the soils derived from the granitic rock iscommonly observed but the degree of augmentation varies considerably.In general, the soils with a wetter soil moisture regime have a higher claypercentage and also a more pronounced clay distribution curve.In addition, granitic soils near the kopje have a higher clay content thanthe associated soils further down slope (profile 1 versus profile 2). Thisfeature was correlated by Thompson (1961) with differences in themineralogical composition. This was found to be only partly true (Chap-ter III.4).

The most typical Vertisols, profiles 22 and 25, have an amount of claycomparable to most of their counterparts in the rest of the world (Dudal,1965). The difference between the lithomorphic and the topomorphicposition of these soils could be responsible for the slight variation in thedistribution of the clay through the profile, see also fig. 26. Relative tothe particle size distribution, profile 25 closely resembles the margalithicsoil as described by Mohr and Van Baren (1959, p. 261).

91

Fig. 26. Particle-size distribution for selected soils in weight percentages. 1 : coarse sand, 2: mediumsand, 3 : fine sand, 4 : very fine sand, 5 : coarse silt, 6 : fine silt, 7: clay

The relation between the clay and the other fractions was computed forall soils. A very high degree of correlation was found to exist between theclay and the fine silt fraction (r = 0.971); this is mainly caused by the verylow percentage of the 20—2 urn fraction in the soils (see III.2.2.).A fair negative correlation was observed between the clay and the fine +medium sand (r = —0.772).

The shape of the clay distribution curve was found to be closely related tothe differences in the soils. The relevance of this curve is supported by thefact that the majority of soils are developed on homogeneous parentmaterial. Four types of clay curves are recognized: 1) clay-constant,2) clay-bulge, 3) clay-pyramid, and 4) clay-step(s).

92

The application of the type nomination may vary if used for the total soildepth and is here limited to the clay distribution in the A and B horizonsto a depth of 1 50 cm; if the C horizon is included no depth limitation isused (fig. 21).

% clay

0

50

100

depthcm

10

• i•i•

iVi

(16)|

1

1

|

1

20

\\'\

(15) \

\

30 40 5C

\ N\ X

\ X

\ > v

s (29 i \

V iA \./ '• \

è

type 1

34

prof i le no in ( )

Fig. 27. Clay curves

Type 1, clay-constant: this curve is encountered in coarse textured soils ofthe Inceptisol and Entisol Order, such as profiles 5, 16, 20 and 30. Theabsolute increase of clay with depth varies between 0 — 2.5% clay, whilethe relative increase in clay may be as much as 50%. This is due to thevery low clay percentage of the soils. The increase in clay in absoluteterms seems therefore more appropriate than the relative indication. Theconstant trend of the clay curve in the Vertisols is merely attributed to

93

the effect of churning that causes a fairly even distribution of the claywith depth. Sharp changes in the amount of clay in these soils arerestricted to the topsoil (Type 4, clay-step). Similar observations weremade by Smyth (1963) and Fölster et al. (1967), who comment thattextural differentiation not only increases with the age of soil formationbut also with the rising clay content of the parent material. The impover-ishment of clay in the surface horizon is ascribed to surface wash.

Type 2, clay-bulge: this type occurs in the medium textured soils derivedfrom granitic parent material; the following profiles (all Alfisols) arerelevant: 3, 6, 7, 8, 9, 23 (weak), 29 and 32. The absolute clay increasewith depth may be as much as 30% clay, but commonly varies between10 — 20%. The relative increase in clay in the B horizon varies between20—50%, while a subsequent decrease in the lower part of the B differsless than 20% from the clay percentage in the upper part of the B. Hereagain it is preferable to indicate the clay increase in absolute rather thanrelative terms.The "clay-bulge" has been widely recognized and used to designatetexture B-horizons (Bt); its occurrence became almost synonymous for aclay illuviation horizon in the early years of European soil science. As soilgenesis was better understood it became evident that weathering of soilmaterial in situ could also cause such a distribution (Laatsch, 1937).Similar soils with this type of clay distribution curve were reported inother parts in Southern Africa (Thompson, 1 965; Van der Eyk, et al., 1971).

Type 3, clay-pyramid: soils with this type of clay distribution are com-monly found on granitic parent material and include the profiles 1, 2, 4,10, 13 and 33 and is weakly expressed in profile No. 11. The positivecurve indicates the increase of clay with depth while the negative curverepresents the regular decrease of clay with depth. The absolute increasein clay varies between 15 and 40%; a subsequent decrease or increase inthe C-horizon may equal this amount; if expressed relatively, the increasein clay may reach 50—100% clay. A combination of this type and theclay-constant is sometimes observed (profiles 12, 15 and 21); after aregular increase of the clay percentage with depth the amount staysconstant. The pyramid type of clay distribution has been recognized byseveral scientists; its development has been attributed to pedogenesis(Scholz, 1968) as well as to internal stratification of parent material(Fölster et al., 1967).

Type 4, clay-step(s): the clay curve reflects the sudden increase or de-crease in the clay percentage followed again by either increase or decrease.It often occurs in combination with other types. If applied to the control

94

section from 0 — 100 cm the profiles 15, 25, 26, 31 and 32 are considered.The change in clay percentage is variable but is often more than 10% clay(absolute).In the studied soils the sudden change in the clay curve is caused byadmixture from allochthone material (sand), or removal of soil materialthrough the effects of wind and water erosion on the topsoil, which mayresult in profile truncation. In some horizons the change is due todifferent stage of weathering. This type of curve frequently occurs inalluvial soils, then resulting from differences in the dynamism of sedimen-tation.

The pedogenetic processes that cause differentiation in the clay distribu-tion of the soils under consideration are illuviation, vertical as well aslateral, and formation of clay minerals (in situ), in some parts of thesolum i.e. subject to more intense chemical weathering of primary miner-als (Scholz, 1965). A more detailed account of these processes is given insection V.l.if the criteria are applied as suggested by the USDA 7th Approximation(1970) with regard to the clay movement in the argillic horizon, thefollowing remarks can be made. In soils of which the eluvial horizon hasless than 15% clay, the argillic horizon must contain at least 3% clay(absolute) more than the eluvial horizon. This is the case in profiles 2, 4,7, 10, 11, 13, 14, 18, 21, 29, 32 and 33. If the eluvial horizon hasbetween 15—40% clay, the ratio between the clay percentages in theargillic and the eluvial horizon must be 1.2. This is the case for profiles 8,9, 12, 15, 23, 24, 25 and 31, but not for profile 22. Above 40% theabsolute increase of 8% is considered to be sufficient for an illuvialhorizon to be classified as argillic. Neither conditions are met for profiles5, 18, 16, 20 and 30 and the lithic Entisols.How far the suggested criteria are representative for a clay illuviationresulting in an argillic horizon is discussed in Chapter IV.It is however pointed out that the limits as set by the 7th Approximationdo not cover the clay distribution below the control section (0 — 100 cm),which may be an important soil characteristic.

III.3. Elemental composition of the fine earth

III.3.1. Major ElementsNine major elements were determined by Röntgen fluorescency techniquesof the fine earth fraction, in addition to a number of trace elements dealtwith in Chapter III.3.2.The elements consituting the largest proportion of the fine earth are Si, Al

95

and Fe; these are followed by the oxides of Ca, Mg, K and Na. Titaniumand phosphate occur only in subordinate amounts. The amount of SiO2

ranges from 62.3% to 100% for all soils. The lowest values are encoun-tered in the hydromorphic Alfisols and the highest percentage occurs inthe granitic Entisol. The sharpest contrast in SiO2 concentrations isobtained within the Entisol order, notably the granitic Entisols versus thebasic ones. Similar trends hold true for the amount of Al2O3 and Fe2O3 .

The influence of the rock suites is clearly evident in the concentration ofthe major elements in the Entisols (basic). Mutual differences on a meanbasis do not show any marked variations. The concentration of Al2O3

varies between 1.1 and 24.4%; the Vertisols have values in the higher levelof this range (7.2-16.5%) in addition to Inceptisols (8.2-17.7%) and theEntisols on material rich in bases (17.8-20.1%). The concentration ofFe2O3 varies little in the soils, major variations occur in the Entisolsdeveloped on granitic material (0.8 — 1.2%) and those developed on doleriticmaterial (10.7-10.9).

Table 26. The concentration of Si'O2, Al2O3 and Fe2O3 in the soils (% by weight)

soil

VertisolsInceptisolsEntisols (acid)Entisols (basic)Alfisols (Haplustalf)Alfisols (hydromorph)Alfisols (Rhodustalf)all soils

SiO2

range

69 .0-68 .0-

85.890.4

83.6-100.063 .0-63 .2 -6 2 . 3 -7 0 . 1 -

65.695.289.183.3

62.3-100.0

A12O3

range

7.2-16.58.2-17.71.1-10.7

17.8-20.14.6-24.46.1-21.08.7-19.11.1-24.4

Fe2O3

range

2 .0 -2 .0 -0 . 8 -

6.88.21.2

10.7-10.91.0-1.9-2 .4 -

8.48.97.4

0.8-10.9

Changes in the concentration of SiO2, Al2O3 are reflected in the SiO2/Al2O3 , the SiO2 /R2O3 and the Al2 O3 /Fe2 O3 ratios.Highest SiO2 content is usually encountered in the A-horizon(s) resultingin high ratios. The concentration of SiO2 commonly decreases with depth,while the concentration of Al 2 O 3 increases, thus giving rise to consider-ably lower SiO2/Al2O3 and SiO2 /R2O3 ratios. The amount of iron,indicated as ferri-oxide in all analyses, shows usually a regular increasewith depth. Corresponding A l 2 O 3 / F e 2 O 3 ratios are fairly constant andonly in some cases increase in the soil, as for example in profile 15.The concentration of elements other than silicium, aluminium and iron is

96

greatest in the Vertisols (7.4%) and basic Entisols (6.4%) and smallest inthe Inceptisols (2.0%) and hydromorphic Alfisols (1.6%) and ranges be-tween 3.2% and 4.3% in the other soils.More information concerning the above is presented in the followingtable.

le 27. The concentration of bases and Ti and P in the soils (% by weight)

TiO2

>up range

0.40-0.780.49-1.060.18-0.401.06-1.240.25-1.280.31-1.030.39-0.79

s 0.18-1.28

Cao

range

0.19-90.26-20.09-01.32-50.10-20.18-20.44-0

0.09-9

.66

.02

.24

.31

.52

.02

.70

.66

MgO

range

0.00-2.920.31-2.320.00-1.962.75-3.380.00-3.350.42-3.730.00-1.41

0.00-3.73

K2O

range

0.67-4.892.91-4.290.27-5.230.48-2.040.00-4.710.00-4.182.80-4.00

0.00-5.23

Na2Orange

0.00-2.280.00-1.550.00-1.530.00-0.280.00-4.490.00-1.010.00-2.08

0.00-4.49

P2O5

range

0.03-0.120.02-0.200.00-0.100.04-0.540.00-0.220.04-0.190.03-0.12

0.00-0.54

The information does not differ as much as could be anticipated from thecomposition of the parent rock.The amount of potassium is considerably higher in soils derived fromgranitic material. The concentration of the various elements dependslargely on the composition of the parent rock which in turn is reflected inthe assemblage of primary minerals. The data obtained on the composi-tion of the fine earth of the investigated soils correspond well with valuesgiven by Jackson (1969) and Mohr et al. (1959, 1972). The relationshipbetween the elements and the grain size fractions may provide useful

Table 28. Correlation coefficient between Si'O2, AI2O3, Fe^O^ and some grain sizefractions

grain-size

clayclay + fine siltc. silt + fine andmedium sand

SiO,

0.700.870.69

A12O3

0.820.870.78

Fe2O3

r

0.890.860.61

97

indication for the evaluation of the soils. Computations were thereforecarried out to see if such correlations exist. This was found to be onlypartly the case (see table 28).Of the found correlations one is visualized in figure 28. The graph isconstructed for all samples except those from the Entisols and profiles 25and 26.

y =0.112 » • 0.798

f = 0.B9

clay %

Fig. 28. Scatter diagram and regression line for concentration and clay percentage

In general, the finer fractions are more readily correlated with the ele-ments than the coarser grain size fractions. The high correlation foundbetween the three elements and the clay + fine silt fraction is partly dueto the very low amount of silt in the soils, resulting in a high correlationbetween the clay and the clay + fine silt fraction (r = 0.97).Although other workers (Buursink, 1971) found more statisfactory corre-lations between the various elements and the textural composition of thesoils, these studies were mainly conducted on alluvial soils, which have infirst instance an inherited grain size distribution that is often influencedby the nature of the solid sediment load. The correlation between Si, Al

98

Table 29. Correlation coefficient and regression equations for the oxides of Si, Aland Fe, as calculated for the fine earth of all samples

SiO2 A12O3

SiO2 Fe2O3

A12O3 Fe2O3

regression equation

-0.88 y =-0.495 x+51.680-0.87 y = -0.263 x +24.805

0.82 y = -0 .915x+ 2.371

st. err. x st. err. y

3.730% 2.138%4.228% 1.220%2.641% 1.329%

and Fe was found to be of greater significance for the investigated soils.Results are given in table 29 and illustrated for one case (SiO2—Al2O3) infigure 29.In combination with table 29, the above-quoted correlations do point tothe differentiation between the soils and are indicative for some soil-forming processes as dealt with in Chapter V.

y = -0.495 x + 51.680r =-0.88

Fig. 29. Scatter diagram and regression line for the concentrations of AI2O3 and S1O2

99

III.3.2. Trace elementsA number of trace elements were determined in order to evaluate the soilsand to indicate the differences between the soils encountered. Apart fromresearch by Nilsson (in Siderius, 1972), no data of trace element concen-trations in Botswana soils were available. The information (presentedcompletely in Appendix I) denotes the following range in trace elementsin ppm: Ba, 100-1720 (mean 515); Co, 2-77 (mean 21); Cr, 4-300(mean 81); Cu, 5-58 (mean 22); Mn, 4-2830 (mean 350); Ni, 4-104(mean 27); Sr, 2-250 (mean 29) and Zn, 2-86 (mean 20).The variation in concentrations occurs within the "limits" proposed byMitchell (in Bear, 1969), but the mean values indicate that the concentra-tions are in general on the low side, which is illustrated in figure 30.

frequency70 -i

0-10 10-20 20-30 30-40 40-50 50"60 60"70 70"80 >80(Co ,N i ,Sr ,Cu ,Zn) ppm

0-100 100-200 200-300 300-400 400-500 500"600 600-700(Mn.Cr) ppm

I I I ' I I I0-200 200-400 400-600 600"800 800-1000 »1000

(Bo) ppm

Fig. 30. Frequency distribution of trace elements

100

The nature and content of trace elements in the investigated soils can betraced back to their parent material, data relative to the latter are given inChapter 1.3., which confirm the findings of Apostolakis et al., (1970).However, the various trace elements are not uniformly distributed amongthe different rock types and many show a preference for a certain rock(Mitchell, 1969). This trend can often be observed in soils that are derivedfrom a certain rock type, although this does not necessarily mean that theamount of trace element of the soil can be readily deducted from the parentrock (Oertel, 1961). Investigations indicate that a relationship does existbetween the concentration of the trace elements and the soil in whichthey occur, as is demonstrated in the following table.

Table 30. Relation between trace elements (ppm) and selected soils

Traceelements

BaCoCrCuMnNiSrZn

Vertisols

range

100-17201 2 - 585 - 2245 - 58

370-12508 - 682 - 595 - 60

Alfisols (UlticHaplustalf)range

100-26002 - 565 - 2565 - 504-28302 - 852 - 803 - 86

Alfisols (hydromorph)

range

100-12602 - 58

25- 3525 - 54

86-25083 - 1042 - 455 - 45

While keeping in mind that the Vertisols are derived from the doleriticrock suite and the Alfisols (Ultic Haplustalfs) from the granitic rock, it isseen that the concentrations of Co, Cr, Mn and Ni are relatively high inthe soils developed on basic material. The concentrations of Cu, Sr and Znare more or less equal in the different soils, but Ba is well concentrated inthe Alfisols.Tç> a certain extent the hydromorphic Alfisols indicate the influence ofpedogenetic processes with regard to the trace element distribution, asthey have most likely undergone changes in the original soil material beingsubject to a more variable soil moisture regime.An extremely low content of trace elements is recorded in the sandyKalahari soil (Typic Ustipsamment, profile 30), derived from the CaveSandstone.Our data corroborate results reported by Mitchell (1969) and do not varywidely from trace element data given in Chapter I.

101

The computed correlation between the amount of trace elements anddifferent grain size fractions was applied to all samples, but yielded littleresult. The following statement can be made.The bivalent trace elements (Co, Cu, Ni and Zn) show a good negativecorrelation with a) the coarse sand fraction (r = —0.85), with b) the me-dium sand fraction (r = —0.80) and with c) the fine sand fractions (r =-0.75) .The positive correlation is recorded when dealing with the fine fractionsbut only for some elements, notably Mn, Ba and Sr. These elements have apositive correlation with the coarse silt fraction of respectively 0.83, 0.98and 0.79.The clay fraction correlates poorly with the various elements, but the finesilt + clay fraction has good positive correlation with the bivalent traceelements, especially if applied to the Vertisols (r = 0.81).The affinity of the trace elements for the finer grain size fractions,particularly fine silt and clay, in the investigated soils is evident andsupported by the findings of Krauskopf (1972).

The distribution of the trace elements in the soils is variable. For Mn andBa a decrease in the concentration with depth is noted in the majority ofsoils. The opposite is the case for Cu, Zn, Cr and Ni, their concentrationsshow a trend to increase with depth in most soils, although an initialincrease may be followed be a decrease in the deep subsoil. No clearpicture is obtained for Co and Sr, the amount of element varies irregu-larly.A clear change in trace element concentration in the soils is oftenconnected with the B horizon and corresponds with the textural variationthat does occur in that part of the solum, thus indicating the translocationof trace elements caused by migration of the finer particles (Norman,1963).Relationships between the trace elements themselves and some majorelements are known to exist in a number of cases. According to Krauskopf(1972), the similarity in ionic radii of Fe, Mn and Zn and the strongcovalent bonds that Cu and Zn form with S are mainly responsible for thisinterrelationship.A computed correlation between Zn and Co, Cu and Ni is shown infigure 31.

The high positive correlation between Zn and these bivalent elementsimplies the relationship between them and indicates that nutrient applica-tion should be balanced with regard to all elements present.

102

250PPTr

200-

150-

100-

50

y - 2.633 x * 34.224

r = 0.88

TO" 20 30 40 50 60 zn 70

Fig. 31. Scatter diagram and regression line for the concentration of Zn and some bivalent traceelements

III.4. Mineralogy of special constituents

III. 4.1. Mineralogy of the sand fractionMineralogical research of the sand fraction of soil materials is widelyapplied to characterize the relation between soils and their parent materialand is a useful guide as to the uniformity of the material from which thesoil has developed. In addition it may yield valuable information relativeto^ the utilization of soils (Sherman, 1971).The sand fraction contains most of the primary minerals to be found. Aconvenient way of separating lighter and heavier minerals is to use thespecific gravity of these minerals as compared to bromoform (s.g. 2.89).The variation in heavy minerals is much greater, a fact that is particularlyadvantageous in comparison to the few members of the light mineral suite,although both groups may be indicative of the source of the minerals.

103

The sand fraction commonly used (50—500 microns) for heavy mineralinvestigations was found to be of little value, as the coarser sand(200—500 microns) yielded practically no such components. The fine andmedium sand fractions were therefore investigated (50—200 microns), thisbeing similar to the procedure applied by Watson (1965). Regular checkswere carried out in relation to the coarser material to make sure that nodeviations of the applied research technique occurred. The mineral com-position of some soils is given in table 31.

III.4.1.1. Heavy mineralsInvestigations of the heavy mineral composition of the soils led to therecognition of five assemblages (fig. 32).Assemblage 1 is characterized by the abundance of green hornblende,followed by epidote and zoisite, while garnet and staurolite occur insubordinate amounts. The presence of zircon and tourmaline is negligiblewhen compared with their occurrence in assemblages 3, 4 and 5. Augitedoes occur as trace or in very small amounts, its presence is however ofgreat importance as it is indicative of the proximity of basic rock. Theamphiboles, in our counting identified as green hornblende, are firstlyascribed to the transformation of pyroxenes into amphiboles (Chap-ter 1.2.2.), while epidote is also mainly derived from ferromagnesiumminerals. In addition, clino-zoisite is regarded as a secondary product oftransformation from pyroxenes or of lime-soda plagioclase feldspar (Mil-ner, 1962).The presence of garnet further proves the influence of metamorphosedgranitic rock. The heavy mineral suite as described above is commonlyencountered in soils derived directly from dolerite, such as in profiles 14,23, 27 and 28. In addition this assemblage may be found in soils amidstthe granitic region. In the case of profiles 12, 24 and 13, the suite isindicative of the presence of intermediate or basic bodies of rock withinthe granitic complex, as for example dikes as referred to in Chapter 1.2.1.In assemblage 2 the same minerals as in the previous suite are present. Theoccurrence of the large amount of epidote however together with consid-erable amounts of amphibole, zoisite and staurolite, warrants its separa-tion from the former suite. The high amount of epidote may also bederived from the transformation of plagioclase feldspar which may subse-quently have caused the low percentage of this feldspar in the correspond-ing light mineral assemblage (see III.4.1.2.).

The occurrence of garnet further indicates the nearness of gneisses andschists as recorded in the Basement Complex. Pyroxenes occur but astrace or in very small amounts. The staurolite, finally, is indicative ofdynamo-metamorphic influence.This assemblage is common in soils derived from transported weathering

104

profile 2320 40 60 80 100

profile 25

profile 10

5-

21.5-

43-

62,5-

81,5-9 1 -

i:::;:;:i:jfl:j:i|v

»:':';3i;l|j|j|jl - =j

profile 140 20 40 60 8,0 100

20 40 60 80 100

49,5

81,5

119

166

profile300 20 40 60 80 100

11,5

42,5

100 H 5

100

142,5

192,5

toermahne I epidote

amphibole

hornblende)

dysthene(kyanite) I garnet miscellaneous

Fig. 32. Heavy mineral composition of representative soils from each assemblage

105

products of doleritic rock, such as in profiles 20, 25 and 26. It is alsoencountered in profiles 1, 17 and 22 located in the chiefly granitic area. Ifsuch is the case the assemblage testifies the proximity of more basic rock.Assemblage 3 is typified by an increasing amount of zircon in the heavyfraction; this mineral is distinctive for assemblages 3 and 4. On the otherhand the fairly large amount of minerals typical for the groups 1 and 2,such as amphibole and epidote, suggest that this assemblage may also betransitional to the first two groups. This is also deduced from the presenceof small amounts of zoisite, rutile and staurolite. Soils with this heavymineral composition are commonly situated in lower lands such as profiles4 and 15. Other profiles, namely 2, 7, 16 and 18, are fairly close to thosesoils in which assemblage 1 or 2 is found. The most distinct changes in theamount of zirvon is often related to the A-horizon.Assemblage 4 is considered typical for the soils derived from the CaveSandstone or which are under strong influence of the sandveld. This groupis characterized by high percentages of zircon and tourmaline with sub-ordinate but significant amounts of rutile, staurolite and kyanite (profile30). Similar observations were made by Wayland et al. (1953) and Polder-vaart (1957) relative to the heavy mineral composition of Kalahari sand.The latter author further notes the possible occurrence of epidote, am-phibole and garnet in fringe areas. Investigations on a number of profiles,that is 29, 31 and 32, located in such an area (sample area D) indeedshowed the presence of these minerals in the heavy fraction.The amount of tourmaline decreases notably away from the sandveld aswas observed in profile 33, while the amounts of epidote and greenhornblende increase. This soil is thus considered to belong to assem-blage 3.The presence of rutile is particularly indicative of eolian deposits of theKalahari (Poldervaart, 1957, Harmse, 1967).Kyanite, often associated with staurolite, is indicative of the presence ofmetamorphic rocks, especially schists and gneisses. The heavy mineralcomposition of the Kalahari sand (profile 30, Typic Ustipsamment) pointsto the source of the variation of the associated soils in the catchment ofthe Mahalapshwe and Bonwapitse rivers. The admixture mainly consistingof zircon and tourmaline is however commonly restricted to the topsoil.Assemblage 5 is characteristic for the majority of soils derived fromgranitic rock. Typical is the very high percentage of zircon (varietymala con).Poldervaart (1956) reported similar percentages in granitic soils in South-Africa. Zircon and tourmaline are considered most stable minerals and areable to survive several erosional cycles. The presence of metamorphosedgranitic rock and the admixture of other minerals is testified by thepresence of epidote and amphibole, respectively rutile and staurolite.Sillimanite and andalusite occur as traces.106

The status of weathering is also implied in the percentages of opaqueminerals and alterites; whereas the former are iron compounds (mainlyhematite), the latter are minerals altered to such a degree that their charac-teristics are concealed. The percentage of opaque minerals was countedseparately as is common usage in the European school of sand mineralogy(Carroll, 1962; in Milner).The amount of opaque minerals is high and ranges from 91% in the deepsubsoil of profile 7 to 28% in profile 1. High percentages are recorded alsoin profiles 11, 19 and 32 of respectively 70, 71 and 71% (arithmaticmeans), the majority of the soils contain between 50 and 70% opaques,while in the remaining soils this percentage varies between 30 and 50%.There is a trend that soils with a high opaque content have a lowpercentage of alterites. For example low alterite percentages occur inprofiles 1, 11, 31 and 33 (1 — 2%), while a high amount of alterites iscounted in profiles 2 and 26, respectively 16 and 15%.The majority of the soil materials however have an alterite percentage ofbetween 2 and 5%. Although the exact nature of most of the alteritescannot be traced, there are indications that a fairly high percentage is infact strongly weathered green hornblende.In general the original rock composition is well reflected in the heavyminerals observed in the investigated sand fraction, taking into accountthe vulnerability of these minerals to weathering. Thus a decrease inpyroxenes, apatite, biotite in the rock weathering products was observed.No studies were conducted on the affinity of the various heavy mineralsto particle sizes.Research on this subject (Dougles, 1940; Van Andel, 1952) on alluvial pisediments, gained variable results.Our findings indicate that almost the total heavy mineral load is limited tothe fine and medium sand fraction, thus enhancing statements made byPettijjohn et al. (1972) relative to fractionated mineral research.

If we pursue research as carried out by Brewer (1950), assuming equalheavy residues for both the topsoil and the subsoil for profiles 1 to 4, wearrive at the following conclusions (see fig. 33).

107

1220

s c a l e 1

4

: 50.000

prof tie

_ . tourmal ine

gar net

epidote.zois i te

amphibole

t _ topsoi l

s _ subsoi I

Fig. 33. Variations of the heavy mineral composition in the profiles 1 to 4

The heavy mineral assemblage runs parallel in the top and the subsoils,indicating its uniform distribution. Furthermore, a sharp relative increasein zircon from profile 1 to 2 occurs, whereafter the percentage stays fairlyconstant, but for the change in topsoil versus subsoil. The increase in thetopsoil of profile 4 is ascribed to lateral supply in this drainage channel. Aregular increase in tourmaline from profile 1 to 3 is seen, but also asubsequent decrease in profile 4. The percentage of epidote-zoisite de-creases sharply from profile 1 to 2 and further decreases, followed by asmall rise, in the subsoil of profile 4.However green hornblende increases relatively downslope from 1 to 2,followed by a decrease and subsequent increase in profiles 3 and 4respectively.Garnet is present in considerably amounts in profile 1 only, but occurs insubordinate quantities in the other soils.

108

The largest changes occur in the zircon, tourmaline and garnet percent-ages.Taking into account the dominance of granitic rock, the changes in heavymineral composition partly confirm findings by Thompson (1965). Hestates that "at the base of the (granitic) kopje there would be a localconcentration of the heavier feldspatic and ferro-magnesium minerals" . . ."subsequently the weathering products of these minerals would give riseto a textural variation between the members of the transition downslopein such a way that the finer textured soils occur at the upper end of thesequence and the coarse textured ones at the bottom part."The grain size distribution in the profiles 1 to 4 partly shows a differentpicture in that the finest textured soil (profile 4) occurs at the lowest partof the transit. In this case lateral supply of finer constituents musthowever not be excluded. The coarsest textures are found in profile 2encountered at the middle upper slope of the pediplain, while a sandy clay(loam) texture is encountered also in profile 1, it being the upper memberof the sequence.

III.4.1.2. Light mineralsThe soil mineralogy is supplemented by the knowledge of the lightmineral composition of the sand fraction (s.g. < 2.89). Investigationsthereof were carried out on 50 selected samples. The result complementsthe data obtained from the heavy mineral composition and is summarizedas follows: quartz, orthoclase, anorthite and albite are the dominant lightminerals; oligoclase, microcline, muscovite, sericite and volcanic glassoccur as accessory minerals. The presence of volcanic glass was alsoreported in soils in the Karroo, South Africa (Cox et al., 1967).Statistical calculations carried out on the dominance of the light mineralsrevealed the relationship with the heavy mineral assemblages, as given insub a. On basis of the mean values of the percentages of minerals occur-ring in the investigated profiles, it appears that assemblage 1 is supple-mented by quartz, anorthite and orthoclase in the light fraction, respec-tively 70, 15 and 8%. The presence of lime-bearing plagioclase feldsparimplies the presence of basic rocks and as such confirms the conclusionsbased on the study of the heavy mineral assemblage. Very large amountsof quartz are further observed in the light mineral fraction associated withassemblage 2. The percentage orthoclase is low, while anorthite may occuras trace. The light mineral suite of the other three assemblages indicateshigh amounts of quartz and orthoclase with minor occurrence of albite.The percentage of orthoclase is about 20% in group 3 and 25% in group 5.The percentage weathering products is variable, the highest amount wasencountered in the C material of profile 25 (45%), but normally variesbetween 0—10%. The investigations evince that the majority of these

109

Table 31. Mineral composition of the fine and medium sand fractions (50—200 micron)in mutual percentages for selected soils

SoilProfile

25

20

30

28

14

11

15

10

Depth

0 - 33 - 33

3 3 - 676 7 - 9696-142

142-190

0 - 3030- 65

0 - 232 3 - 5252- 8080-120

120-165165-220

0 - 38

0 - 1818- 5656- 9090-110

0 - 1616- 4545- 8282-108

108-140140-173

0 - 22 - 43

4 3 - 6969-9191-127

127-158158-180

0 - 1010- 3333- 5353- 7272- 9191+

Vol

e, g

la—

-

-

_

-

tr

_

_

-

—

tr—

tr

_

_

—

-

_

tr

tr

Qua

rtz

9190

43

6447

94

92

64

53

5558

79

8181

81

76

73

77

73

79

82

77

Sä

Ort

hocl

a

67

6

1628

4

5

7

29

2922

17

1514

15

21

23

21

24

18

15

18

Alb

ite

11

-

53

2

3

-

11

46

2

22

2

2

2

2

2

2

2

3

Light

Mic

rocl

ir

tr

tr

1

_

-

_

-

-

1

2-

1

11

1

1

1

tr

1

1

1

1

mineral composition

U 01 U

Olig

ocla

s

Ano

rthi

t

Mus

covi

t

1 — —1 -

same as abovesame as abovesame as above

- tr tr

2 tr1 2

tr — -same as abovesame as abovesame as abovesame as above

tr — -

3 20 tr

2 - -same as above

t r — —

2 - -

_ _ _

same as above1 - -2 - -

same as above^

tr - -same as above

1same as above

tr — —same as above

tr - -

_ _

same as above_ _ _

same as abovesame as above

- - -

Bio

tite

tr

tr

-

413

_

-

tr

1

—

1

tr

tr—

-

tr

tr

tr

tr

—

-

Ser

icit

e

—

-

tr

-

_

-

-

_

-

_

—

—

-

_

_

—

-

—

-

Phy

toli

tl

—

-

-

tr

tr

tr

1

_

-

tr

tr

tr

tr

tr

tr

tr

. tr

tr

tr

-

c

Wea

ther

ipr

oduc

ts

1

2

45

96

—

-

6

2

1011

1

——

-

_

_

-

-

1

1

1

110

L»J Os © en S) 4*.

S3 Os Os 00 O H-*

I I

I I I I I

CO 00 W NJ 00

I 1

! I t

I I 1 I

e»j o N I S3 en i-> oo

I ! I

I I I

I I !

I 1 I

« ? S? S) H-

L»J SJ \O Os -b-

I I I I I

oo o> m * NI to

I I

P l» U H

I I

I I I I I

I I

I I I I I

' Si S

OJ >—» U>

I I I

Ln Os H- OO

I I

M W *Û W

I I ( ^ I

O S3 Os 4».

I I I I

OO

to00

Os

sO

toO

toOO

OsO

to

en

toOs

OssO

to

to

I I I I

I I I I I I

00 so ^1 Ui Ui OS

Os 00 O H-» 00 h-

I I

I I

I-» H- U>

I I I

I | I ! I

I I I

I I I

I I I

I i

I i

I i

00 so so OS ^1 to

-fr. OJ Os O OS

N W H N M

I I I

OS OS ^. so

P N H H M

I I I I I

I I I I I I .

I I I I I I

I I I I I I

I I I I I I

H s] U CO Ui

Opaque

Zircon

Tourmaline

Garnet

Rutile

Titanite

Staurolite

Kyanite

Andalusite

Epidote

Zoisite

Gr. Hornblende

Augite

Hypersthene

Monazite

Sillimanite

Alterite

weathering products are altered biotite and intermediate and basic plagio-clase. . • .The composition of the light mineral suite of the sand fraction once morefocusses the attention on the relation between the dolerite and thegranitic rock suite and further points to the strong degree of weatheringthat has taken place (see also Holmes, 1969).Relative to the first remark, the occurrence of basic plagioclase, anorthite,is only observed in material directly derived from the doleritic rock.The second observation leads to the conclusion that, of the light minerals,only those with a low to very low weatherability have persisted in theenvironment, such as quartz and orthoclase.The distribution of the heavy and the light minerals with depth in the soilsdoes not show any marked changes other than some minor variations thatmay occur in the topsoils of soils in sample area D (profile 29) and in thesoil developed on alluvial of the Bonwapitse River (profile 22).

In conclusion, the mineralogical research of the sand fraction confirms ourfindings of Chapter 1.3. relative to the origin of the soil materials andfurther explains the small variations that may occur.The percentage of weatherable minerals is low, only the more persistentminerals are encountered in large numbers.The uniform distribution of the minerals in the soils enhances and pro-motes sound interpretation of the chemical data with reference to the soildevelopment (Reynders, 1964).

III.4.2. Mineralogy of the clay fractionIn the former sections of this chapter and also in Chapter II.3. theimportance of the clay fractions with relation to soil properties andcharacteristics has been emphasized.Not only the amount of clay (III.2.3.) but especially the type of claymineral determines to a large extent a number of evaluation criteria, suchas cation exchange and nutrient status, while clay mineralogy is alsoapplied in the classification of the soils (USDA, 7th Approximation1970). Further investigations warrant a better insight in the genesis of thesoils. This research was carried out by means of X-ray diffraction tech-niques on all samples, while from 85 selected samples of A, B and Chorizons the elemental composition was determined through X-ray fluo-rescency. An additional 29 samples were subjected to special treatments.Although suggestions were made on the identity of a number of clayminerals in Botswana soils (Siderius, 1972), these were hitherto notconfirmed by data. Reports from adjacent territories indicate the presenceof the commonly expected minerals in the clay fraction encountered

112

under similar conditions and derived from similar parent material. Theycomprise kaolinite, illite and montmorillonite (Harmse, 1967; Watson,1965; Scholz, 1963, 1968).This general picture is confirmed by our investigations but some elabora-tions are necessary.Quantitative measurements oiLclay minerals would be premature in thelight of the performed analyses. However, norm analyses carried out on anumber of soil clays allowed a tentative approach (personal communica-tion De Rooij). Weight percentages of the constituents of the clay fractionwere recorded as follows: trace (tr), subordinate (10—40%) and dominant(more than 50%); for quartz a different standard was used: trace (tr),moderate amount (1 — 10%) and large amount (10 — 20%). Feldspar, mainlyorthoclase, was not indicated separately but may occur in considerableamount in profiles 2, 7, 11, 14, 15, 16 and 17. Indications of the presenceof iron compounds (hematite, goethite) was observed in profiles 13, 14, 21,23, 24 and 27.The main source for the formation of clay minerals is provided by theweathering products of the primary minerals (Chapter III.4.1.). These inturn depend on the parent rock composition as was indicated in theprevious section, the main minerals include pyroxenes and amphiboles,some biotite as well as feldspars and quartz.Their weathering products are the basis for the formation of kaolinite andmontmorillonite as well as for the genesis of a number of clay mineralsthat occur in lesser amounts such as illite, vermiculite, chlorite and mixedlayer minerals (Worrall, 1968; Scheffer and Schachtschabel, 1970).This expectation is further supported by the considerations of Mohr andVan Baren (1959, 1972) with relation to the basic and acid milieu specificof the development for the various clay minerals. In this context Kraus-kopf (1967) comments that "acid solutions favour the formation ofkaolinite while basic solutions are a suitable environment for the forma-tion of montmorillonite".The mineral composition of the investigated clay fraction of the soilsconfirms to a large extent these anticipations.Kaolinite, with subordinate amounts of illite, is observed in profiles 3, 5,11, 17 and 32, all situated in the granitic area in addition to profile 30developed on material from the Cave Sandstone.Kaolinite, with subordinate amounts of illite and montmorillonite, isfound in other profiles located in the granitic region.A similar combination with lesser amounts of traces of mixed-layer clayminerals and/or chlorite and/or vermiculite is encountered in most of thehydromorphic Alfisols and in the subsoils of profiles 1, 2, 4, 15, 19, 20and 24. In the doleritic region, the dominancy of montmorillonite wasdemonstrated in the Vertisols and in the subsoils of profile 10, together

O

with subordinate amounts of kaolinite and illite; some vermiculite and/orchlorite may also be present. Mixed-layer clay minerals mainly belong tothe Mt-I type.

The distribution of the clay minerals in the soils is uniform in general;greatest changes in composition occur in the hydromorphic Alfisols and inthose soils with contrasting subsoil material such as profile 10, Rhodustalf.The dominance of kaolinite in the soils derived from granitic material isindicative to the later stages of weathering (Watson, 1965).

The identification of the clay minerals is mainly based on the interpreta-tion of the diffractograms. These graphic representations of the intensityand peak form of the basal spacing and other spacings serve as anindication for the internal structure of the various sets of planes in thecrystal. The method is widely applied to identify minerals of knownspacing (Worrall, 1968). The normal basal spacing of the 1 : 1 layerAl-silicate kaolinite at room temperature is 7.15 Â, while a second orderreflection was occasionally observed at 3.57 Â (002). The normal basaldistance between the crystal planes of illite is 10.3 Â, a reflection ofsecond order at 4.98 Ä was observed in a few cases (002).Montmorillonite, vermiculite and chlorite show reflection at 14 Â. Charac-teristic peak areas for chlorite further occur at 7.00 Â and 4.72 Â.In general, the peak areas are well outlined, while the background is fairlylow. Special treatments were carried out on 29 soil clay samples toconfirm the interpretations made from normally treated soil clays.These treatments include 1) Mg saturation and ethylene-glycol solvation atroom temperature; 2) the saturation of the clay specimen with potassiumat 33° C. and 3) the subsequent heating of the K saturated sample to550° C (Jackson, 1969). These experiments have one thing in common,namely the alteration of the crystal structure due to the exchange ofmetal cations in the lattice and/or possible sorption of organic compoundsfollowed by decomposition or transformation of the clay mineral causedby heat. X-ray diffractograms of the clay specimen treated in such a wayshow basal reflections known to occur in the different clay minerals andas such are applied to identify the composition of the clay fraction in theinvestigated soils. The selected samples showed high reflection intensitiesat random between 10 and 16 Â or irregularities in this range, as well asthose clays that show weak developed peak areas at 7 and 10 Â. Proce-dures of the treatments and their purpose are outlined by Jackson (1969)and adapted as follows.

Mg saturation and following glycol solvation at 25° C induces the increaseof the basal spacing of the Mg montmorillonites, the type as encounteredin the investigated soil clays, to 18 Â, thus providing a clear differentia-

114

tion between montmorillonite on one side and vermiculite and chlorite onthe other.Saturation of the clay with K at room temperature followed by heating isan additional check on the closure of vermiculite and montmorilloniteinterlayers, while the chlorite phases remain expanded. In addition, the10 Â reflection peak of illite is better pronounced due to better basalorientation caused by improved dispersion. Heating of the K saturatedsamples to 550° C causes the decomposition of kaolinite, while mont-morillonite and vermiculite close to 10 Â each.In addition, the 14 Â peak of chlorite is reinforced while the 7 Â reflec-tion has weakened or disappeared. The mixed-layer or interstratifiedminerals (Mi) can be detected further by heating of the K saturatedsample to 550° C. They proved to belong dominantly to the Mt-I type.The mixed-layer clay minerals Mt-V and Mt-I close to 10 Â, but Mt-Chl mayvary between 10-14 Â. Glycol treatment brings about an increase in basalspacing between 14-18 Â for Mt-Chl and Mt-V, and between 10-18 Â forMt-I.These methods proved successful for the differentiation between thevarious clay minerals. Additional investigations were carried out on sam- ß^ i«.pies from soil clays which were treated with diluted (2 N) HCl and H 2 O 2

and comparisons between these diffractograms and the ones derived fromthe "normally treated" clays were made. The treatment causes the re-moval of sesquioxidic coatings, carbonates as well as organic substancesand a fairly clean but possible corroded clay is obtained. In less than 50%of the cases the diffractograms of the pretreated clays showed a muchhigher background (up to twice as high) for the clays of the B21 horizonsfrom profiles 3, 4, 6, 9, 12, 18, 31 and 33. Considerably lower intensitieswere measured for the same horizons in profiles 10, 17 and 22. The basalreflections for the montmorillonite were enhanced from none or poorreflection to distinct peak areas in all Vertisols and the Typic Haplustalf(respectively A and B21 horizon). The reflections of kaolinite in thepretreated samples were much more evident than in the normal soil claysin the B21 horizons of profiles 2, 3, 4, 6, 11, 15, 26, 30, 31 and 33.Much lower intensities however were observed in the pretreated samplesof B21 horizons in profiles 7, 12 and 14, all Ultic Haplustalfs.The results imply that the removal of concealing material improves thereflection of montmorillonite considerably, while the clay mineral is goodcrystalline, as it is assumed that the very acid environment of the treat-ment is in contradiction to the field in which it is stable (neutral toalkaline).

It is noteworthy that a relatively high amount of organic matter wasrecorded in these soils (Chapter III.5.).Also the reflections of kaolinite show much better or remain the same. Inthose samples were a decrease in intensity was observed, a fairly broad

115

peak area also occurs, which indicates poor crystallization of the mineral.These clay minerals are therefore more liable to be broken down in thecourse of treatment.The several types of clay distribution curves (Chapter III.3.1.), werecompared with the clay mineralogy of the clay fraction with specialreference to the clay pyramid, type 3. It is noted that in the soils wheresuch a distribution occurs kaolinite is the dominant clay mineral butsubordinate amounts of illite, montmorillonite and chlorite are present. Inaddition, the pH in these soils increases with depth from slightly acid toneutral or alkaline, while an increase in moisture was also noted in thefield.This environment is thought favourable for the neo-formation of the 2 : 1lattice clay minerals directly from the primary minerals as suggested byMillot (1972).A subsequent increase in the clay percentage may therefore be anti-cipated.In conclusion it appears that kaolinite with subordinate amounts of illiteare the dominant clay minerals in the granitic region, whereas montmoril-lonite and related 2 : 1 clay minerals are dominant in the doleritic area.However, montmorillonite, chlorite, vermiculite and mixed-layer mineralsmay be present in the subsoils of some granitic Alfisols if conditionsrequired for their formation are met.Further elaborations concerning the genesis of the various clay minerals isgiven in Chapter V.

III.5. Organic matterThe soils have a low organic carbon and nitrogen content, the values rangebetween 0.15% and 1.84% organic carbon and between 0.02% and 0.25%nitrogen. If a correction factor of 1.3 for the organic carbon is applied, asis suggested by American workers since with the wet Walkley-Blackmethod only 77% of the carbon is being oxidized, the organic carbon datarange between 0.2 and 2.39%. Although some doubt may exist as to thejustification of this correction factor for semi-arid soils (Harmse, 1967),it has been used in this study to comply with the criteria for the USDAclassification.The mean organic carbon content for 63 topsoil samples (average depth42 cm) is 0.65% and the nitrogen percentage 0.06. Both values decreaseconsiderably with depth in all soils except in the Vertisols, where mulch-ing and churning provide a fairly regular distribution throughout theprofile.When the soils are grouped as outlined in Chapter II, the following pictureis obtained concerning the organic matter.

116

Table 32. Organic matter in soils.

Soil Organic carbon % Nitrogen %

mean range mean range

C/N

range

VertisolInceptisolEntisol(acid)Entisol(basic)Alfisol(ultic)Alfisol(oxic, typic)Alfisol

0.630.420.27

1.14

0.39

0.33

0.59(hydromorphic)Alfisol(Rhodustalf)

0.74

0.43-0.830.33-0.550.15-0.35

0.94-1.28

0.16-0.48

0.08-0.76

0.30-1.84

0.37-1.26

0.100.050.03

0.19

0.04

0.05

0.06

0.09

0.07-0.120.03-0.060.02-0.04

0.10-0.25

0.03-0.07

0.02-0.08

0.04-0.17

0.05-0.14

7(10)

( 8)

7

( 9)

( 8)

(10)

8

6 - 78-118 - 9

5 - 9

6-10

7 - 9

8-12

8 - 9

The highest organic carbon contents occur in soils derived from basicparent material, followed by red Alfisols and those with hydromorphiccharacteristics. The percentage nitrogen corresponds with this trend.Extremely low figures for the obtained organic matter correspond withthe Entisols, the Inceptisols and most of the other Alfisols.Taking into account the analytical error in the nitrogen determination, theC/N quotients are placed between brackets if the nitrogen percentage isbelow 0.05%. Comparable data on organic matter for soils in SouthernAfrica were made available by Harmse (1967); they correspond wellwith our information. The variations in organic matter content are in firstinstance ascribed to the different types of vegetation as outlined inChapter 1.4.

The Vertisols mainly support Acacia species, while the Entisols, Incepti-sols and Ultic Alfisols often carry deciduous Terminalia and Combretumspecies. The remaining Alfisols support dominantly a Mixed Acacia Wood-land vegetation. In addition, the grass cover, being a most importantsource of humus, is not identical on the different soils and may even beabsent; see also Chapter 1.4.Thus the occurrence and nature of the organic litter is variable anddepends largely on the interrelationship between the soil and the vegeta-tion. The amount of organic matter is small when compared with areas inthe humid temperate areas and the wet tropics (Kononova, 1961; Mohrand Van Baren, 1959).

117

Of the soil climatic conditions reduced and seasonally bound soil moistureis the most severe limiting factor for micro-biological activity.Taking into account the distinct dry and wet seasons, as well as the dryspells within the rainy season, the processes that affect the raw organicmaterial may be considered as follows (Harmse, 1961; Finck, 1963; Pauli,1964).During the dry season the soil surface temperatures are very high and apart of the organic material is indeed burned (loss of NH3, H2O andCO2).Deeper down where the soil temperatures are somewhat lower and somemoisture may still be available, mineralization (chemical weathering) ofthe (fragmented) organic matter on a restricted scale may take place, eventhough little or no microbiological activity is anticipated in the topsoil.The activity of micro-organisms, which cause the litter to be broken downinto elemental compounds, is greatly accelerated by improved moistureconditions during the rainy season. At the start of the dry period,however, microbiological activity is suppressed, while the disorganizationof the enzymatic systems cause a more intensive humification to takeplace while the soil dries out (Harmsen, 1959; Kononova, 1961). Thepresence of humus could be verified in thin sections thanks to theoccurrence of very dark brown to black coloured soil material. The brown-ing of topsoils is presumably caused by the coating of mineral grains byorganic matter.A third process deserves attention: the fragmentation of organic matter.This may be caused by physical breakdown and/or by soil animals. Theresult of this activity (especially of mites) is seen in thin sections inparticular. These Orbitatid mites, are also an indication for the poorquality of the raw organic material and/or the poor conditions underwhich it is formed (Bal, 1970). They prepare mainly the way for betterhumification at a later stage.

III.6. SalinityThe presence of salts in the soils of eastern Botswana giving rise tosaline/alkali conditions is not a common feature. Extensive saline areas arehowever encountered in the central northern part of the country, notablythe Makgadikgadi depression.In our soils the EC5 of the 1 : 5 waterextract was determined to indicatepossible salts. Solubles were determined if the value was 0.1 mmhos/cm orhigher.This condition was met for the Vertisols, most of the hydromorphicAlfisols and the basic Entisols.Calcium, sodium, chloride and sulphate appear to be the main ions. The

118

total amount of solubles present in most soils is low as is indicated by thesum of the cations and anions, that vary respectively from 0.3 — 5.1 and0.2-4.9 meq/100g.In addition the ECS of the 1 : 5 waterextract does not rise above1.0 mmhos/cm for all 41 samples analysed.In fact, in three soils only, an ECS value of 0.5 or higher was measured.Determination of the solubles carried out on the saturation extract ofthese soils are given in table 33.

ible 33. Electric conductivity of the saturation extract, ESP and soluble (meq/1 )

mple

.71

.72

.73

.74

.93

.94

.95

.78

!.79,80

Depth

6 9 - 9191-127

127-158158-180

6 4 - 9797-127

127-170

6 7 - 9696-142

142-190

Ecmmhos/

cm.

3.43.43.03.8

5.05.85.8

3.13.33.9

ESP

21

232627

4

76

97

10

Ca

1.91.6

1.01.2

16.515.016.0

3.53.55.0

CationsMg

1.21.0

0.91.1

17.217.2

18.9

3.33.74.5

K

0.40.4

0.50.6

0.70.70.8

0.4

0.50.5

Na

34.336.135.237.8

29.637.436.1

24.327.032.2

HCO3/

co3

1.6

2.52.74.5

4.33.43.6

3.63.22.3

Anions

Cl

31.034.7

33.732.6

55.560.459.8

21.424.830.4

SO4

0.3

0.50.4

0.3

2.74.54.3

5.65.4

6.3

The subsoil of profile 15 (Typic Natraqualf) is classified as alkali (ECe < 4and ESP > 15).The subsoil of profile 22 (Typic Pellustert) is saline (ECe > 4 andESP < 15), while the salinity and alkalinity as expressed in the ECe andESP value are not sufficiently high in profile 25 (Typic Pellustert) tosatisfy either definition (see also USSLS, 1954).The sodium chloride salinization as encountered in profiles 15 and 25accompanies a high amount of sodium on the exchange complex of theNatraqualf because of very low Ca, Mg and K contents.In the topomorphic Vertisol of the Bonwapitse flood-plain, profile 25, Caand Mg carbonates are encountered in addition to the chlorides and somesulphates. A similar trend is expressed in the Vertisols closer to theShoshong Hills. The data confirm the varying degree of salinity, whichchanges considerably from one locality to another.The source of the solubles in case of the Vertisols is correlated with thedoleritic rock and its weathering products, such as calcrete.The presence of sodium is partly accounted for as a weathering product ofNa bearing feldspars such as albite (Leneuf, 1959).However the accumulation of chloride solely from a local source seems

119

unlikely, as the amount of chloride in rocks in the area is presumably toolow to account for the amount encountered.An additional allochthone origin is therefore feasible. A second possiblesource of chlorides is the extensive depression located 100 — 150 milesnorth to northwest of the survey area. According to Bawden (1965) NaClis the dominant salt of these former lake bottoms.Salt particles taken up during the dry season and carried by the dominantnortherly to northwesterly winds may subsequently be deposited in south-ern regions.Thereupon carried in solution to low-lying areas, the accumulation of thesodium chlorides causes the saline/alkali conditions as met for example inprofile 15. In granitic areas in eastern Botswana saline/alkali soils aremerely restricted to the bottomlands. Their occurrence is patchy. Soilerosion often causes the exposure of the columnar natric B horizons, afeature also reported by Harmse in South Africa (1967).Summarizing a sodium chloride salinization seems dominant in the gran-itic area of the survey region, whereas carbonates, and to a lesser extentsulphates together with chlorides, form the major solubles in the doleriticregion.

120

CHAPTER IV

ARRANGEMENT OF SOIL CONSTITUENTS

[V.l. Micromorphological observations

For the descriptions of the samples the terminology from Brewer [1964)is used, although in some cases deviations from that system were made iffound desirable. The arrangement of the soil constituents was studied in20 mammoth-sized ( 8 x 1 5 cm) thin sections made from undisturbedsamples. The sample sites were selected according to macromorpholoçicalinformation obtained in the field. A comparison between this informationand the micromorphological data is given in section IV.2. A continuousseries of samples was not taken and in some profiles only one sample wascollected to represent a particular phenomenon such as clay illuviation,mottling, or animal activity.The descriptive information relative to the thin sections can be found intable 34; remarks on the possible genesis of the soil material is tfiven in thefollowing paragraphs. The soil classification as outlined in Chapter 11 isemployed to designate the soils.

From a Vertisol. profile 25, three samples were collected, rcspectivelvfrom the Al 3 (44-58), the ACca (74-88 cm) and from (109-123 cm)'.The pedality is visible as subangular peds of about 300 micron, size andgrade of the peds decrease with depth. Plasma separations around the pedsand occasional voids are the main cause for their detection. The develop-ment of the void patterns seems to be related to a great extent to the swelland shrink properties of the soils and is especially clear in the upper partof the soil.The related, distribution of the plasma with reference to the skeletongrains is porphyroskelic.Considerable difference in the concentration of skeleton crains howeverdoes occur in isotubules, in which the skeleton grains and the plasma arenot aggregated. In addition, a large amount of coarse material is encoun-

DC? o f

tered in aggrotubules. while in the deep subsoil a granotubule is observedwherein the skeleton grains are free of plasma, or where the plasma occursin the form of pedological features.The reason for the local concentration of the grains in these tubules is noteasily explained, but may be related to the action of animals (termites)which consume the finer soil constituents but could not manage the largergrains. The plasma fabric of the Vertisol is expressed in the upper part of

121

lig. 34. Mascpic skehepic and hisepic plasmic fabric and stress cutans, 't'y pic Pelhtstert (profile 25,depth 76 cm). 'Hun section wnii'r crossed polarizers. 100 x.

I:ig. 35. Skehepic plasmic fabric, Typic Pellustert (profile 25. depth 56 cm). 'Fliin section undercrowd polarizers, 100 x.

122

the solum as plasma separations, called plasma reorientations by Slageret al. (1970), see also Stephen (1960), with striated orientation that occurin isolated patches or islands (insepic plasmic fabric). In the lower part ofthe soil plasma separations occur also as zones within the s-matrix (masepicplasmic fabric), (fig. 34).Skelsepic plasmic fabric occurs throughout the soil and may be inducedby the shearing of the soil material, especially between skeleton grains(fig. 35). The boundary of the plasma separation in this case is gradual, incontrast to the transition such as occurs around embedded grain cutans.The occurrence of neocutans along voids is also caused by stress.Vosepic plasmic fabric such as observed in the Vertisols of the Sudan(Blokhuis et al., 1970; Buursink, 1971) is very limited which is partlyascribed to restricted action of the swell and shrink.Important pedological features are pedorelicts. They have a light browncolour and are densely packed, their boundary with the s-matrix is sharp,but may be gradual in some cases. The latter is often associated with adecomposition of the pedorelict. The relict may sometimes be envelopedby a compound cutan consisting of an argillan, an organo-argillan and askeletan. The presence of these pedorelicts, noteworthy in the top 90 cmof the soil, indicates: a) admixture of foreign soil material in the soiland/or b) former soil processes leading to their formation. Both arepossible according to the physiographic position of the soil (Chapter II)and the changing climatic conditions (Chapter I).Movement of plasma from the s-matrix is thought to be the cause of thedevelopment of neoskeletans. Illuviation of plasma is seen sporadically asvery thin void argillans in the A13 horizon. The occurrence of embeddedgrain cutans in the lower part of the soil brings up the question of theorigin of the cutanic material and/or the enveloped grain. It is notexcluded that its formation can also be derived from an eroded skelsepicpart of the matrix, whose present form was initiated by the movement ofthe soil material.Carbonate nodules with a crystic fabric occur throughout the soil butincrease in amount with depth. They are similar to the ones described byBlokhuis et al., (1968) but their occurrence is linked to other processesrather than released from the calcareous subsoil by churning, as thisprocess is not considered to be very active.The origin of these nodules is associated to: 1) an allochthone source, forexample exposed calcrete, whose weathering products were in due coursetransported into the soil material, 2) to pedogenetic processes causing thenodules to form out of a solution enriched by released carbonates fromthe soil matrix.Biologic activity in the Vertisol is very intense in comparison to forexample similar soils in the Sudan (Buursink, 1971) and occurs in a much

123

larger degree than was anticipated from field observations. There areindications that voids and channels are transformed into tubules throughanimal activity as single and welded faecal pellets are concentrated inthem (Ghitulescu, Stoops, 1970). The shape, size and the occurrence ofthese pellets testifies to the presence of Oribatid mites and possibly verysmall earthworm species (or termites), (Bal, 1970, 1973). The formerseem to consume organic matter as well as mineral matter, a feature notknown hitherto. The faecal pellets of the earthworms are about 200—400micron. The pellets are often concentrated in tubules, but are not neces-sarily restricted to them.The amount of organic matter is remarkably high throughout the solumand was found to be fairly evenly distributed, although the greatestamount occurs in the topsoil, which is in agreement with the chemicalanalyses. The distribution pattern of the organic material is ascribed to theprocess of churning as well as animal activity.

The calcareous subsoil of a Vertisol was studied in a sample taken fromprofile 24 at the boundary of the ACca and the C-horizon (69—83 cm).The plasmic fabric is dominated by plasma separations that occur as amosaic (mosepic plasmic fabric), secondly skelsepic plasmic fabric occurs(fig. 36).The occurrence of rounded pedorelicts, presumably existing out of ses-quioxides, is again ascribed to the admixture of allochthone materialand/or former soil forming processes. The relicts have a similar appearanceas the ones described in profile 25, but are more red in colour. Inaddition, some of these nodules do not contain plasma and they may beenveloped by a ferri-argillan. In general the pedorelicts have a higheramount of skeleton grains and are poorer in organic matter than thesurrounding s-matrix. The plasmic fabric is described as silasepic. Car-bonate nodules with a crystic fabric occur increasingly with depth, theyare rounded and do not seem to have been derived from weatheredcalcrete rock formed in situ, thus enhancing the hypothesis relative totheir origin as proposed for their occurrence in profile 25.

The Udic Rhodustalfs are represented by profile 23 and the Typic Rho-dustalfs by the subsoil of profile 10. Three samples were taken from thefollowing horizons of profile 23, the Al (1 — 15 cm), the B2t (51—65 cm)and theB23 (89-103 cm).The pedality of the soil material is variably expressed, usually as small(100—200 micron) weakly developed peds in the upper part of the solum.In the B23 horizon some larger peds were observed (fig. 37). The relateddistribution of plasma with reference to the skeleton grains is uniformthroughout the soil, however, concentration of skeleton grains is occasion-

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Plate I.I'dic Rhoditstalf,profile 23.

Plate 11. Iron stained material of the C horizon from profile 7, also showing compound channelferri-argillans, depth 72 cm. Thin section under crossed polarizers. 48 x.

Plate Hi. Graded compound cutan, Typic Natraqualf (profile 15, depth 58 cm). Thin section undercrossed polarizers. 120 x.

Plate IV. Skelinsepic plasmic fabric, compound void ferri-argillans and papules. "Aquic"Haplustalf(profile 4, depth 115 cm). Thin section under crossed polarizers. 120 x.

Hg. 36. Skelsepic plasmic fabric with occasional embedded grain cutans, Typic Chromustert(profile 24, depth 73 cm). Titin section under crossed polarizers- 100 x.

Fig. 37. Peds in an Udic Rhodustaif (profile 23, depth 97 cm). Thin section under crossedpolarizers. 100 x.

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ally observed in aggrorubulcs. In the topsoil there are virtually no plasmaseparations but anisotropic domains that arc not orientated in regard toeach other; the matrix has a flecked distinction pattern, resembling thesilasepic plasmic fabric as described by Brewer (1964). In the subsoilplasma separations in zones are more common, giving rise to masepic andrelated kinds of plasmic fabric. It is ot interest to no re that in theB23 horizon zones occur in which a small random crack pattern (crazeplanes) is developed, thus giving rise to extremely fine (less than 50 mi-cron) peds. Evidence ot clay illuviation, seen as void argillans, is limitedand mainly restricted to the B2t horizon. The cutans are otten discon-tinous, slightly adhesive and constitute only a subordinate amount ofplasma. Embedded grain cutans are more common. Closer observationreveals that a gradual transition to the s-matrix occurs occasionally.In this case the cutanic material is relatively thick and may be deformedto a varying degree, which may be the cause of a gradual transition of thecircumference to the s-matrix, thus causing it to resemble a skelscpicplasmic fabric.Noteworthy are also neoskeletans along channels and vughs; these areprobably formed by a loss of plasma from the matrix which was sub-sequently deposited elsewhere. An increase in plasma may also be derivedfrom tiie local weathering of minerals in the soil giving rise to skelscpicfabric and ultimately to the formation of papules. These arc occasionallyobserved in the B23 horizon.Sesquioxidic nodules are mainly restricted to the B2t horizon; theirboundaries vary from sharp to diffuse. The nodules are in general denselypacked and have a pronounced porphyroskelic internal fabric. In thereddish brown s-matrix of some nodules, fillings of plasma may beencountered. In this case the internal fabric of the nodules is similar to theone of the s-matrix. The occurrence of the plasma fillings points to aperiod in which the movement of colloidal material was stronger.The amount of organic matter is relatively high in the Al and is well distri-buted, the amount clearly decreasing with depth. Evidence o^ animalactivity is greatest in the topsoil but still considerable deeper down in thesoil. This can be deduced from the occurrence of single and welded faecalpellets (up to 300 micron in diameter), which are mainly produced bymites and earthworms. The pellets are found in varying degrees of decom-position and are finally taken up in the soil matrix. Their occurrence ismainly restricted to the aggrotubules.The transition in a Typic Rhodustalf (profile 10), from the dark reddishbrown sandy clay loam to the highly calcareous C-horizon, is demon-strated in thin section taken at a depth of 84—98 cm. The amount ofplasma decreases considerably with depth in the area of study, but theamount of carbonate nodules increases. Plasma is accounted for as free

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grain argillans but also occurs frequently as embedded grain argillans.Some scsquioxidic nodules occur and the related distribution of theplasma with referencc to the skeleton grains in these nodules is por-phyroskelic. They are characterized by their reddish colour. The car-bonate nodules have a fine to coarse crystic fabric and contain consider-ably less skeleton grains than the surrounding s-matrix. The nodules aremainly sub-angular. In addition calcium carbonate may be present incal cans.The animal activity is clearly noticeable because ot the occurrence ofsingle and welded faecal pellets and the result of mining, mainly concen-trated in aggrotubules. Some voids may have been developed throughsolution of the calcrete, tor winch the term "solution voids" is proposed.The micromorphology of some Ultic Hap lu s tal f s was investigated bymeans of a number of thin sections from profiles 2) . 14 and 2. The firstbelongs to the sandy family and the last two to the coarse loamy family.Samples from profile 21 were taken from the B21 and the B22 horizons atrespectively 49—63 and 96 — 106 cm depth.Being ag^lomcroplasmic, the related distribution of the basic fabric of thes-matrix deviates from those generally described hitherto, thus indicatingthe influence of the soil texture on the related distribution o^ the plasmawith reference to skeleton grains. The amount of intertextic is subordinatebut relevant in this context. The plasmic fabric of the B21 is seen as isolatedpatches or islands within the dominantly flecked plasma (insepic plasmicfabric), while in the lower part of the B horizon the plasma has a fleckedorientation pattern expressed as two sets of short discontinuous plasmaseparations, usually perpendicular to each other (lattiseptic plasmic fa-bric).The presence of sesquioxidic nodules with a distinct porphyroskclic re-lated distribution and of a dark red to reddish brown colour, indicates theadmixture of foreign soil material and/or former soil processes. The size ofthese nodules may reach 0.5 cm. The second important pedological fea-ture constitutes free as well as embedded grain cutans. which are classifiedas ferri-armllans.The amount of illuviated plasma is estimated less than \% in the studyarea. In addition, a certain amount of plasma may have been derived fromthe weathering of primary minerals, notably plagioclase and orthoclase.The distinction made by Mermut et al. (1971) between proper illuviationcutans and cutans developed in situ, based on the presence of small piecesof primary minerals in the clay cutan could not be verified. The degree oforientation of the ferri-argillans is variable. In addition, their thickness canhardly be described to illuviation only, which is thought to be very limitedat present.Animal activity is accounted for by the presence of single faecal pellets,

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i:tg. 38. Detail oj d gratwtubule, Ultic Haplustalf (profile 21, depth 59 cm). Thin section under

partly crossed polarizers. 40 x.

mainly from Oribatid mites, while possible termite activity is acknow-ledged in channels and may also be testified for in aggrotubules andgranotubulcs (fig. 38). The amount of organic matter is extremely low,which is confirmed by the laboratory data.

The B3 horizon of profile 1 4 (Ultic Haplustalf. coarse loamy) was sampledfrom 68 — 80 cm. The plasmic fabric is characterized by dominantly aniso-tropic plasma with anisotropic domains that are unorientated with regardto each other and is therefore classified as argillasepic. Plasma is furtherobserved around single skeleton grains (free grain-araillans and fern-argillans), while some embedded grain ferri-argillans also occur. The pre-sence of free grain cutans is regarded as proof of illuviation of plasma,after which the enveloped grain was embedded in the s-matrix. Typical arcfurther matrans around (weathered) rock fragments, which are indicativeof mass movement of soil material, most probably associated with thenature of the rainfall. There is a great deal of animal activity observedindirectly as single and welded faecal pellets, mainly confined to aggro-tubules.

Additional information about the subsoil of a coarse loamy Ultic Hap-lustalf was obtained from a thin section prepared from a sample taken at adepth of 97-111 cm in the B23 horizon of profile 2. The arrangement ofsoil constituents is similar as described for profile 14. Moreover thepresence of sesquioxidic nodules with varying circumference is noted.

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Within the s-matrix well orientated plasma fillings are clearly recognized,indicating sufficient moisture to induce these flows (fig. 39).The dating of these nodules is subject to speculation. Considering thephysiographic position of the profile, it is assumed that they were formedduring a different stage in the soil formation and are regarded as pedo-relicts.

The natric B2g horizon and the B2g horizon from profile 1 5, a TypicNatraqualf, are the next subject of study. The samples were collectedfrom 51—67 cm and from 97—111 cm respectively.The structure expressed in individual units, showing a diameter of about1 50 to 200 micron, is better developed in the B2g horizon than in thenatric B2g. Planes, as well as reorientated plasma, outline the structuralelements. The basic fabric is complex. The plasma is marked by distinctplasma illuviation and deformation (fig. 40), random plasma separationsand outspoken plasma fillings. In addition mass illuviation occurs resultingin compound cutans.In the subsoil, reorientated plasma as well as larger amounts of plasmaresults in a dominantly mosepic plasmic fabric, in contrast to the mainlyskelscpic plasmic fabric in the upper part of the B.

l:ig. 39. Oriented plasma filling in an Ultic Haplustalf (profile 2, depth 107 cm). Thin section inplain light. WO x.

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Fig. 40. Plasma illuviation and deformation. Typte Natraqiialf (profile 15, deplh 63.5 cm). Thnsection under crossed polarizers. 40 x.

Fig. 41. Void and channel argillans and sketinsepic plasmic fabric, "Aquic" Haplustalf (profile 4.depth 11 6 cm). Thin section under crossed polarizers. 40 x.

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The amount of small skew planes increases considerably in the B2g; theirpresence corresponds with a change in the mineralogy of the clay fraction.Papules, often as large as 500 micron, are frequently encountered. Em-braced by the s-matrix, they occasionally arc deformed to such an extent,better regarded as patches of reorientated plasma.Illuviation void (fcrri-) argillans arc clearly visible, but they do not occurthroughout the whole area of study.Mass illuviation occurs as compound cutans in addition to the typementioned above. These cutans somewhat resemble the agricutans de-scribed by Jongerius (1970). but differ in the gradual zonation of thecutanic material that becomes coarser towards the s-matrix (Plate 111).This feature is similar to the graded bedding as described by Kuenen(1953) and resembles on a micro-scale the first layer in the completesequence of a turbidite, known as "graded interval" (Bouma, 1962). Thesedimentolo^ical processes that caused the development of these macrofeatures are however hard to visualize in the soil material considered. It isnot held impossible that animal activity may have played a part in thegenesis of these graded cutans. Roads et al. (1965) describe the formationof biogenic graded bedding by deposit- feeding organisms in intertidal andshallow subtidal environments. Although these conditions are ratherdifferent from the one described by this author, the fact that animalacitivity may have been an additional cause to the development of thisfeature should not be excluded.

Biological activity is considerable in the soils in which these cutans occur,in addition their physiographic position implies that sufficient moisture isavailable during some time of the year to induce the mass transport of soilmaterial.Plate 111 shows a graded compound cutan, having an argillan on top of thegraded material. However, if we assume that two processes are responsiblefor the genesis of these features, they are more appropriately classified asgraded complex cutans.Apart from the physical factors that induce slaking and subsequent massilluviation, the movement of plasma and skeleton grains in this soil is alsocaused by the peptization of the clay, due to the high amount of sodiumon the exchange complex. The deformation is more strongly developed inthe lower part of the soil, in this part more planes are also observed. Thesefeatures are correlated with the types o( clay mineral (Chapter 111.3.1.).Iliuviation contributed to the formation of free grain argillans and indi-rectly to the occurrence of embedded grain argillans.

A feature of special importance are ncoskelctans, occurring frequentlyalong voids; their plasmic fabric may be described as silasepic and cor-respond with the "flecked background" as reported by Brewer (1964).The concentration of these lighter zones caused by subsequent closing of

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i'iy. 42. Irregular sesquioxidic nodule ("pedorelict") with sharp circumphvrence, Aquic Haplustalf(profile 8, depth 91 cm). Thin section in plain light. 40 x.

I:ig. 43. Sesquioxidic nodule with gradual boundary, Aquic Haplustalf (profile 8, depth 56 cm).Triin section in plain light. 40 x.

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the voids may result in fairly large, pale coloured areas.Sesquioxidic nodules do occur and sometimes resemble the pedorelicts asfound in the Vertisols and Haplustalfs. A marked difference is howeverthat in the samples from this soil the boundaries of the nodules are oftendiffuse which could be indicative for their present (de)formation. Somemay have a weak concentric structure.

Information on the organization of soil material in some Aquic Hap-lustalfs was obtained from thin sections taken from profiles 8 and 4. Fromprofile 8 two samples were collected from the B2g horizon, at a depth of44—58 cm and 79—93 cm respectively. The increase in clay and its orien-tation causes the development of mainsepic plasmic fabric in the lowerpart of the B2g. The moister soil conditions may eventually lead to a morepronounced movement of soil constituents causing the genesis of void ar-gillans (fig. 41) as well as graded compound cutans as described for theNatraqualf, but may be deformed in the lower part of this horizon (seealso Plate IV). These cutans are more abundant in the upper part of theB2g. According to Brewer (1968) clay illuviation plays a subordinate rolein particle size differentiation in soil profiles; the formation of clay in sitybeing foremost the cause for an increase in this grain size.Sesquioxidic nodules are red to reddish brown and may have sharp togradual boundaries (fig. 42, 43). Some may contain flow structures. Feld-spatic weathering occasionally gives rise to the formation of skelsepicplasmic fabric.Especially in the lower part of the soil, pale coloured areas occur, whichmay be caused by the reduction of iron compounds and its subsequentremoval from the matrix. These areas are most evident along voids. Thereis a great deal of animal activity in the soil mainly concentrated inaggrotubules (up to 1 cm long). Single and welded faecal pellets as well asmining are caused by the action of mites, earthworms and termites. Theiractivity is also widespread in the subsoil as confirmed by field observa-tions.Additional information from an "Aquic" Haplustalf (profile 4) does notdeviate widely from those obtained from profile 8. The plasmic fabric isbetter expressed and animal activity is less intense in the upper 90 cm ofprofile 4. Carbonate nodules with a crystic fabric occur in the deepsubsoil.

An example of weathered granitic rock, saprock according to Bisdom(1967), was taken from profile 7 (60—74 cm). The original rock structureis only present in some rock fragments, which are also weathered to aconsiderable degree.In its totality the material has a porphyroskelic related distribution, voids

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occur as planar voids and channels. Weathering of the rock is evident inthe (micro-) cracks and along joints. The planar voids may contain avariety of material such as a) breccia-like material, b) compound ferri-argillans, c) yellowish void argillans or, d) they may be empty. Yellowishcutanic material is often observed on top of compound ferri-argillans andmay indicate the last stage of clay illuviation (Plate II).X-ray analyses indicate the dominance of kaolinite with subordinateamounts of illite as minerals of the clay fraction. Heavy mineral composi-tion denotes a large amount of zircon (55%) and amphibole (33%) withsome tourmaline (8%), epidote (7%) and staurolite (2%), while orthoclasedominates the light mineral fraction. The weathering of the feldspar issimilar to that outlined by Bisdom (1967); the significance of the (micro)cracks is evident.In comparison to the overlying B3, the fine earth of the C has a markedincrease in total iron from 1.7 to 5.9%, while total SiO2 decreases from85.8% to 78.0%.The amount of A.I2O3 increases from 9 to 11%. A similar trend isobserved in the elemental composition of the clay fraction for Fe2 03 andSiO2, while Al2 O3 stays fairly constant.Iron stains are clearly visible in cracks and joints of the mineral fragmentsand in the breccia-like material between the rock and the mineral frag-ments and in plasma fillings (fig. 44). The related distribution of thes-matrix closely resembles some of those encountered in sesquioxidicnodules.The amount of iron in a number of plasma fillings was measured byelectron micro probe. Areas for selective countings were chosen on basisof colour, as seen in thin section (see also LaFleur, 1970). Material with avery dark colour was found to contain about 30 to 35% of iron and thesomewhat browner material between 25 to 30% of iron. Countings werealso conducted on a sesquioxidic nodule in soil material, results weresimilar; in addition, yellowish material was found to contain about 5%iron.An X-ray scanning picture of a plasma filling in the C-horizon of thegranite shows the alternating lighter and darker areas (lighter colouredareas contain more iron), (Fig. 45.) Microscopic investigation of the thinsection implies the darker material to consist mainly ofJiematite, whilethe lighter coloured areas contain more hydrated iron compounds, such asgoethite. No binding comments on the relation between the kind of ironand the percentage as found can be given, as no measurements wereconducted on the other elements.The occurrence of well-orientated plasma fillings points to a moister soilregime, in which these flow structures could form.In the upper part of the study area (60-65 cm), pedological features

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lig. 44. Iron stained plasma filling in weathered granite, C horizon oj profile 7 (L'ltic Haplitstalf,dvpth 12 cm). Thin HTtion in plain light. 100 x.

lig. 45. X-ray scanning picture of iron distribution in a plasma filling oj weathered granite, detailof fig. 44, (Profile 7. depth 72 cm). 750 x.

include aggrotubules with organic matter, compound papules, skeletansand void argillans. According to Schmidt-Lorenz (1971 and personalcommunication), the material has been subject to laterization, althoughthere is no evidence that this process is active at present.

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IV.2. Evaluation ot the Held data

The description of the soils in the field is supplemented by the micro-morphological data. Macroscopical observations are limited by the meansused and often deviate considerably from the information obtained tromthe study of thin sections. A tew examples arc mentioned to illustratethese differences.The pedality. described as "the physical constitution of a soil material asexpressed by the size, shape and arrangement of peds" (Brewer, 1964)accommodates the concept of the USDA Soil Survey Manual (1951). Inthis handbook peds are described as part oi the soil structure defined as"the aggregation of primary soil particles into compound particles, orclusters of primary particles, which are separated trom adjoining aggre-gates by surfaces of weakness". Many soils arc described as structureless orapedal (massive-coherent soil material, single grained-non coherent soilmaterial), however there are some remarkable exceptions (fig. 46). In thinsection however pedality is often visible as small natural aggregates out-

Fig. 46. Coarse columnar structure in profile 15 {Typic Natraqualf).

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lined by plasma reorientations or is less often indicated by voids. Therelated distribution of. the plasma with reference to skeleton grains isdifficult to assess in the field, although the textural composition of thesoils is readily obtained. This may aid to evaluate the amount of skeletongrains in relation to the amount of plasma and, as such, points to the typeof related distribution to be expected (Brewer et al., 1969).The void pattern as observed in the field comprises biogenic pores andcracks; only macrovoids (larger than 75 micron) can be detected orimplied by the presence of pores. Most of these were classified as tubular(FAO, 1966). A significant aspect of the microscopical investigationsappeared to be the activity of animals which are responsible for a largeamount of pores or their deformation.The plasmic fabric does not lend itself to field description with the meansat hand. Related stress-induced features such as vosepic and masepicplasmic fabrics may be partly deduced from the presence of stressphenomena (slickensides, pressure faces), and to a certain extent by theilluviation of plasma. The presence of smooth ped surfaces does notnecessarily imply clay illuviation (Reynders, 1972). Such surfaces as wellas "cutans" were observed in the field around peds, grains and in voids.Micromorphological investigation of the thin sections revealed the limitedoccurrence of true illuviation cutans, although embedded grain cutanswere readily observed, especially in most granitic soils. The majority ofdistinct ferri-argillans is confined to the B-horizons of most Aquic Hap-lustalfs.Reviewing the plasmic fabric of some oxic and argillic horizons, Bennemaet al. (1970) conclude that there is no close relationship between theplasmic fabric and the morphology of these B-horizons. Our observationsindicate also that plasmic fabric may be expressed differently in thevarious argillic horizons.In the majority of cases the appearance of mottles could be correlated,after microscopical investigation, with the presence of sesquioxidic no-dules with diffuse boundaries. In addition the mottling may be attributedto the presence of "pedorelicts" in some soils where no hydromorphicconditions are anticipated.Animal activity was observed in the field as related to millipedes andtermites. This picture was elaborated largely by thin section examinationsand indicated the presence of mites and earthworms as well. The biolog-ical activity is far greater than was to be expected from field observationsand contributes to a higher evaluation of the soils. In addition, thepresence of tubules widess the concept of homogenization as described byHoeksema (1953) and may also be valid for soils in semi-arid areas. Thisprocess may eventually be responsible for the fairly uniform grain sizedistribution, as for example in profile 8.

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Table 34: Micromorpliological description

Profile Classification Horizon Depth Related distribution of Plasmic Fabricplasma with referenceto skeleton grains

25 Typic Pellus- A13 44-58 porphyroskclictert

insepic with some mase]and skelsepic

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AC c a 74-88 porphyroskelic skelmainsepic

ACca 109-123 porphyroskelic skelmainsepic

24 Typic Chro-mustert

ACca/C 69-83 porphyroskelic skelmainsepic

23 Udic Rhodus- Al 1-15 porphyroskelictalf

silasepic with some inse

B 2 t 51-65 porphyroskelic insepic intergrading tobimasepic and molattisC

B23 89-103 porphyroskelic insepic with some latti-silasepic

10 Typic Rhodus- B3/C 84-98talf

porphyroskelic inter- skelmainsepic with somgrading to agglomero- lattisepicplasmic

21 Ultic Haplus- B21talf (sandy)

49-63 agglomeroplasmic with insepic with some lattissome porphyroskelic orintertextic

B22 92-106 agglomeroplasmic lattisepic

general

Pedological features

biogenic activity-

Remarks

some carbonate nodules with crystic common welded and single faecalfabric; occasional thin void argillans; pellets; few isotubules, somefew skeletans and occasional aggrotubulespedorelicts

common carbonate nodules with few single and welded faecalcrystic fabric; some pedorelicts; few pellets; few aggrotubulesneoskeletans; occasional neoargillan

common distinct subangularpeds of about 300 micron;much organic matter; somemeta planes and skewplanes

as above

stress argillans; occasional neo-skeletan and calcans; some embed-ded grain cutans; many carbonatenodules with crystic fabric

common single and weldedfaecal pellets; few granotubulesand few aggrotubules

few fairly distinct peds ofabout 150 micron; commoncalcrete fragments

abundant carbonate nodules withvarying crystic fabric; roundedpedorelicts; occasional compoundcutans, papules and embedded graincutans; common lithorelicts

occasional embedded grain cutansand ped cutans

few faint void argillans; someembedded grain argillans and pedcutans; some channel ferri-argillansand some sesquioxidic nodules;

some ped cutans and embeddedgrain cutans; some channel ferri-argillans; occasional papules andneoskeletans

common single and weldedfaecal pellets; few aggrotubules

some single and welded faecalpellets; few aggrotubules

some single faecal pellets;common aggrotubules

faint peds of about 100micron; much organicmaterial; common vughsand channels

fairly distinct peds ofabout 100-150 micron

moderate peds of about200 micron, occasionally1200 micron; commonsmall voids; some crazeplanes

some sesquioxidic nodules; common some single faecal pellets;embedded grain cutans; some free common aggrotubulesgrain cutans; occasional calcan;abundant carbonate nodules withcrystic fabric, slightly rounded

some peds of about 1 50 micron;some simple and compoundpacking voids; common calcretefragments

occasional embedded grain cutans;common free grain ferri-argillans;some sequioxidic nodules

some sesquioxidic nodules; somefree and embedded grain ferri-argillans; some papules

few single faecal pellets; someaggrotubules, occasionalisotubules and granotubules

occasional single faecal pellets;few aggrotubules

weak peds of 75-100 micron;common compound and singlepacking voids; some orthovughsand channels

common single and compoundpacking voids; some orthovughs

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Table 34 (continued)

Profile Classification Horizon Depth Related distribution of Plasmic Fabricplasma with reference toskeleton grains

14 Ultic Haplus- B3 68-80talf (coarseloamy)

agglomeroplasmic with:rr argillasepicsome porphyroskelic

Ultic Haplus- B23 97-111 porphyroskelictalf (coarseloamy)

argillasepic

15 Typic Natra- B2g 51-65qualf

porphyroskelic with skelsepic with vosepicsome agglomeroplasmic and insepic

B2g 97-111 agglomeroplasmic with mosepic with some skel-some porphyroskelic in-and vosepic

Aquic Haplus- B2gtalf

44-59 porphyroskelic inter-grading to agglomero-plasmic

argillasepic with somemasepic

B2g 80-95 porphyroskelic mainsepic, some silasepii

"Aquic" Haplus- B22 62-76talf

porphyroskelic mainsepic with locallysome skelsepic

B2g 81-95 porphyroskelic bimasepic with some latand skelsepic

B2ca/C 104-118 porphyroskelic bimasepic, some latti- arskelinsepic

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Pedological features Remarks

general biogenic activity

some sesquioxidic nodules; many many single and welded faecalfree and some embedded grain ferri- pellets; common aggrotubulesargillans; some matrans around rock and occasional striotubulesfragments; occasional lithorelicts

common packing voids andchannels

some pedorelicts and lithorelicts;occasional free grain ferri-argillans;some void argillans; occasionalpapule and compound cutans;fillings of plasma

some aggrotubules some orthovoidsand orthochannels

many papules; common (deformed)embedded grain cutans; commonsesquioxidic nodules; commonchannel ferri-argillans; some gradedcompound cutans

some embedded grain cutans; somesesquioxidic nodules; commonpapules; some free grain argillans,some void argillans; occasional neo-skeletan

some single faecal pellets; somestriotubules and occasionalisotubules

few single faecal pellets; occa-sional striotubules

few weak peds of about 100-150micron; some planar voids andskew planes

moderate peds of about 150-200micron; some orthochannels;many skewplanes; strongilluviation and deformation

some embedded grain argillans; fewgraded compound cutans; somesesquioxidic nodules; occasionalvoid argillans

many single and welded faecalpellets; common aggrotubules

some fairly distinct angular pedsof about 100 micron

occasional compound cutans; some as abovepapules; some embedded and freegrain argillans; occasional voidargillans; some sesquioxidic nodules;occasional papules; some neo-skeletans

some orthovughs and somechannels

common embedded grain argillans;occasional papules; some sesquioxi-dic nodules; occasional channelargillans and stress argillans

some sesquioxidic nodules; occa-sional embedded grain argillans andskeletans and neo (stress) argillansand papules

common embedded grain argillansand sesquioxidic nodules; someskeletans, void and free grainargillans; common papules; somecarbonate nodules with crysticfabric

some single and welded faecalpellets; occasional aggrotubules

weak angular peds of about 100-150 micron; some orthovughs andchannels

common single and welded faecal as above with some planar voidspellets; few aggrotubules

many single and welded faecalpellets; few aggrotubules

common skewplanes, craze planesand channels; some vughs

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CHAPTER V

ROCK WEATHERING AND SOIL FORMATION

The environmental conditions in central East Botswana induced variousstages in rock weathering as well as in soil formation.Physical weathering, or the breakdown of the rock into smaller fragments,leaving the original mineral composition virtually unchanged, occurs main-ly as a result of the variation in the diurnal temperature.Chemical weathering requires the presence of water, not only to impel thechemical attack on the fresh rock, but also to carry the released com-ponents away. In addition, the wind-action and biological activity mayplay an active role in the disintergration of material.Water is a limited commodity in the pedosphere and its supply dependssolely on the rainfall. Hence most intensive chemical breakdown of min-erals and neo-formation of minerals is to be expected during the rainyseason from November to April, and is then enhanced by favourable hightemperatures.A wide range of materials is simultaneously affected by the action of theatmosphere. In the following sections the alteration of the fresh rock (R)as compared to the weathered rock (C), is dealt within section V.l., whileprocesses that are active in the soil are reviewed in part V.2.

V.l. Rock weathering

The combined physical and chemical agents induce the disintegration ofthe fresh rock.The assessibility of water in cracks and joints, the structure, the textureand the mineral composition of the rock determine mainly the rate ofweathering. Several forms of weathered granite are mentioned in Chap-ter 1.2. In addition, Pettijohn et al. (1972), remark on the presence of"granite wash" in the immediate surroundings of granitic hills, a featurealso observed in corresponding locations in the survey area.In terms of soil formation the chemical weathering of the fresh rock andthe constituents derived therefrom will be emphasized.This process comprises the transformation of minerals through a) theadsorption of water and/or hydration on the mineral surface, b) thehydrolysis of these surfaces caused by the dissociation of water in H3Oand OH~ ions and c) liberation of the ions from the mineral structure.The dissociation of the water is considerably enlarged by higher tempera-tures; sixfold by a temperature increase from 1.0° C to 30° C (Mohr et al.,1959).

147

The presence of carbon dioxide in the atmosphere and its ionization in thesurface waters caused its pH to decrease to about 5.7. This in turn makesnatural water a better solvent of minerals (Krauskopf, 1967).The behaviour of minerals exposed to such a solvent is primarily depen-dent on the build up of the crystal framework.Early research, published by Goldschmidt in 1937, on the principles ofchemical elements in minerals and rocks revealed not only that atoms andions are combined in a regular arrangement to form lattices, but moreoverthat their formation implies a sorting of elements according to their ionicradii and valency properties. For the most common elements in rocks andsoils these values are given in table 35, according to data from Loughnan(1969).

Table 35. Ionic radius, radius ratio cation I oxigen and Zlr values for some commoncations in soils and rock.

ionic radius(Â)ratio/radiuscation/oxigenZ/r ratio

Si4*

0.41

0.299.5

Al3*

0.50

0.366.0

Fe3*

0.64

0.464.7

Mg2*

0.65

0.463.0

Ti4*

0.68

0.495.9

Fe2*

0.76

0.542.7

Mn2*

0.80

0.572.5

Na*

0.95

0.681.0

Ca2*

0.99

0.712.0

K*

1.32

0.950.75

Similarity in ionic radius (r) and ionic charge (Z) may promote isomor-phous substitution, but only if the valency does not differ more than oneand the lattice structure of the mineral is suitable.The formation of rocks from silicate melts was commented on by Bowen(reviewed in Bear, 1969 by Barshad and Jackson). The crystallization ofthe various minerals caused by decreasing temperature and pressure wasfound to take place according to the degree of polymerization of the SiO2

tetrahedron, which is considered to be the basic structural unit of mostsilicate minerals (Loughnan, 1969).In general two series are recognized: 1) the discontinuous mafic series and2) the tektosilicate series (Jackson, 1969):

olivine -> pyroxene -*• hornblende -*• biotite.quartz, K-feldspar

anorthite -»• oligoclase ->• albite -^(Ca-plagioclase) (Na-plagioclase)

From left to right in the series the temperature and the basicity decrease.The stability in circumstances of 1 atm. pressure and at room temperature

148

increases if the difference with conditions at time of forming lessens. Thearrangement of the SiO4 tetrahedron, in which Al may substitute Si,becomes exceedingly complicated in the later stages of crystallization.Its substitution causes the addition of, for example, Ca2+ or Na+ toguarantee electric neutrality of the mineral.Seven groupings of the tetrahedrons are visualized, from single tetrahe-drons (neo-silicates) to the complex continuous framework of tetrahedronsin three dimensions (tekto-silicates), (Holmes, 1969). The formation ofthese clusters depend on the measure in which the O-atoms share one ormore tetrahedrons.

Some attention was given to the structural framework of the silicateminerals as it determines to a large extent the vulnerability of the mineralto weathering. Goldlich (referred to by Loughnan, 1969) found theweathering sequence for the common rock forming minerals almost thesame as the one given for the crystallization of minerals from a silicatemelt.The tighter the oxygens are packed around the cations in positions otherthan tetrahedral, the more stable the structure at low temperatures andpressure, (Barshad, 1969.) In addition, the stability of the configuration ismaintained if the sums of the positive and negative charges equate(Pauling, quoted by Loughnan, 1969). This is illustrated by the ratioradius cation/oxygen and the ionic potential (Z/r). Thus it is feasible toassume that the breakdown of the mineral structure will most likely takeplace where the weakest cation-oxygen bond occurs.The energy to induce liberation of the ions from the mineral structure issupplied if the equilibrium between the rock and its immediate environ-ment is disturbed. This may be caused by hydrolysis, causing the disrup-tion of the weakest bond in the structure and is further promoted by achange in redox potential through the oxidation of Fe2 + to Fe3+. Thereactions continue as long as released ions form a more stable system in acertain environment than the one from which they are liberated.The loss of constituents was found to follow the sequence Ca > Na > Mg> K > Si > Fe > Al. The seemingly anomalous position of K is explainedby its being readily entrapped in secondary formed minerals, such as illite(Loughnan, 1969) as the ion does not readily hydrate in water, thusdeveloping a field to weak to annex water molecules (Millot, 1970).An impression of the change in rock composition for the two major rocktypes in the area, e.g. granite and dolerite, is obtained while comparing thefresh rock specimen and its weathering products. The rock samples 87B(leucogranite) and 12B (dolerite) were taken while the C-horizons fromprofiles 7 and 13 (respectively sample Nos. 7.5 and 13.63) and profile 24(sample 24.84) were used.

149

The soils 7 and 13 are classified as Ultic Haplustalfs while the C-horizonfrom profile 24 underlies a Vertisol.Two reservations as to the validity of the results are appropriate; firstly asa result of the analyses method, iron is given as Fe2 O 3 , and secondly thefresh rock was not obtained from the same profiles as the C-horizon.For the calculation of the various oxides the procedures as outlined byKrauskopf (1967) are applied. This implies that the aluminium concentra-tion is assumed constant, as the element is considered rather inert toweathering cycli if the environmental pH is between 4.5 and 9 (Loughnan,1969). Objections against the use of TiO2 as reference oxide may beraised on the basis of the very small amounts in which this element occursin the investigated material, thus enforcing the analytical error.To arrive at the persentage loss or gain of constituents in the weatheredrock as compared to the fresh rock (R), the following steps are carriedout.

Table 36. Calculation of gains and losses during weathering

SiO2

A12O3

Fe 2 O 3

TiO2

CaOMgOK2ONa2O

P2O5

1

leuco-granite(87B)

76.410.10.80.281.912.185.402.710.18

2

weatheredrock (7.5)

77.010.7

5.80.380.170.874.230.800.08

3

changeAl = c

72.710.1

5.50.360.160.823.990.760.08

4

loss orgain

- 3 . 70

+ 4.8+ 0.08- 1.75- 1.36- 1.41-1 .95-0 .10

5

percentage lossor gain to 1

5

0+ 575+ 29- 92- 62- 26- 72- 61

Column 1 gives the composition of the fresh rock (R) and column 2 thecomposition of the weathered material (C) in weight percentages. In bothcases the analytical error and the trace element concentration are takeninto account, thus adding the totals up to .100%. Column 3 results from thecalculations of weight in grams from each oxide in the fresh rock (R) onthe assumption that Al is constant. The losses or gains of the oxides givenin column 4 are computed by substracting the values in column 3 fromcolumn 1. Column 5 gives the same loss or gain in percentage of theoriginal amount in R. In a similar way the variations from R relative to theC was calculated for the C-horizon in profile 13. The following values are

150

obtained: SiO2 : -36%; A12O3 : 0%; Fe2O3 : t275%; TiO2 : +46%; CaO:-89%; MgO: -80%; K2O: -64%; Na20: -79% and P2 OS : -72%. Firstly,it is noticed that the mobility of the macro-elements such as CaO, MgO,Na2 O is in accordance with the expectations raised above, while losses ofSiO2 are also relatively high.Secondly, the increase in Fe2 O3 is manifest in the C material of theinvestigated profiles. Its rise can hardly be explained as to be derivedsolely from the weathering in situ of Fe bearing minerals such as biotite, ofthe granite. Micromorphological observations confirm the concentrationof iron mainly as staining of the material (Chapter IV). Transport of ironfrom elsewhere as iron-rich plasma, also noticed in plasma fillings, istherefore feasible. There is no conclusive evidence, however, that the ironis derived from vertical transport through the soil as the enrichment mayas well be caused by lateral supply. If the changes in composition of theC-horizon are taken as an indication for a process, it resembles closely theabsolute accumulation of iron as described by D'Hoore (1953). Theposition of the profiles on to the fairly flat land surfaces in which drainageis restricted and fluctuating water tables (did) occur, is thought to befavourable for an accumulation of iron compounds (Magnien, 1959,Alexander et al., 1962, Sombroek, 1962, Stephens, 1971).This requires however a wetter moisture regime than is presently encoun-tered. These conditions did exist in the past (Chapter 1.2.), hence theformation of the iron enriched material is thought to be a paleo-feature.A different picture is obtained from the comparison between the doleriteand its weathered counterpart (table 37).

Table 37. Gains and Losses of dolerite

RC%

(12B)(24)loss or gain

SiO2

50.9068.4+ 80

Al2

15.11.

0

O 3

35

Fe2O3

9.84.0

- 4 6

TiO2

0.630.51+ 8

CaO

11.288.81

+ 5

MgO

8.291.87- 7 0

K

0.2.+

2O

8665313

Na2O

2.822.26+ 8

P;

0.0.+

lOs

101258

The position of this profile differs considerably from the granitic ones asit is located at the bottom of the Shoshong Hills and, as such, subject toappreciable movement of constituents.The breakdown of minerals rich in bases led to a transport of Ca, Mg, Feand Al. The tekto-silikates (Na and K-feldspars mainly) weather less easily,which result in a relative enrichment of Si, Na and K. The encounteredweathering products i.e. calcrete and montmorillonitic type of clay mine-rals confirm these assumptions.

151

V.2. Soil Formation

The formation of soil is induced by factors such as climate, parentmaterial, biological activity and topography, which instigate the soil-forming processes (Jenny, 1941). These processes operate in the soil; thesoil constituents being the reagents.Inherent to the formation-of soils is the development of soil horizons; alayer of soil or soil material approximately parallel to the land surface anddiffering from adjacent genetically related layers. Recognition of thesehorizons and their subsequent definition led to a) a better understandingin the various processes operating in the soil, b) a basis for soil classifica-tion.The interrelationship between the various soil-forming factors is not astatic one and is not always dominated by one and the same factor, forexample climate. Thus similar soil-forming processes are not restrictedgeographically but occur where the operation conditions are fulfilled. Thisnotion stimulated genetic soil studies on a world-wide scale and lead tobetter insight in notably the controversial concepts of processes as lateri-zation and podzolization (Andriesse, 1971).The movement of soil constituents leading to the differentiation of soils isthe subject of the next section. After general remarks on the mobility ofthese constituents, the genesis of soil is approached according to thelandscape entities in which they occur.

The mobility of the components depends primarily on the presence ofwater. If this commodity is present, the constituents may go into solutionas ions, as colloids or as complex-compounds. Early research by Gartledge(1928) and Goldschmidt (1937) on the mobility of ions, was reviewed byLoughnan (1969) and Millot (1970).Ions are thus divided into three groups based on their ionic potential (Z/r)and their ionic radius (r):Group 1: the hydrated ions; these are ions with a weak ionic potential

(Z/r <3.0) such as Ca2+, Mg2+, Na+and K+, which tend to passinto true ionic solution during the weathering processes;

Group 2: the ions with an intermediate ionic potential (Z/r between 3.0and 9.5), such as Al3+, Fe3 + and Ti4+, which are precipitatedand become concentrated in the residue;

Group 3: ions with a high ionic potential (Si, P and N) may form solublecomplex anions.

The solubility of the various ions may equally be influenced by a numberof variables such as the pH and the redox potential (Eh) of the liquid-liquid and the liquid-solid phases, which are dominant in soil materials.

152

A summary on the solubility of the common cations released by silicateminerals in soils was, amongst others, given by Reynders (1964),Loughnan (1969), Millot (1970) and Segalen (1.971). Within a pH range ofabout 4.5 to about 9.0 which covers the values encountered in theinvestigated soils it appears that:Ca2+, Mg2+, Na+ and K+ are readily lost under leaching conditions; the rateof loss from the environment may be retarded for K+through fixation inthe illite structure; Fe2 + may be lossed but the rate of loss depends on theredox potential and the degree of leaching, while Si4+ is slowly lost ifadequate leaching occurs; Ti4 + is considered immobile in the TiO2 form;Fe3 + is immobile under oxidizing conditions and Al3+ is regarded virtuallyimmobile in the given pH range.Whereas the alteration of the fresh rock implies a loss of constituents, theB3 horizon may be marked by loss as well as a gain in components tovertical and/or lateral transport of materials. This fact concerning somealfisols is illustrated in table 38.

Table 38. Gains or losses of metal oxides in the B3 horizon as compared to the C.(in % of)

OxideProfile

7 (Alfisol)12 (Alfisol)32 (Alfisol)10 (Alfisol)

SiO2

+ 33+ 45- 2 5+ 3

Fe2O3

- 6 5+ 8- 7- 1 2

CaO

+ 24- 3 9- 7- 9 6

MgO

- 2 6- 2 4- 3 8_ 2

K

++_+

7481527

Molar

SiO2

A12O;

C

12.14.25.89.5

ratios

i

B3 -

16.36.25.59.8

A12O3

Fe2O3

C

2.94.54.45.7

B3

8.14.85.95.3

The variation in the SiO2 , Al2 O3 and Fe2 O3 percentages is reflected inthe SiO2 and the Al2 O3 ratios. An increase is noted for SiO2 in the B3

Al2O3 Fe2O3

for all B-horizons except profile 32 while also losses are recorded for MgOourlined above.The relation between the landscape and the soils was found to be ofsignificance in dealing with the soil-forming factors (oilier, 1959), hencespil genesis is discussed according to the locations of soils in the sampleareas. (Oilier, 1959)..In sample area A the toposequence of profile 1 to 4 is of special interest asthe soils occur on a cross-section through' the Late Tertiary pedeplain. The

153

Sample area A

Sample area B

Sample area C

Sample area D

19 20

Sample area E

Fig* 47. Schematic diagrams of soils in relation to topography.

154

soil material has been subject to intense weathering, such as can bededuced from the mineral composition. In addition leaching caused theconsiderable loss of the more mobile constituents, resulting in low CECand a low base saturation. Barridge et al. (1965). Comment that texture,drainage and topographic position mainly govern leaching, assuming thepresence of sufficient moisture. Exceptions herefrom are encountered insoils near a fresh supply of primary minerals (profile 1) or where anaccumulation of the leached constituents could take place (profile 4).The migration of clay is closely associated to the movement of water inlateral and/or vertical direction, subsequently the accumulation of clay inthe B-horizon was partly ascribed to this process. It is particularly clear inthe lower B-horizon of the "Aquic" haplustalf. Changes in the concentra-tion of some major soil constituents were indicated by comparing thecomposition of the deepest B-horizon with a topsoil, preferablethe upper B. To limit the influence from surface wash and wind erosion ordeposition, the topmost horizon was not taken. The changes were calcula-ted as outlined by Krauskopf (1967) taking Al2 O3 concentration as thereference oxide.The high concentration of MgO in the B22 of profile 1 is ascribed to itsposition, being close to the source of ferro-magnesium minerals (Chap-ter 3). The loss of Fe2 O3 may have led to subsequent enrichment of thelower B in profile 2 through lateral drainage. Overall losses in the B23 ascompared to the Bl are reported in profile 3, which is located in thecentral part of the toposequence, whereas a sharp increase in the concen-tration of more soluble elements is noted in profile 4, positioned in adrainage channel as lowest member of the sequence. In this profile, an"Aquic" Haplustalf, a change in clay mineral composition as well as anincrease in the amount of clay is encountered with depth. This presume-bly caused the relative enrichment of MgO in the B2ca horizon. Restricteddrainage provokes grey colours in the subsoil horizons of the AquicHaplustalfs (Young et al., 1965).Changes in profile 11 are similar as indicated for profile 3, apart forFe2 O3, while variations in profile 10 show the calcareous C material.

•i, > ! • • ( blockage by :

( loamy) sand •.;.;.• iron mottles s s t O n e s , gravel

P—I sandy Loam ++

+ calcareous g granit ic rock

Il l l l l l (sandy) clay Loam 6 gley d doler i te

: fSX^ (sandy) clay • drainage c ca lcrete

cracks • surface, wash sc schist en

155

Table 39. Gains and losses into

Profile

1,2,

3,4,

10,11,

* u

B21-B22B1-B3B1-B23Bl-B2caB1-B3B1-B3

= upper B

theBl)

SiO2

+ 1- 15- 2 9- 4 7- 2 6- 2 6

1 = lower

Fe2O3

- 10+ 17

0- 2- 12- 11

B

the B horizon as a

CaO

- 2 1- 6 9- 3 3+ 13+ 23- 2 0

MgO

+ 191- 45- 70+ 233- 35- 46

result of

K2O

- 5- 17- 3 9- 4 6- 4- 2 9

soil-forming

Molai

SiO2

A12O

u

7.517.313.310.313.233.8

' processes (in %

r ratios

3

1

7.514.4

9.58.09.8

25.1

Al2

Fe2

u

4.97.35.84.74.66.9

o3

o31*

5.55.95.84.55.37.3

The development of profile 10, a Rhodustalf, was found to be incidentalon the pediplain and its occurrence mainly ascribed to those processesthat led to the genesis of its counterpart profile 23.In sample area B, part of the late Tertiary erosion surface, the profiles 5and 7 are located on slightly higher topographic positions, whereas profi-les 6, 8 and 9 are situated in the lower bottom-lands and drainagechannels.It is thought that a temporary water-table could develop on the flatinterfluves under sufficient rainfall. Presently however, a surplus of wateris not observed in these areas due to high evaporation rates and decreasingprecipitation (Chapter 1.2). A perched water-table may occur nowadays inthe soils located in depressions, to which the lateral drainage is directed.This has in addition, led to a change in environment inducing the neo-formation of minerals and resulting in a higher base saturation and cationexchange capacity in these soils.The occurrence of graded compound cutans in Aquic Haplustalfs (Chap-ter IV) implies that the soil dries out sufficiently in the dry spells, to allowfor the transmittance of saturated soil material through voids duringsubsequent wetter periods.Following the procedure as applied above for the calculation of the gainsand losses for the C-horizon as compared to the upper B, we arrive at thefollowing table for the profiles 5 to 9.

156

Table 40. Gains and losses in the C or lower B horizon as compared to the Bl (in %)

Molar ratios

SiO2

A12O3

u 1

AI2O3Fe2O3

u 1*

5, B-C7, Bl-C6, B1-B38, B21-B2g9, Bl-C

10 + 5 0 - 2 1 - 5 0 +3 17.6 15.926 +219 - 3 0 + 4 6 - 2 4 10.9 9.216 + 1 0 2 + 5 - 1 2 - 1 5 10.9 9.222 + 2 4 + 3 7 + 3 0 - 18 10.7 8.3

- 4 1 34 _ 13 + 29 n.d." 10.9 6.5

5.35.95.95.06.2

3.52.92.94.04.7

* not determined** u = upper B

1 = lower B or C

As for the soils in sample area A, there is an increase in SiO2 towards thesoil surface. According to the results of Chapter 3, a rise in the concentra-tion of SiO2 is correlated with a decrease in the content of Al2 O3. Markeddifference with the better drained profiles (such as 1, 2, 3, 11) on thepediplain, is the sharp increase in the concentration of Fe2 O3 in the C-horizon of all profiles encountered. Whereas in the profiles 5 and 7 theprocesses that led to this increase are hard to visualize in the presentenvironmental conditions (see V.I), it may be envisaged in the bottomland soils. Conditions in the subsoil of these "Aquic" Haplustalfs mayinduce the release and redistribution of iron in reduced milieu during wetperiods and its dehydration and crystallization in the drier spells (vanSchuylenborgh, 1971).This has led to the occurrence of mottles in the subsoil of the investiga-ted profiles (6, 8 and 9). These mottles were identified mainly as ses-quioxidic nodules with sharp and diffuse boundaries. Similar features wereobserved by Blume et al. (1969), who remark on the accumulation of Feand Al in these mottles. A fact confirmed for Fe by electron microprobeanalysis of a "nodule". It is noteworthy that the movement of iron oxidesor hydroxides could not be verified in thin section, for example as ferrans.In the only proper Aquic Haplustalf (profile No. 8) the material from theC-horizon is richer in CaO and MgO, probably caused by lateral addition.The relative loss of K2 O in the subsoils is ascribed to leaching ot thisconstituent from these horizons.Sample area C represents the eroded part of the Tertiary plain. Reliefdifferences are greater and drainage channels more pronounced. A number

157

of soils (Nos. 12 to 15) are located on a toposequence, whereas profiles 16and 1 8 occur in associated positions. The calculated changes in elementalcomposition for the upper B and the C-horizon reveals the following:

Table 41. Cains and losses in the C or B3(s) horizon as compared to the upper B or Ahorizon (t)

Profile

14, B2-C16, B21-C12.B21-C13, B21-C15, A12-C18, B21-C15, A22-B312, B21-B3

SiO2

- 5- 33- 49- 39+ 163- 31+ 20- 2 6

Fe2O3

+ 59+ 118- 4+ 9- 62+ 20- 47- 8

CaO

_ 2+ 31+ 58- 3 6- 7 3+ 17- 70+ 4

MgO

+ 27+ 7+ 232+ 221- 17- 35- 34+ 152

K2O

- 1- 23- 44- 44+ 373- 30+ 167- 18

Molar

SiO2

A12O-

t

8.617.38.3

13.47.4

12.47.48.3

ratios

)

s

8.2

11.74.28.2

19.48.65.96.2

A12O3

Fe2O3

t s

4.915.34.95.73.05.63.04.9

3.16.84.95.37.04.85.74.5

Soils on the interfluves, such as the profiles 14 and 16, have a similarprofile development as the ones in analogous position in sample area B.Noteworthy is however the increase in MgO in the B3 or the C horizon ofprofiles 12 and 13, presumebly caused by the (lateral) supply of weather-ing products from Mg-bearing minerals, such as amphibole.The specific environment in profile 15; high pH values, anaerobic conditi-ons caused by wetness in the rainy season, may have induced the move-ment of SiO2 and FeO, although redistribution of only the latter wasobserved in thin section. The considerable increase in K2 O in the C andthe B3 horizon is partly expained by the variability in the concentrationof Al2 O3 in the fine earth, as such once more indicating the reservation tothe use of this element as a reference. For the greater part, however, thehigh amount of K-feldspar (40% of the light mineral fraction) in the C andB3 horizon is the cause for the reported enrichment.The increase in sodium has led to saline/alkali conditions, such as de-scribed in Chapter II, which led is its turn to movement of plasma.Especially in the lower gley horizons of the Typic Natraqualf there isstrong evidence for the illuviation of clay as well as its deformation. Thisis ascribed to the clay mineral composition in these sou horizons (mont-morillonite and chlorite in addition to kaolinite and illite) and to animal

158

activity. In analogy to the described wetter bottom-land soils in thegranitic region a redistribution of iron is visible as mottles in the field.This was confirmed in thin sections through the occurrence of palecoloured areas in the B2t horizon and accumulation of iron in nodules.The variations in iron context are rather similar to those observed byAhmed et al. (1962). Who found an average of 2% Fe2 O3 in bleachedlayers and about 24% Fe2O3 in mottled horizons (see also IV.1). Modifi-cation of the soil material in the form of sesquans or ferrans was howevernot noticed in the thin section.

Soil transitions in sample area D differ considerable from the hithertodescribed ones because of contrasting parent material and drainage. Aswas outlined in Chapter II, the weathering products of the sills give rise ontheir south side to the formation of soils rich in bases, whereas soils on thenorth side of these sills are in general poor in bases, for example profile21. Intensities on the element concentrations are given in table 42.

Table 42. Cains and losses in subsoil horizons (s) as compared to topsoil horizons (t)(in %)

Molar ratios

SiO2 Al2 O3

Profile SiO2 F e 2 O 3 CaO MgO K2O A12O3 A12O3

t s t s

19, Al-20, B1221.B1-B323, B21-B324, A l - C25, A11-(II)C26, A12-B2ca

+ 33- 7+ 18- 11- 2 0- 7

- 15+ 3- 1 0- 14+ 24- 7

+ 74+ 23+ 23+ 469+ 47+ 31

- 5 6+ 17n.d.+ 13+ 21- 2 5

+ 139- 15+ 28+ 30- 49- 10

5.316.5

6.511.411.6

9.5

7.115.3

7.710.2

9.38.8

2.95.13.93.96.02.8

3.55.04.44.54.93.0

The relative gain in K2 O in the subsoil of profile 20 is ascribed to the highpercentage of K-feldspar and (weathered) biotite in this horizon.The nature of the parent material represents itself in the enrichment ofCaO and MgO in most soils, where poor drainage induces the formation ofmainly smectite minerals in the profiles 24 and 25. The high amount ofCaO in the C horizon of profile 24 testifies to the presence of a partlyconsolidated CaCO3 accumulation. The development of the Rhodustalf(No. 23) is attributed to a slightly different topography, somewhat betterdrainage and different parent material. Under the prevailing soil climateiron is released from ferro-magnesium minerals through hydrolysis in the

159

wet season and leaching of the iron compounds as organo-metal comp-lexes may take place on a restricted scale in the topsoil horizons (SiO2

/Al2 O3 ratio in the A = 8.4 and in the B21 = 6.5). However due to littleorganic matter in the deeper horizons leaching is virtually stopped and theiron compounds impregnate the soil. A partial dehydration of the ironcompounds is induced by the dry season, thus resulting in a reddishcolour.The intensity of this process is mainly controlled by the duration andamount of precipitation.These toposequences of red and black soils are well known in the tropics.The clay mineral genesis of the members is governed by the prevailingpermeability and leaching conditions, as well vertical as lateral (Mohr etal. 1959, 1972; Kantor et al. 1 972).Soil formation in sample area E is complex due to contrasting parentmaterial and strong differences in relief. Schist forms the parent materialfor profile 29, the weathering products of sandstone for profile 30, meta-dolerite for profile 31, while the profiles 32 and 33 are situated in thegranitic region of the upper catchment of the Mahalapshwe River.Variations in elemental concentration for the upper and lower soil hori-zons are given in the following table:

Table 43. Gains and losses in the (A)C horizon(s) as compared to the upper B or A(t)(in %)

Profile SiO2 Fe2O3 CaO MgO K2O

Molai

SiO2

A12O

t

: ratios

3

s

Al2

Fe2

t

O 3

O3

«

29,B21-C - 6 2 - 5 2 - 4 6 +867 - 7 3 15.2 5.8 7.7 15.830, A12-C - 1 - 7 - 10 n.d. - 38 100 100 3.4 3.431.A12-AC 0 - 1 - 2 1 n.d. + 5 16.5 15.2 4.0 4.032, B21-C - 9 + 1 7 + 8 - 1 5 - 1 7 6.4 5.8 6.4 5.533, Bl-C - 1 2 +17 - 3 9 - 76 n.d. 11.9 10.5 7.1 6.0

The enrichment of MgO in the C-horizon of the developed Alfisol, profile29, is ascribed to lateral supply of weathering products from ferro-magnesium minerals and the high amount of weathered biotite. The smallchanges in the element concentration in the Entisol is inherent to itsformation. The variations of the CaO concentration in the Vertisol (31) isopposite to the one encountered in the other Vertisols (profiles 24 and

160

25). This is thought due to a) the poor supply of bases from thehinterland and b) topographic position. The relatively small changes incomposition are further ascribed to churning. Soils in the granitic areasuch as profiles 32 and 33 are similar in their development as the ones inanalogous position in sample area B and C.

The clay fraction of the soils was found of special importance (Chap-ter III). From recent reports (Proc. Int. Clay Conf. Madrid, 1972) it isevident that a common numerator has yet to be found to explain theoccurrence of the various clay minerals in soils.According to Millot (1972) among the three mechanisms originating clayminerals-heritage, transformations and neo-formation- the weathering zoneis at first characterized by neo-formation and secondarily by transforma-tion. Clay minerals in the investigated soils are considered to originatethrough the last two processes. Barshad (1969) suggests the formation of

these minerals to depend on the Al _— ,-, ratio of the medium, itsv Al2 O3 + Fe2 O3

pH and the presence of bases in solution.Thus a ratio of between 2 and 4, a pH of 7 or more with a high content ofCa and Mg in solution would induce the formation of montmorillonite;the same values but with a high amount of K in solution would produceillite and a ratio of 2 or less, a pH of 7 or less with a low concentration ofbases would give rise to the formation of kaolinite. If Mg would bedominant in the solution under the same conditions then vermiculitewould form.No actual measurements were carried out to verify this hypothesis, how-ever, an approach was made on the basis of the SiO2 /Al2 O3 Molar ratioand the quotient K2O + MgO + Na2O/Al2O3 of the clay fraction, toinvestigate the possible correlations as mentioned above. In addition thecorrelation between the SiO2 /Al2 O3 ratio and the pH-KCl was found tobe high (r = 0.84), while the one between SiO2 /R2 O3 and pH-H2 O is alsogood (r = 0.81). The findings, presented in figure 47, confirm largely thetheory of Barshad and accommodate the statements made in ChapterIII.2.3.

The SiO2/Al2O3 and the SiO2/R2O3 ratios are more than 2 in all clays.High SiO2 /R2 O3 ratios, of about 3 to 4 are encountered in the Vertisols,while lowest values are found in the Oxic Haplustalfs (ratio of about 2).The ratio decreases in all soils with depth except in the Vertisols and theTypic Natraqualf.The variation of the SiO2 /Al2 O3 ratio within and between the soils is asfollows:

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Table 44. Variation of the S1O2 /Al2 O3 Molar ratio of the clay fraction

Soil

VertisolsInceptisolsEntisols (basic)Entisols (acid)

mean

4.43.03.43.1

range

3.9-5.13.0-3.03.1-3.92.9-3.2

Soil

Oxic HaplustalfsAquic HaplustalfsRhodustalfsUltic HaplustalfsTypic NatraqualfTypic Haplustalf

mean

2.63.03.12.93.33.9

range

2.3-3.02.5-3.52.7-3.82.5-4.03.2-3.53.4-4.6

SiO,AI2°3clay

5.0

45

4X3

as

ao

2.5

20

y =4.47 x + 2.19

r =0.88

AI2°30 0.10 020 030 0.40 Q50 0.60 0.70 clay

, , . . „ „ , . , . , . . , . . , .. SiO, , K,O + MoO y Na, O ( ,, ,big. 48. Relationship between the Molar ratios . ' and —1 An — °* tne c'ay~

fraction

162

The SiO2 /Al2 O3 ratio of the clay fraction increases in the Vertisols withdepth and may increase in some Alfisols after an initial decrease. For themajority of soils however the value decreases with depth, thus confirmingthe findings relative to the elemental composition of the fine earth fraction.In an account on the importance of the use of these ratios to identifysoil-forming processes in the (sub)tropics, Schmidt-Lorenz (1971) com-ments that if these ratios are more than 2, the soil material is considered"siallitic", thus indicating that the stage of weathering is not advancedenough to deplete the amount of SiO2, which concentration is still highwhen compared to the contents of Al2O3 and Fe2 O 3 . However the fairlylow SiO2/R2O3 ratio may be indicative for a fersiallitic trend in theweathering of the soil materials, thus giving rise to an enrichment ofespecially iron in some soils. As such most of the soils developed from thecrystalline rocks of Basement Complex approach the concept of TropicalFerruginous Soils (D'Hoore, 1964; Buringn, 1970; Duchaufour, 1970).In particular the movement of SiO2 either caused by present or pastprocesses is or great significance in the tropics and requires furtherconsiderations. As was indicated in Chapter III an increase in clay withdepth is clearly noticeable in many soils and was typified as a "clay-bulge"or "clay-pyramid" distribution curve. For a number of granitic soils withsuch a clay distribution the gains and losses were calculated according tothe principles outlined above. Considered are the top of the B-horizon andthat part of the B where a maximum of clay is encountered.

Table 45. Gains and losses in the B2t horizon as compared to the B(2)l (in %)

Profile

3. Bl-B2t4. Bl-B2g

12. B21-B2t29. B21-B2t

SiO2

- 3 9- 4 4- 2 4- 3 2

Fe2O3

+ 1- 3+ 6- 7

Molar

SiO2

A12O3

B(2)l

1 3 . 3 -1 0 . 3 -

8 . 3 -1 5 . 2 -

ratios

B21

- 8.2- 8.0- 6.3- 10.3

(fine earth)

SiO2

R2O3

B(2)l

11.4 -8.5 -6.9 -

13.4 -

-7.0-6.6-5.2- 9 . 2 "

A12O3

Fe2O3

B(2)l

5.8 - 5.84.7 - 4 . 84.9 - 4 . 87.7 - 8 . 2

clay percentage

B(2)l

20.225.029.621.6

B2t,g

38.143.543.032.6

The data show a similar trend as those obtained from the topmost B andthe B3 horizons. As can be seen from the elemental composition of theclay fraction, there is little variation in the concentration of SiO2 andAl2 O3 in the considered horizons in the same soil. In addition it wasfound that the amount of SiO2 and Al2O3 changed little in the non-clay

163

fraction of the horizons in the investigated soils, for example in profile33: 83% and 83%; in profile 29: 94% versus 95%; in profile 12: 86%versus 87% and in profile 4: 94% and 96%.Taking into account that the concentration of SiO2 in the clay fraction islower than in the non-clay fraction, it is concluded that the considerableincrease in clay in the lower B causes a decrease in the amount of SiO2 in thishorizon of the Alfisols considered.

The variation in soil profile development as outlined above reveals theconsiderable influence of the parent material and the topography assoil-forming factors.Leaching governed by rainfall, topography and drainage varied under theclimatic modulations.This process was however not so intensive that it led to complete deple-tion of SiO2 and bases. Soils in intermediate positions in the graniticregion however may show a trend into this direction as is indicated by the"Oxic" and "Ultic" subgroup nominations of a number of Alfisols.Zones of accumulation are encountered in the bottom lands of thegranitic pediplain and peneplain. Soils encountered in these positions aresubject to impeded drainage, which under prevailing climatic conditionsmay lead to the redistribution of iron. The removal of constituents iscounteracted by lateral supply of bases, which in many cases provokes theneo-formation of clay minerals. In extreme conditions, the excess ofsodium in particular, may induce salinity.

Higher clay percentages in the B horizon of Alfisols are only partly causedby accumulation through clay migration. This process is limited at presentand probably was at a maximum in the past. The formation of clay in situin thought to be an important factor to the clay increase as noted in manyhydromorphic Alfisols.The presence of argillic horizons in soils that have at present a very drymoisture regime, indicates that the climatic conditions preserve ratherthan destroy these features.The pronounced dry season impels the formation of voids in the soils, adevelopment enhanced by animal activity. The mass movement of soilmaterial caused by heavy and erratic rainfall is thus made possible.The presence of iron enriched material of the C and B3 horizons ingranitic soils at flat position on the present interfluces is furthermore anindication of other conditions than encountered today.Also the release of calcium from the doleritic rock and its subsequentaccumulation at the base of the zones of departure (the doleritic hills)points to more favourable leaching conditions than met with nowadays.The poor drainage of the clay soils in the present position further lessensthe removal of this constituent from these soils.

164

The enrichment of potassium and ocasionally magnesium in some subsoilsis associated with the high amount of orthoclase and weathered biotite inthese horizons.In the sense of Ruellan (1971) many soils contain relicts, especially thosethat have known a wetter soil moisture regime.However sharp transitions between the various horizons are mainly restric-ted to the B3 en C horizons; in the remaining part of the solum gradualtransitions prevail; indicating the homogeneous and far reaching develop-ment of the soils. In general it is thought that the processes that inducedthese relicts have weakened rather than that they have stopped altho-gether or have been replaced by controversial ones.

165

CHAPTER VI.

SOIL CLASSIFICATION AND LAND USE

VI. 1. Soil classification

The approach to the classification of soils according to the absence orpresence of diagnostic horizons is based on the classical concepts on soilformation. These concepts were conceived in Russia and documented byDokuchaev in 1886 (Tiurin, 1965), while during the same period (1892)E. W. Hilgard made known similar ideas on soil formation and classificationin the United States (in Jenny, 1961).Continued research on and surveys of soils, especially in the WesternHemisphere, led to a better understanding of soil forming factors andsubsequently to improved definitions of units of various soil classificationsystems.That soil developed as a result of the interaction of a number of factors,such as climate, parent material, vegetation, topography and time, wasclearly recognized by Jenny (1941), and this synthesis was admirablyapplied by Mohr and Van Baren in their studies of soils in Indonesia(1959).The publication of the 7th Approximation in 1960 by the Soil SurveyStaff of the USDA introduced another dimension to the existing systemsof soil classification, namely clearly defined morphometric criteria toidentify the various diagnostic surface horizons (epipedons), as well as anumber of diagnostic subsurface horizons. The principles of the systemare universally applicable, thus providing a first outline for an inter-national classification system as well as procuring a basis for world-widesoil correlation (Smith, 1965). That the system is subject to critisism, notonly because of its linguistic ingenuity, but also with regard to its multipleattributes and class structure (Webster, 1968), would be foreseen. Inreviewing the system, Cline (1963) points out the genetic aspects inrelation to soil properties when he says, "soils thought to have had similargenesis are placed in the same group; the groups are defined in terms ofsoil properties . . . " and further "thus the basis on which the classes havebeen formed are genetic considerations while the criteria by means ofwhich the classes are differentiated are soil properties."Similar systems were brought out by the FAO/UNESCO (1968, 1970) inan attempt to correlate the different soil classification systems throughoutthe world in order to establish a legend for the Soil Map of the World.Regionally, the study of Van der Eyk et al. (1969) focusses attention on

167

Table 46. Soil Classification and Correlation

ProfileNo.

Order Subgroup Family FAO/UNESCC

12345

6789

1011121314151617181920212223242526

2728

2930313233

AlfisolAlfisolAlfisolAlfisolInceptisol

AlfisolAlfisolAlfisolAlfisolAlfisolAlfisolAlfisolAlfisolAlfisolAlfisolEntisolEntisolAlfisolEntisolInceptisolAlfisolVertisolAlfisolVertisolVertisolAlfisol

EntisolEntisol

AlfisolEntisolVertisolAlfisolAlfisol

Ultic HaplustalfUltic HaplustalfUltic Haplustalf"Aquic" HaplustalfLithic Ustropept

"Aquic" HaplustalfUltic HaplustalfAquic Haplustalf"Aquic" HaplustalfTypic RhodustalfOxic HaplustalfUltic HaplustalfUltic HaplustalfUltic HaplustalfTypic NatraqualfTypic UstrorthentLithic Ustipsamment"Aquic" HaplustalfLithic UstorthentUdic UstropeptUltic HaplustalfTypic PellustertUdic RhodustalfTypic ChromustertTypic PellustertTypic Haplustalf

Lithic UstorthentLithic Ustorthent

Oxic HaplustalfTypic UstipsammentEntic ChromustertOxic HaplustalfUltic Haplustalf

siliceous, fine loamysiliceous, coarse loamysiliceous, fine loamykaolinite, clayeysiliceous, coarse loamy,shallowsiliceous, fine loamysiliceous, coarse loamysilieceous, fine loamykaolinite, clayeysiliceous, fine loamysiliceous, coarse loamykalinite, clayeysiliceous, fine loamysiliceous, coarse loamysiliceous, fine loamysiliceous, sandysiliceous, sandy, shallowsiliceous, fine loamymontm., clayey, microsiliceous, coarse loamysiliceous, coarse loamymont., clayey, calcar.kaolinite, clayeymontm., clayey, calcar.montm., clayeysiliceous, fine loamycalcareous

montm., clayey, shallowmont., clayey, shallow,calcareoussiliceous, fine loamysiliceous, sandymont., clayey, calcar.kaolinite, clayeysiliceous, coarse loamy

Chromic LuvisFerric AcrisolChromic LuvisGleyic LuvisolDystric Cambi:

Gleyic LuvisolOrthic LuvisolGleyic LuvisolGleyic LuvisolChromic LuvisiOrthic AcrisolChromic LuvisiChromic LuvisiChromic LuvisiGleyic SolonetCambic ArenosDystric RegosoGleyic LuvisolLithosolChromic CambOrthic AcrisolPellic VertisolChromic Luvis(Chromic LuviscPellic VertisolChromic Luvisc

LithosolLithosol

Orthic AcrisolCambic ArenosChromic VertisFerric LuvisolFerric Luvisol

168

the application of the concept of diagnostic horizons to local conditions.Prior to the work carried out by this author this new approach had notbeen applied in Botswana.Previous soil classification systems used in the country by Van Straten(1959), Van Straten and De Beer (1959), Bawden and Stobbs (1963) andmore recently by Blair-Rains and McKay (1968) are mainly derived fromthe system developed by D'Hoore (1954). Many of the older classificationsystems are still extremely valuable although they may not do enoughjustice to the fact that the soil is an intergrated part of the biosphere.(UNESCO, 1968).In the following paragraphs the latest issue of the 7th Approximation isemployed (1970) while correlations are made with the FAO/UNESCOsystem (1968, 1970) and other soil classifications if found necessary. It isnot the intention to repeat all the differentiating criteria of the classifica-tion, but to point out some characteristics relevant to the soils. Usefulinformation relative to the agricultural use of the soils may be gathered fromthe classification on Family level. Comprised information on the classifica-tion of the studied soils is given in table 46.Four soil Orders were distinguished: Entisols, Vertisols, Inceptisols and Alfisols

Entisols are mineral soils that show little or no evidence of developmentof pedogenic horizons, other than an Ochric epipedon or Histic epipedonor an Albic or Spodic horizon with or without a salic horizon, sodiumsaturation of 15% or more, a calcic or gypsic horizon, plinthite, burieddiagnostic horizons or ironstone.The occurrence of Entisols in the area is attributed to the nature of theparent material, and not to the recent age of the material in which theyare encountered. The parent material consists mainly of quartz grainsresulting in coarse textured soils. The latter is the main criterion for thesubdivision of this Order on suborder level, dividing the very coarse texturedones (Psamments) from the coarse loamy and loamy ones without furtherdifferentiation (Orthents). Soil depth and soil moisture regime furtherdetermine classification on the lower levels.The Vertisols are mineral soils that have no lithic, paralithic contact orpetrocalcic or duripan within 50 cm of the soil surface and have:a) 30% or more clay in all subhorizons down to 50 cm or more after the

upper 18 cm are mixed,b) at some periods in most years, cracks that open at the surface and are

at least 1 cm or more wide at 50 cm depth, unless irrigated,c) gilgai and/or slickensides at some depth between 25 and 100 cm

and/or wedged-shaped natural structural aggregates with their longaxes tilted 10 to 60 degrees from the horizontal.

In the area under consideration, the Vertisols are developed from rock

169

rich in bases and have inherited most of their properties from the rockweathering products. Environmental conditions led to the formation ofdominantly montmorillonitic clay minerals.The Vertisol Order is subdivided according to a certain rhythm subject tothe opening and closing of cracks and the soil moisture regime. The soilcolour provides further divisions on the lower levels (Great Group). ThusPellusterts have chroma of less than 1.5 in some part of the matrix and ofthe upper 30 cm in more than half of each pedon. Chromusterts have amoist chroma equal or superior to 1.5. In large parts of Southern Africathese soils are classified as Subtropical Black Clay Soils (Van der Merwe,1969).Inceptisols are mineral soils with altered horizons that have lost bases oriron and aluminium but retain weatherable minerals, and that lack illuvialhorizons either enriched with silicate clays that contain aluminium, orthose enriched with amorphous mixtures of aluminium and organic car-bon.In the study area these soils have an Ochric epipedon overlying a Cambichorizon. They are classified as Tropepts on basis of the soil temperatureregime and are subdivided on a lower level on basis of the soil moistureregime and the occurrence of a lithic contact. One profile was consideredto belong to the not yet defined "Udic" subgroup in analogy to its use inthe Alfisol Order.These soils are tentatively correlated with the Grey Ferruginous Lateriticsoils as described by Van der Merwe (1969).Alfisols are mineral soils that have an argillic horizon or a natric horizonand have a base saturation by sum of cations that is 35% or more at adepth of 1.25 m below the upper boundary of the argillic horizon or1.8 m below the surface of the soil, whichever is shallower.Most Alfisols encountered are classified as Ustalfs because of their presentsoil moisture regime and the properties of the epipedon. As in other partsof the world, these soils tend to form a belt between the Aridisols on theone side, and the soils of warm humid regions on the other side. On GreatGroup level the soils are classified as Haplustalfs. Their definition readsthat they are freely drained, moderately deep to deep soils with an argillichorizon of loamy or clayey particle-size, some 2 : 1 clay minerals and ahigh base saturation. On subgroup level the Ultic, Oxic and Aquic adjec-tives were appropriate to distinguish soils from the central concepts of theTypic Haplustalfs.The Ultic Haplustalfs refer to those soils that do not have a base satura-tion percentage in the argillic horizon of 75% or more in some part,and/or have no calcic horizon or soft powdery lime within a certain depth.The Oxic Haplustalfs have less than 24 meq cation exchange capacity per100 g clay (with reservation to the cation retention from NH4Cl, that was

170

not determined). The definition for the Aquic subgroup is fully condi-tioned in profile 8, but is not satisfied for Haplustalfs in similar topo-graphic position such as profiles 4, 6, 9 and 18. Differences with theoriginal concept of "Aquic" are minor however. For example, in profile 4mottles with chromas of 2 or less occur at a depth of 78 cm instead of75 cm. Therefore the soils with gley horizons are indicated as "Aquic" inthe text.Based on the colour of the matrix and associated mottles one Aqualf wasrecognized, which because of its natric horizon is classified as a TypicNatraqualf (profile 15). Rhodustalfs are differentiated on basis of theirred colour. Two soils were thus classified (profiles .1.0 and 23). The Udicsubgroup notation for one of them refers to the presence of a calcichorizon or soft powdery lime.

VI. 2. Land use

At present the majority of the land in the investigated area is used forgrazing while subordinate parts are cultivated under rainfed conditions. Inthe past, however, cropped areas occupied a fairly large acreage despitethe fact that most preparing of the land was done by hoe (Fosbrooke,1972). The main crops included sorghum, melon, pumpkin and at presentalso maize, groundnuts and beans. Little was done to maintain the fertilityof the soil in the early days of the settlers, who for the largest part camefrom South Africa.The departure of a large number of inhabitants from the Shoshong area inthe latter half of the last century is partly attributed to the depletion of thesoil and the shortage of water. Water is still the crucial factor for thedevelopment of the land. The unreliability of the rainfall (Chapter I) doesnot encourage the cultivation of crops on other than subsistence level. Theholding of cattle is more appropriate to the environment although theirnumber is restricted not only by the grazing potential of the land, but also bythe availability of watering points and their location.Water assessment studies in the area are not optimistic with regard to alarge aquafer that as yet may be tapped for domestic, agricultural andindustrial use (Hyde in Fosbrooke, 1972).The complexity of land development is not only caused by physicalfactors, but even more so by human attitudes and economic considera-tions. Although difficult to evaluate, the attitude of the populationtowards development is of great importance and its misjudgment had ledto failure of many a scheme. The economic factors are readily comprisedin the terms "input" and "output" such as proposed for the development ofa land utilization system, instigated to embrace all factors relevant to thedevelopment of a certain area (FAO, 1972; Beeketal., 1972).

171

The subject under study concerns physical factors of the land only, asexplained in the preceding chapters.During a semi-detailed soil survey of the Shoshong area particular atten-tion was paid to a number of physical soil factors such as soil depth (d),soil texture (t), drainage (w) and erodability or effects of past erosion (e).Chemical analyses completed the picture relative to the elemental composi-tion of the soils, their exchange phenomena, fertility level, salinity problemsand related characteristics and properties.In dealing with some of the main aspects of land use the division inlandscapes as outlined in Chapter II is appropriate. In sample area A, for themost part sufficient soil depth is observed inherent to the soils developed onthe pediment. The texture of some soils may not be optimum with regard tothe water holding capacities, whereas exchange capacities are variable, beingclosely related to the amount of clay. Soils in the depressions, such as inprofile 4, may be subject to a water surplus during the rainy season, whereasin addition salinity and clay mineralogy do not encourage cropping in theseareas.Most soils in sample area B have a limited soil depth due to erosion orshallow profile development. However, in this area belonging to thegranitic peneplain, the slightly lower parts have a greater soil depth, inaddition to a better soil moisture regime. Gley features in these soils areexpressed, but often not clearly, within 60 cm of the surface thus indica-ting a reasonable part of the soil to be suitable for plant root penetration.Areas in the immediate vicinity of profiles 8 and 9 have a regular croppingschedule. In this region especially the danger exists that, through thepressure on the land, less suitable soils may be included in the cultivationpattern, thus increasing the hazard of soil erosion and land misuse.In the more eroded part of the catchment area, sample area C, somecultivation is being carried out at present on parts of the broad interfluves;however the greater part of this area is mainly suitable for cattle. Inaddition to the problem of soil conservation there is another factor, namelythe accumulation of salts in the lower lying areas. The saline areas occur inpatches which makes a precise location of them is necessary. Moreover, theworkability of the bottom-land soils is limited due to the presence of 2 : 1clay minerals in addition to the fine texture of the soils, causing poordrainage and a surplus of water over prolonged periods. Much erosion isobserved along old paths or cattle tracks. In several instances roads had to bere-routed, because gullies and exposed rock had made them impassable. Soiltransition is fairly gradual in this area, with exception of the boundariesbetween the soils of the slopes and the ones of the valley floors.This picture is further complicated in sample are D, where the black claysoils of the Bonwapitse flood-plain are associated with similar or reddish soilon the pediment. Soil boundaries are sharp and clearly visible on the aerial

172

photographias well as otherwise. Only the soils which are fairly easy to workarc being cultivated at present, although there are indications that in earlytimes the finer textured soils were cropped. The latter can be managed onlywith the right amount of moisture, i.e. when of a friable consistency.The Vertisols however offer excellent possibilities for grazing as the grasscover is fairly thick and rejuvenates soon after the dry season. The sandiersoils on the pediment in certain localities have potential for cropping butneed special crops that mature and ripen in a short time, before the soildries out (fig. 49). The infiltration rate of most of these soils is such that areasonable amount of water can be stored at greater depth on conditionthat it is consumed fairly quickly. The depletion of soil fertility seems tobe the foremost reason for the abolishing of a number ol fields, on most ofthe sandly or coarse loamy Haplustalfs.Sample area E is used for pasture, mainly because the area is too far awayfrom population centers to make cropping a feasible proposition.Of special interest is the sandy soil of the Kalahari, as such and similarsoils cover a tremendous acreage in the interior of the country. As wasshown in the investigations, the soil is poor in almost all substances that

tig. 49. Poor sorghum crop on an Ultic Haplustalf (coarse loamy), in the background theMarutlwe Hill.

] 7 3

are needed kir plant growth. Thanks to its particular texture it is never-theless capable ot holding ;i fairly larme amount or water for a given time.Tliis allows decprooted plants to survive in the semi-arid conditions. Thusone point in the soil to its favour is its depth. Soils similar to the describedPsammctK, but of a coarse loamy texture arc reported to sustain a fairlyhigh yield under minted conditions and with some application of fertili-zer. One aspect is the problem of wind erosion which deserves specialconsideration. Windbrakes arc a must, unless very small areas arc cropped.One property of most of the granitic soils is the hia;h amount ot quartzpresent which caused plow shares and other farm equipment to blunt in ashort time. The data on the soil fertility indicate that the amount ofnitrogen is low, whereas potassium is slightly higher. Available phos-phorous was also found to be extremely low. The application of com-pound fertilizer yielded good results under sufficient precipitation, but ifthe rain fails or is not adequate for the crop to mature or even for theseeds to germinate, much effort is in vain.Experiments carried out by the FAO/UNDP in analogy to but on abroader basis than some local farmers, showed high yields under irrigationon coarse and fine loamy Haplustalts. thus in a way creating an additionalcontroversial viewpoint, concerning priorities as to the use of water.Considering the physical factors of the area, emphasis should primarily bebrought on beef production in combination with subsistence farming insmaller areasThe delicate balance between the use and misuse ot the soil is a challenge tothose who depend directly upon it for their existence. An exact appreciationof this equilibrium, which so much reflects the situation in the biosphere,should also be a major issue to all who might tend to forget the fundamentalimportance of soil for life.

174

APPENDIX I.

METHODS

I.I. Survey methods

1.1.1. Office methodsA first impression of the area was obtained from aerial photographs (scale 1 : 40.000),looked at through a Wild stereoscope. The photographs were printed on glossy,double-weight paper. Landforms, geology and vegetation formed the basis for areas inwhich different soils were expected. These were outlined on the photographs with waxpencil. Map legends were finalized after field surveys and a first draft of the soil mapprepared on an uncontrolled mosaic, after which additional field checking procuredthe final legend and map.

1.1.2. Field methodsSoils were studied in the field in soil pits, road and river cuts and from materialbrought up by soil auger. Soil pits were 2 m deep unless a (para)lithic contact wasencountered within this depth. The length of the pit was orientated to the North. Theriver-side or Thompson auger with a core diameter of about 7.5 cm was frequentlyused. Auger observations were set through to 125 cm but occasionally deeper ifthought necessary. Soil descriptions were based on the FAO Guidelines for SoilDescription, the Munsell Soil Colour Charts were used for the colour identification.Samples were exclusively taken from soil horizons as visible in the pits. Soil observa-tions were made within the units as outlined on the aerial photographs and followedno fixed grid pattern. The density of observations is about one per hectare. Vegetationcounts were carried out by Van Rensburg (1971) on a quadrant basis set up aroundeach soil pit. Most of these data are summarized in Chapter 1.4. The survey procedurefollows largely the suggestions made by Vink (1963).

1.2. Laboratory methods

1.2.1. Particle-size analysesThe determinations were carried out on the fine earth of all samples by the pipettemethod and by sieving. Air dry samples were gently crushed, if necessary, and sub-sequently sieved through a 2 mm sieve. Particles coarser than 2 mm were retained andindicated as coarse fragments. Of the remaining material 20 g was used for theparticle-size analyses. Organic matter was removed through treatment with H2O2;carbonates and iron coatings were destroyed by 0.2 N HC1. The thus created soil pasteswere neutralized by successive washing with distilled water and wet sieved through a50 micron sieve into a 1000 ml cylinder. A solution of 0.1 Mol. sodium-pyrophosphateand 0.04 Mol. sodium carbonate was used as dispersion agent. The cylinders weretopped up to 11 mark with distilled water and thoroughly shaken for good dispersion.The size analyses determination continued than according to standard procedures. Forclay mineralogical analysis samples of the fraction smaller than 2 micron were ob-tained from not pre-treated soil. Dispersion was obtained with the solution as men-tioned above, while subsequent precipitation was induced by adding a solution of1.4 N Ca-acetate and 0.3 N acetic acid. The main particle-size nomenclature is ob-tained from the USDA system with additonal limits at 500, 200, 100 and 20 micron.

175

ƒ.2.2. Determination of the pH-water and the pH-KClThe soil was shaken for two hours with distilled water or 1 N KCL in a 1 : 5 soil-waterratio c.q. 1 N KCl mixture. Readings were done in the suspension with a pH meterwith glass electrode on all soil samples.

1.2.3. Bulk density valueUndisturbed soil clods of about 7 g were taken from 46 samples. The paraffine waxmethod was used as coating material (Black, 1965) for the volume-weight deter-mination; the results are presented in g/cm .

1.2.4. Water soluble salts and ions in soil-water 1 : 5 extractThe soil was shaken with water (soil-water ratio 1 : 5) for two hours and the extractobtained by filtration. Salt content (EC5 ) was measured by electrical conductancewith a conductivity meter, results expressed in mmhos/cm at 25 C.The following ions were determined: calcium, sodium, potassium, magnesium, (bi)carbonate, chloride, sulphate and nitrate. Ca, Na and K were determined by flame ;

photometer and the Mg colorimetrically with thyazol yellow. The amount of (bi)carbonates is determined by titration with 0.1 N HC1; chloride is measured with a"Chlor-o-counter" (Marius), nitrate is determined colorimetrically with brucine andsulphate turbidimetrically. These analyses were carried out on 41 soil samples whichhad an EC5 of 0.1 mmhos/cm or higher, results are expressed in meq/100 g soil on anoven-dry basis. \

1.2.5. Water soluble salts and ions in the saturation extractWater was added to the soil till a saturated paste was obtained. An extract wasobtained by centrifuging with a high speed centrifuge. In the extract the ECe wasmeasured with a conductivity meter and results expressed in mmhos/cm at 25 C. Ca,Na and K were determined by the flame photometer and Mg colorimetrically withthyazol yellow; carbonate and bi-carbonate and chloride were determined titri-metrically and sulphate determination was done turbidimetrically. These analyses werecarried out on 10 samples all having a EC5 of 0.5 mmhos/cm or more. The results areexpressed in meq/1.

1.2.6. Phosphorous determinationThe method as outlined by Olsen was followed. The soil was shaken with 0.5 Mol.NaHCO3 for 30 minutes in a 1 : 20 soil-water ratio. In the extract, obtained byfiltration of the suspension, P was determined by means of molybdenum blue/ascorbicacid. The results are given in ppm.

1.2.7. Free calcium carbonate determinationThe fine earth fraction was subject to treatment with 20% HCL, and the soil was thenshaken for 1 hour in a "Scheibler" apparatus and the volume of evolved CO2 wasmeasured. Results are given as %CaCC>3 of oven-dry soil, however MgCC>3 may also bepresent.

1.2.8. Organic carbon determinationThe soil organic matter was oxidized with potassium-di-chromate and sulphuric acidwithout application of external heat (Walkley-Black method). The amount of potas-sium-di-chromate used was determined by titration with ferrous sulphate. Accordingto Jackson (1969) only 77% of the carbon in the organic matter is oxidized ascompared to the dry combustion method, for most soils of the temperate humid zone.

176

Usually the factor 1.3 is therefore applied to calculate the amount of organic carbon.The organic carbon percentage was determined on all soil samples.

1.2.9. Percentage nitrogen determinationThe micro-Kjeldahl method was followed, the results are expressed as %N in oven-drysoil. The analyses were carried out on all topsoil and some subsoil samples, as thepercentage was found to be extremely low in the deeper soil horizons.

1.2.10. Exchangeable metallic cation determinationThe soil was mixed with purified sand and put into percolation tubes and leached with50% alcohol to remove the water soluble salts. Subsequently the soil was percolatedwith IN NH4 acetate 96% alcohol (1 : 1) atpH 8.2. In the leachate Ca, K and Na weremeasured by flame photometer, while for Mg the colorimetric thyazol yellow methodwas used. The results are expressed in meq/100 g soil on an oven-dry basis.

1.2.11. Cation exchange capacity determinationFollowing the treatment of the soils as outlined under 1.2.10., the soil material waspercolated with IN Na-acetate of pH 8.2 to saturate the soil complex with sodium. Theexcess sodium was then washed out one or more times with 96% alcohol. Lastly theabsorbed sodium was replaced by leaching with IN NH4-acetate of pH 8.2. Theabsorbed sodium was then determined in the leachate by flame photometer. Theresults in meq/100 g of oven-dry soil give the CEC value at the equilibrium pH of 8.2.

1.2.12. Heavy and light mineral analysisPre-treatment with H2O2 and HCl was carried out on 50 g of the bulk fine earth.Samples of the 50 to 200 micron sand fraction were obtained, as it appeared that thecoarser sand yielded virtually no heavy residue. This material was cone-quartered downto amounts required for separation by bromoform (s.g. 2.89) and for mounting of themineral grains on slides. Determination and line countings of the mineral species werecarried out by means of polarizing microscope according to the methods as outlinedby Kerr (1959) and Milner (1962). Of the heavy fraction the mutual percentage ofopaque and transparant grains were determined first and subsequently 100 transparentgrains identified; of the light minerals 100 grains were counted per sample.

1.2.13. Preparation of thin sectionsThe procedures for the preparation of thin sections follow the outlines by Jongeriusand Heintzberger (1964). The unsaturated polyester resin synolite 544 was used forimpregnation. Samples were sawn and polished to a thickness of about 15—20 micron.Investigations on 20 mammoth-sized thin sections were done by means of transmutentand polarized light through a Leitz Orthoplan microscope.

1.2.14. Clay mineralogyMaterial from the not pre-treated clay fraction (see 1.2.1.) was dried either in a stove at40 C or freezedried, and if necessary, ground. Random powder specimen of allsamples were analysed using Philips X-ray diffraction equipment consisting of: agenerator control cabinet (PW 1320/00), a stabilized generator (PW 1310/00) withwide range goniometer (PW 1050/25), a basic recording unit (PW 1352/10) withsealer-time combination (PW 1353/100) and a pulse height analyses combination (PW1355/10).

177

The experimental conditions to obtain the pen-recorded X-ray diffractograms are:radition: CoKaHigh voltage: 35 kVcurrent: 30 m Afilter: Fedivergence slit: 1receiving slit: 0.3 mmscatter slit: 1

detector: proportional detector probescanning speed: 1° (20) per minutefull scale: 400 counts per secondtime constant: 2 secondssample holders: flat aluminium holder or glass slides

Additional diffraction data were obtained from 29 parallel orientated specimens. Thesaturation with Mg and ethylene-glycol solvation, and saturation with K for heatingtreatments, were performed according to procedures outlined by Jackson (1969).

1.2.15. Elemental analysisThe concentration of 17 elements was determined with X-ray spectrometric analysesand atomic absorption spectrophotometer investigation. Determined with the formertechnique were the oxides of the elements: Si, Al, Fe, Ti, Ca, Mg, K, Na and P.The following trace elements were determined by means of the latter method: Mn, Cu,Zn, Cr, Co, Ni, Ba and Sr.The analyses were carried out on the fine earth of all samples, while in addition, theoxides of the major elements were determined on 85 samples of the clay fraction.

1.2.16.a. X-ray fluorescency determinationAnalyses of the major elements were done by means of a Philips 1410 X-rayspectrometer with chrome tube consisting of the same basic units as described in1.2.14. The results were recorded on paper tapes for computer analyses by means of aTape Punch Control (PW 4206/01).To prepare the sample for analysis 4—6 g of the air dry sample was heated at 900 Cfor two hours and the loss on ignition calculated. From this sample 400 mg was takento which as added LiOH and B2O3. The fusion reaction was made at 1300 C in afurnace in which the crucible made of platimum +3% gold was plced. The thushomogenized soil material was retrieved as a solid button on cooling. The sample fluxratio is 1 : 10.The experimental conditions may be summarized as follows:

element

Na backgroundNa peakMg backgroundMg peakP backgroundPpeakSi peakAl peakK peakCa peakTi peakFe peak

20 position

54.0053.1044.5043.5090.5089.30

108.95144.75136.70113.1586.1857.52

crystal

KAPKAPKAPKAPPEPEPEPELI FLIFLIFLIF

collimator

coarsecoarsecoarsecoarsecoarsecoarsecoarsecoarsefinefinefinefine

pre-set time (seconds)

100 standard, 40 sample100 standard, 40 sample100 standard, 40 sample100 standard, 40 sample40 standard, 10 sample40 standard, 10 sample

101010101010

178

The element concentration is calculated according to the formula:9f = i Mi.jl * C(j)U

C(i) = [I(i) - B(i)] x K(i) x Ve x 100 +L Ve J

in which:C = concentrationi = number elementI = peak intensity ) c o r r e c t e d f o r D e a d T i m e

B = background intensity )K = apparatus constant, determined on samples with known concentrationVe= dilution of sampleai,j = correction factor (influence of element j on the measurement of element i)Cj = concentration of element jIn first approximation Cj = o, in the second approximation Cj = Ci of the firstapproximation.The complete procedure to obtain the concentrations of the various elements bymeans of a computer programme, was developed by De Rooij (1973, in preparation).

I.2.16.b. Atomic absorption spectrophotometrieThe method of analyses was outlined by De Vries (1971), the procedures applied wereverified through analysis of U.S. Geological Survey rocks (second series; Flanagan,1969), which are used as geo-chemical standards.Part of the fine earth of each sample was cone-quartered twice down to 20 g, this wassubsequently crushed in an agate mortar. From it 500 mg was taken for analysis. TheNa2CO3—Na2B4O7 fusion technique was used to prepare the soil sample for flameanalysis. The sample was subsequently dissolved in acid solution according to Meyerand Koch (1959) and was shaken mechanically at room temperature to preventcoagulation of SiC>2.Lanthanum was added to 1% of the solution to be analysed to prevent chemicalinterferences.A Techtron Model AA4 spectrophotometer was used, consisting of the followingmajor units:a) set of hollow cathode tubes, one for each element, as light source emitting the

sharp-line spectrum of the element to be determined;b) a premix burner, featuring a burner chamber in which sample, fuel and oxidant are

mixed before entering the flame where atomic vapor of the sample is produced;c) an Ebert type monochromator as wavelength selector, which can be set to pass any

wavelength of about 1.860 to 10.000 Â; mechanical slits open to a maximum widthof 300 micron;

d) a HTV photomultiplier type R 213, whose output is fed into a A.C. amplifierwhose signal is rectified and fed to a galvanometer as read-out mechanism, coupledto a 1 mV Hitachi recorder.

The concentration of the elements in a sample measured from the atomic absorptionfollowed the addition method of Zeegers (1959).

179

The experimental conditons are summarized as follows:

element

MnCuZnCrCoNiBaSr

cathode current(+ auxiliary current)mA

104

61010 (+400)10 (+400)1010

absorptionwavelengthÂ

2.7953.2472.1393.5792.4072.3205.5364.607

flame

oxidizingoxidizingoxidizingoxidizingoxidizingoxidizingreducingreducing

fuel

C2H2

C2H2

C2H2

C2H2

C2H2

C2H2

C2H2

C2H2

oxidant

airairairairairairN2Oair

mechaislit wieu

10050

300100

255050

100

1.2.1 7. Electron microprobe analysisThese analysis were carried out on selected areas of thin sections from the samples7.20 and 8.55, with a Cambridge apparatus (type Geoscan) according to methodsoutlined by Veen and Maaskant (1971).

180

APPENDIX II

SOIL PROFILE DESCRIPTION AND LABORATORY DATA

DESCRIPTION OF PROFILE NO. 1

Al 0 - 2 0 cm Dark brown (7.5YR3/2) moist and dry, slightly gravellycoarse sandy loam; structureless; dry loose, moist slight-ly friable, wet non-sticky and non-plastic; common fineroots; smooth gradual boundary; (sample 1.11),

B21 20 — 60 cm dark reddish brown (5YR3/3) moist, reddish brown(5YR4/4) dry, slightly gravelly sandy clay loam; struc-tureless; loose dry, friable moist, slightly sticky andslightly plastic wet; few fine roots; gradual smoothboundary; (sample 1.12),

B22 60 — 110 cm reddish brown (5YR4/4) moist and dry, slightly gravellysandy clay loam; structureless; consistence as horizonabove; increase of coarse fragments with depth; (sam-ple 1.13).

Note: blocked by coarse gravel or boulders at 110 cm.

182

Laboratory data of profile No. 1

Particle size distribution (/i) in % weightSand Silt Clay

Sample Depth Hori- %>2 2000- 500- 20Q- FÖcT 50- 20- < 2 p H l : 5

No. in cm. zon mm. 500 200 100 50 20 2 H2O KC1

33.5 23.3 15.6 8.325.0 18.2 14.9 8.020.9 15.9 12.9 8.2

1.111.121.13

0- 2020- 6060-110

A lB21B22

9.05.28.6

566

.4

.9

.5

445

.0

.2

.9

9.922.829.7

667

.8

.7

.1

555

.4

.1

.5

SampleNo.

1.111.121.13

Exchangeable cations in meq/100 g

Ca

2.524.568.16

Mg

0.491.061.54

K

0.310.310.37

Na

trtrtr

Sum

3.325.92

10.07

C.E.C.

6.108.78

12.20

Base

sat. (%)

546883

P2O5

ppm

822

Organic

% C

0.470.450.29

matter

%N

0.060.05

Sample Elemental Composition of the fine earth (% by weight)No.

1.111.121.13

SampleNo.

1.111.121.13

SampleNo.

SiO2

73.070.469.5

Trace

Mn

280320374

A12O3

13.616.015.7

Fe2O3 TiO2 CaO

4.45,04.5

0.45 1.930.65 1.740.54 1.35

element content of the fine ear

Cu

< 54234

Zn

213740

Cr Co Ni

256 24 10112 12 50112 6 28

MgO K2o

0.76 4.370.69 4.711.97 4.37

th (in ppm)

Ba

400540940

Elemental composition of the clay fraction (%

SiO2 Al. Fe2O3 TiO2 CaO

Sr

505865

Na2O

1.581.401.14

by weight)

MgO

P '

0.0.0.

SiO

A l .

9.17.57.5

Os

150911

.o,

K 2 O

Sum

100.1100.899.2

SiO2

R2O3

7.686.216.35

Na2O

Loss onignition %

3.24.95.2

A12O3

Fe2O3

4.94.95.5

P2O5

1.111.13

4749

.3

.827.229.8

1112

.6

.111

.46

.4241

.21

.701.962.21

2.502.09

0.000.00

30

.84

.93

Mineralogy of the clay fraction Molar ratios

Sample K 1 Mt Mi V Chi Q SiO2 SiO2 A12O3N o - ÄL^ 3 R^) 3 Fe2O3

1.11 +1.12 ++1.13 ++

++ tr tr

X

X

tr

3.0

2.8

2.32

2.25

3.7

3.9

183

DESCRIPTION OF PROFILE NO. 2

Al 0 — 23 cm Dark brown (7.5YR3/2) moist and dark greyish brown(10YR4/2) dry, loamy coarse sand; weak fine subangularblocky structure; firm dry, loose moist, non-sticky, non-plastic wet; abundant fine and medium roots; commonfine and medium pores; smooth gradual boundary; (sam-ple 2.25),

Bl 23 - 39 cm brown (7.5YR4/3) moist and (7.5YR5/3) dry, loamycoarse sand; structureless; very hard dry, friable moist,nonsticky and non-plastic wet; common fine and me-dium roots; common faint fine and medium strongbrown (7.5YR5/6-dry) mottles; common fine and me-dium pores; smooth gradual boundary; (sample 2.26),

B21 39 — 62 cm brown (7.5YR4/4) moist and light brownish grey(10YR6/2) dry, coarse sandy loam; structure and consis-tence as horizon above; few fine and medium roots;common fairly distinct strong brown (7.5YR5/6-dry)mottles; smooth gradual boundary; (sample 2.27),

B22 62 - 88 cm brown (7.5YR4/3) moist and pale brown (10YR6/2)dry, fine gravelly coarse sandy loam; structure and con-sistence as Bl; few fine and medium roots; common fineand medium strong brown (7.5YR5/6) and yellowish red(5YR4/6) mottles; common fine and medium pores;smooth gradual boundary; (sample 2.28),

B23 88 - 120 cm brown (7.5YR4/3) moist and pale brown (10YR6/3)dry, slightly gravelly coarse sandy loam; structureless;slightly hard dry, friable moist, non-sticky and non-plastic wet; very few fine and medium roots; commonprominent dark red (2.5YR3/6) mottles; common thinbroken cutans; common fine and medium pores; amountof coarse fragments increasing with depth; smooth grad-ual boundary; (sample 2.29),

B3 120 - 145 cm brown (10YR5/3) moist and pale brown (10YR6/3) dry,gravelly coarse sandy loam; structureless; consistence asB23; faint reddish mottling; abrupt smooth slopingboundary; (sample 2.30),

(II)C 145+ cm coarse to very coarse subrounded quartz gravel; (nosample taken).

184

Laboratory data of profile No. 2

SampleNo.

2.252.262.272.282.292.30

SampleNo.

2.252.262.272.282.292.30

SampleNo.

2.252.262.272.282.292.30

SampleNo.

2.252.262.272.282.292.30

SampleNo.

2.252.282.30

SampleNo.

2.252.262.272.282.292.30

Depth Hori-in cm. zon

0- 23 Al23- 39 BI39- 62 B2162- 88 B2288-120 B23

120-140 B3

ParSand

% > 2 2000- 500- 200- 100-mm. 500 200 100 50

1.9 43.9 23.6 12.0 6.72.3 34.2.0 35.7.8 38.7.0 36.

14.7 35.

9 23.5 14.5 9.58 23.8 13.6 6.44 19.6 11.2 4.83 18.9 8.7 5.96 20.1 10.1 7.4

Exchangeable cations in meq/100 g

Ca Mg

1.11 0.200.80 0.230.50 0.200.61 0.460.81 1.060.61 1.06

K

0.210.210.210.310.550.44

Elemental composition of the

SiO, A12O3 Fe2

87.9 7.0 1.684.4 8.3 1.885.4 8.1 1.888.6 9.1 2.386.8 10.1 2.685.0 10.1 2.7

O3 TiO2

0.330.490.410.490.420.50

Trace element content of the 1

Mn Cu Zn

96 < 5 < 464 12 1012 5 24 < 5 3

18 5 1050 21 15

Cr Co

19 1219 818 1928 1528 15

210 24

Na Sum C.E.C.

tr 1.52 3.50tr 1.24 3.50tr 0.91 3.94tr 1.38 5.71tr 2.42 6.61tr 2.11 5.27

fine earth {% by weight)

CaO MgO K2O Na2O

0.19 0.97 0.00 1.000.36 0.47 3.89 0.310.18 0.75 3.90 0.650.16 0.78 4.02 0.440.14 0.32 3.94 0.800.14 0.27 3.93 0.81

fine earth (in ppm)

Ni Ba Sr

< 2 380 2510 1720 35

< 2 1720 103 380 12

10 <100 1610 380 80

Elemental composition of the clay fraction {% by weight)

SiO2 A12O3

53.8 27.558.1 24.853.5 29.5

Mineralogy of the

K I Mt

++ + +++ + +++ + +++ + +++ + +++ + +

Fe2O3

9.67.5

10.0

TiO2 CaO MgO

1.85 1.95 1.353.24 0.17 1.201.69 1.16 1.23

% weightSilt

50- 20-20 2

3.1 4.64.1 5.64.8 5.64.3 5.83.9 5.54.4 5.6

Basesat. (%)

433523243740

P2OS

0.060.030.030.030.030.04

SiO2

A12O3

21.317.217.916.114.814.3

K 2 O

2.664.412.03

clay fraction Molar ratios

Mi V Chi Q SiO2

A1,O3

xx 3.3XX

X X

tr xx 4.0tr xxtr xx 3.1

SiO,

R,O 3

2.72

3.33

2.53

Clay

< 2

6.17.9

10.015.920.816.8

P2OS

ppm

4tr6122

Sum

99.0100.0101.2106.1105.0103.4

SiOj

R 2 O 3

18.5815.1715.6913.9112.7912.25

Na,O

0.000.110.00

A12O,

Fe,O,

4.5

5.2

4.6

pH 1:5

H2O KC1

6.5 5.06.4 4.65.7 4.25.6 4.05.7 3.96.0 4.1

Organic Matter

% C % N

0.44 0.040.24 0.040.300.270.190.21

Loss onignition %

2.22.32.32.83.02.6

A12O3

Fe 2 O 3

6.97.37.16.26.16.1

P,O,

1.330.380.82

>

185

DESCRIPTION OF PROFILE NO. 3

Al 0 - 21 cm Dark reddish brown (5YR2.4/2) moist, dark brown(7.5YR3.6/2) dry, coarse sandy loam; weak medium sub-angular blocky structure; firm dry, loose moist, non-sticky non-plastic wet; common fine few medium andcoarse roots; common fine pores; very rapid permeable;wavy gradual boundary; (sample 3.19),

Bl 21 - 39 cm dark reddish brown (5YR3/3) moist, brown (7.5YR4/4)dry, coarse sandy loam; structureless; slightly hard dry,friable moist, very slightly sticky and non-plastic wet;common fine and medium roots; few termite nests (upto 10 cm in diam.); common fine and medium pores;smooth gradual boundary; (sample 3.20),

B21 39 - 62 cm reddish brown (5YR3.6/4) moist, yellowish red(5YR4/6) dry, sandy clay loam with about 20% coarsesand; structureless; few fine and medium roots; veryhard dry, friable moist, sticky and slightly plastic wet;few patchy thin cutans often with reddish colour; andgreyish spots of A l l material in root channels; smoothgradual boundary; (sample 3.21),

B2t 62 - 98 cm dark red (2.5YR3/6) moist, yellowish red (5YR4/8) dry,sandy clay with about 20% coarse sand; structureless:very hard dry, friable moist, sticky and plastic wet; fewfine and medium roots; common fine and medium pores;common broken medium reddish cutans; increasingamount of coarse fragments with depth; smooth gradualboundary; (sample 3.22),

B23 98 - ] 18 cm reddish brown (5YR4/4) moist, brown (7.5YR5/4) dry,sandy clay; structureless; slightly hard dry, friable moist,slightly sticky and plastic wet; few fine and mediumroots; common fine and medium and a few coarse red(2.5YR4/8-dry) mottles; few termite channels; commonfine and medium pores; smooth gradual boundary; (sam-ple 3.23),

B3 1 1 8 - 1 3 3 cm brown (10YR5/3) moist, pale brown (10YR6/3) dry,sandy clay loam; structureless; hard dry, friable moist,sticky and slightly plastic wet; common fine and me-dium yellowish red (5YR4/6) mottles; common fine andmedium roots; common coarse sand; few fine roots;common fine and medium pores; abrupt smooth bound-ary; (sample 3.24),

(II)C 133+ cm blocked by coarse angular gravel.

Note: on the hard soil surface common coarse sand.

186

Laboratory data of profile No. 3

No.

3.193.203.213.223.233.24

SampleNo.

3.193.203.213.223.233.24

SampleNo.

3.193.203.213.223.233.24

No.

3.193.203.213.223.233.24

SampleNo.

3.193.213.24

Sample

No.

3.193.203.213.223.233.24

Depthin cm

0-21-39-62-

i .

21396298

98-118118-1133

Hori-zon

A lBlB21B2tB23B3

Particle sizeSand

%>2 2000- 500-mm. 500 200

0.5 32.4 24.90.6 25.7 22.00.6 29.6 18.11.0 26.0 14.20.9 25.3 14.11.5 24.8 14.5

Exchangeable cations in meq/100 g

Ca

1.511.772.033.063.052.80

Elemental

SiO2

85.979.379.476.880.079.7

Trace

Mn

2361569866

11092

Al ,

7.10.

Mg

0.380.630.971.611.641.64

K Na Sum

0.31 tr 2.200.41 tr 2.810.42 tr 3.420.42 tr 5.090.42 tr 5.110.42 tr 4.86

distribution (/i) in

200- 100-100 50

12.9 9.013.2 8.99.6 8.08.0 6.58.3 8.78.9 8.5

% wcightSilt Clay

50-20

5.65.44.93.93.55.5

C.E.C. Basesat

3.95 564.39 645.51 629.09 567.35 706.65 73

composition of the fine earth (% by weight)

O, Fe,l

9 2.21 2.7

13.8 3.715.14.14.

9 4.33 3.81 3.9

0 3 TiO, CaO MgO K,O Na2O

0.46 0.18 0.00 3.12 0.000.55 0.18 10.69 0.18 0

.31 3.01 0.55

.57 2.85 3.580.66 0.27 2.03 2.59 0.000.63 0.16 00.64 0.15 0

.56 2.61 0.99

.25 2.66 0.00

: element content of the fine earth (in ppm)

Tu

121528121515

Elemental

SiO2

52.951.851.6

Zn

51325252521

Cr Co Ni

48 19 2040 24 2070 27 2059 12 2061 19 2085 24 24

Ba Sr

380 25380 40

<100 17<100 18<100 25<100 23

composition of the clay fraction (% by weight)

A12O3

30.933.934.0

Fe2O3 TiO,

9.0 1.769.6 1.539.2 1.35

Mineralogy of the clay fraction

K

++++++++++++

1

++++++

Mt Mi V Chi

CaO MgO

1.37 0.630.11 0.770.08 1.05

Molar

Q SiOj

A12O3

x 2.9X

x 2.6X

X

x 2.6

. (%)

P,O

o.o;0.0cO.Of

o.o:

20- < 2 p M 1:52 H2O KCI

4.3 10.9 6.64.6 20.2 6.34.1 25.7 6.13.3 38.1 5.84.9 35.2 6.24.9 32.9 6.1

Organic matter

% C % N

0.43 0.050.41 0.050.35 0.050.33 0.060.19 0.040.16 0.03

5.25.24.94.64.84.9

1'2OS

ppm

3trtr11tr

5 Sum Loss onignition %

i 99.8 2.7i 97.7 4.0( 104.9 5.6' 102.5 7.7

.0.03 103.1 6.90.0:

S.2O3

A12O.

18.513.39.88.29.59.6

K.

1.1.1.

ratios

1 101.4 6.2

Si2O3 Al,, R ,O 3 Fc2

15.75 5.711.39 5.8

8.32 5.87.00 5.88.13 5.88.17 5.6

2O Na2O

89 0.6275 0.0018 0.00

SiO2 Al2Oj

R

2.

2.

2.

2 O 3 Fe2O3

45 5.4

20 5.6

.19 5.8

o3o3

P2O,

0.880.580.67

187

DESCRIPTION OF PROFILE NO. 4

Al 0 - 1 6 cm Dark brown (7.5YR3/2) moist, brown (7.5YR4/2) dry,coarse sandy loam; weak subangular blocky structure;hard dry, loose moist, non-sticky non-plastic wet; com-mon very fine and fine (mainly grass) roots; commonfine and medium tabular pores; few termite channels;smooth gradual boundary; (sample 4.12),

Bl 16— 43 cm dark reddish brown (5YR3/2) moist, dark brown(7.5YR3/2) dry, sandy clay loam with 25% coarse sand;weak medium subangular blocky structure; hard dry,friable moist, slightly sticky and slightly plastic wet; fewfine, medium and coarse roots; few fine and mediumtabular pores; rapid permeable; smooth gradual bound-ary (sample 4.13),

B21 4 3 - JUrm dark reddish brown (5YR4/3) moist, (5YR3/4) dry,sandy clay loam with an appreciable amount of coarsesand; structureless massive; hard dry, friable moist,sticky and plastic wet; few thin broken cutans aroundsandgrains and along root-channels; few fine, mediumand coarse roots; few termite nests (up to 5 cm indiam.); common fine tabular pores; smooth gradualboundary; (sample 4.14),

B22 61 - 78 cm dark reddish brown (5YR3/4) moist, reddish brown(5YR4/4) dry, sandy clay with an appreciable amount ofcoarse sand; very hard dry, friable moist, very sticky andplastic wet; structureless massive; common small thinbroken cutans; few fine and medium pores; very few fineroots; few faint greyish and brownish mottling in thelower part of the horizon and common distinct yel-lowish red (5YR4/8—dry) mottles; smooth gradualboundary; (sample 4.15),

B2g 78 - 97 cm very dark grey (10YR3/1) moist, dark grey (10YR4/1)dry and dark brown (10YR4/3) moist, dark yellowishbrown (10YR4/4) dry, sandy clay with common coarsesand; structureless massive; consistence as horizon above;common thin broken cutans; few distinct reddish mot-tles on boundary with B22; the grey colours in thishorizon are coarse mottles in brown matrix; smoothclear boundary; (sample 4.16),

B2ca 9 7 - 1 1 2 cm very dark grey (7.5YR3/1) moist, dark grey (7.5YR4/1)dry, sandy clay; amount of coarse sand and structure ashorizon above; very hard dry, firm moist, sticky andplastic wet; common broken cutans; few reddish brownmottles; very few fine roots; few fine pores; very fewcoarse hard calcrete nodules and quartz gravel and com-mon soft and hard CaCO3 accumulations at the bound-ary to the underlying horizon; smooth clear boundary:(sample 4.17),

C 112 —125 cm yellowish brown (10YR5/6) moist, .brownish yellow(10YR6/6) dry, slightly gravelly loamy sand; commondark red concretions; common large CaCO3 concretions;(sample 4.1 8),

R 125+ cm extremely hard granitic rock.

Note: On the very surface of the soil a thin (2—4 mm) layer of coarse sand occurs.

188

Laboratory data of profile No. 4

SampleNo.

4.124.134.144.154.164.174.18

SampleNo.

4.124.134.144.154.164.174.18

SampleNo.

4.124.134.144.154.164.174.18

SampleNo.

4.124.134.144.154.164.174.18

SampleNo.

4.124.144.18

SampleNo.

4.124.134.144.154.164.174.18

Depthin cm

0- 1616- 4343- 6161- 7878- 9797-112

112-125

Hori-zon

A lBlB21B22B2gB2caC

%>2m m .

0.90.92.82.93.94.7

10.0

Exchangeable cations in m

Ca

2.523.565.387.208.79

12.0017.41

Elementa

SiO, Al2

85.0 6

Mg

0.601.822.633.083.715.107.82

compos

O3 Fe2

1 1.980.9 9.2 3.177.3 1273.7 1373.1 1474.0 1571.8 16

8 4.38 4.69 4.98 5.27 5.8

K

0.390.730.600.690.740.861.16

tion of

Particle size distribution (p) inSand

2000- 500- 200- 100-500 200 100 50

37.1 22.5 11.2 8.728.6 18.6 9.8 7.827.6 15.22.3 14.23.1 11.

8 7.7 5.66 7.3 6.43 6.4 5.7

18.1 11.4 6.7 6.417.0 15

eq/100g

Na

0.09tr0.09tr0.090.090.09

% weightSilt

50- 2020 2

5.3 45.3 43.7 54.2 54.2 54.4 7

Clay

< 2

5 10.79 25.02 34.40 40.28 43.52 45.8

5 9.9 7.9 18.4 20.9 10.4

Sum C.E.C.

3.60 4.396.11 6.648.70 10.94

10.97 12.8213.33 14.2518.05 18.1126.48 26.73

the fine earth (% by weight)

O3 TiO2 CaO

0.31 0.200.54 0.300.61 0.280.60 0.320.64 0.370.0.

74 0.5879 0.73

MgO K2O Na,O

0.89 2.85 0.000.65 2.52 0.000.73 2.48 0.001.76 2.69 2.402.24 1.61 0.173.73 2.35 0.001.80 2.53 0.00

Trace element content of the fine earth (in ppm)

Mn Cu

144 12170 9100 12128 21134 24206 21850 31

Elementa

s.o2

55.652.256.0

Zn

8192229283031

Cr

85110

5940406135

Co Ni

27 2031 3524 3158 7727 3524 4229 35

Ba Sr

< 100 12< 100 25< 100 8< 100 20

380 30< 1 0 0 32

380 28

composition of the clay fraction (% by weight

AljO,

26.730.426.9

Fe,0

9.810.2

9.4

3 TiO,

1.961.351.40

Mineralogy of the clay fraction

K 1

++ +++ +++ +++ +++ +++ +++ +

Mt

trtr

Mi

CaO MgO

1.22 1.701.53 1.571.68 2.12

Molar

V Chi Q SiO2

trtrtr

A12O

xx 3.5X

x 2.9X

X

X

xx 3.5

Base P ;OS

sat. (%) ppm

8292808694

10099

P,OS

0.110.050.060.120.130.070.03

Si2O5

AI2Oj

23.714.910.4

9.28.38.17.3

K , 0

2.431.981.81

ratios

SiO,

R2O3

2.87

2.40

2.89

4

trtrtr

tr

Sum

97.697.398.699.998.1

102.4100.2

Si2O3

R ; O 3

14.8312.298.557.516.276.575.96

Na,O

0.000.000.18

A12O

Fe2O

4.3

4.7

4.5

pH 1:5

H2O KC1

6.8 5.86.6 5.16.6 5.26.7 5.27.2 5.67.8 6.38.1 6.8

Organic matter

% C % N

0.44 0.050.47 0.060.39 0.060.31 0.050.25 0.050.27 0.050.11 0.02

Loss onignition %

3.05.27.18.18.69.4

11.2

AI2O3

Fe 2O 3

5.24.74.74.84.74.84.5

P2OS

0.640.780.57

3

189

DESCRIPTION OF PROFILE NO. 5

Al 0 - 2 1 cm Dark brown (7.5YR3/2) moist, brown (7.5YR4/2) dry,coarse sandy loam; weak fine subangular blocky struc-ture; firm dry, loose to friable moist, non-sticky non-plastic wet; abundant fine roots; smooth gradual bound-ary; (sample 5.31),

Bl 21 — 36 cm dark reddish brown (5YR3.5/3) moist, dark reddish grey(5YR4/2) dry, gravelly coarse sandy loam; structureless;consistence as horizon above; few fine and mediumroots; common weathered rock fragments with presum-ebly Fe and Mg cementation increasing in the lower partof the horizon; abrupt wavy boundary; (sample 5.32),

C 3 6 — 5 5 cm weathered granitic rock of very gravelly coarse sandy 'loam texture; (sample 5.33).

190

Laboratory data of profile No. 5

No.

5.315.325.33

Sample

No.

5.315.325.33

Sample

No.

5.315.325.33

Sample

No.

5.315.325.33

Depthin cm

0-2121-3636-55

Hori-zon

AlBlC

%•>->

mm.

2.752.946.4

Particle size iSand

2000- 500-500 200

29.7 20.231.9 22.340.3 19.5

Exchangeable cations in meq/100 g

Ca

2.011.110.91

Mg

0.430.430.49

K

0.310.260.28

Na Sum

tr 2.75tr 1.80tr 1.68

Elemental composition of the Tine earth

SiO2

89.090.487.4

Trace

Mn

274400

52

A12O3 Fe2<

8.2 2.08.7 2.69.4 4.2

distribution (M) in % weightSilt Clay

200-100

17.513.614.6

C.

3.4.4.

(%by

O3 TiO2 CaO MgO K

0.0.0.

56 0.33 0.31 2.49 0.26 0.49 3.51 0.22 0.26 3.

100-50

11.211.1

4.9

Ba:.E.C. sat

50 7938 4183 35

weight)

jO Na2O

91 0.0003 0.5637 0.00

element content of the fine earth (in ppm)

Cu Zn

5 25 1

31 20

Cr

2111•»3

Co Ni

15 < 215 < 277 45

Ba

1720380

1260

Sr

121213

50-20

8.06.96.2

îe

. (%)

P2O5

0.05•0.020.01

SiO2

A12O3

18.417.615.9

20- < 22

5.2 8.24.0 10.24.8 9.7

Organic mal

pH 1:5

H2O KC1

6.8 5.96.7 5.06.6 5.3

tter

%C %N P2O5

0.55 00.33 00.29

Sum

103.4106.5105.4

SiOj

R2O3

15.9414.8212.35

.05 2

.03 tr4

Loss onignition %

3.33.23.2

A12O,

Fe2O3

6.55.33.5

Elemental composition of the clay fraction (% by weight)

Sample SiO2

No.A12O3 Fe2O3 TiO2 CaO MgO K2O Na2O P2O5

5.315.325.33

52.753.953.1

29.430.830.5

9.19.1

1.441.551.36

2.890.091.62

0.551.110.75

1.792.141.64

0.000.590.72

2.480.721.26

SampleNo.

5.315.325.33

Miner;

K

++++++

»logy

I

+++

of the

Mt

clay fraction

Mi V Chi Q

XX

X

X

Molar

SiO2

A12O3

3.03.03.0

ratios

SiO2

R 2 O 3

2.552.502.48

A12O3

Fe2O3

5.25.35.3

191

DESCRIPTION OF PROFILE NO. 6

Al 0 - 2 3 cm Dark brown (7.5YR3/2) moist, ,dark greyish brown(10YR4/2) dry, coarse sandy loam; very weak mediumsubangular blocky; hard when dry, slightly friable moist,slightly sticky non-plastic when wet; many fine andmedium roots; few termite nests and channels; smoothgradual boundary; (sample 6.34),

Bl 23 - 43 cm dark reddish brown (5YR3/2.6) moist, brown(7.5YR4/3) dry, sandy clay loam; structureless massive;very hard dry, ,friable moist, sticky and plastic wet;common fine and medium roots; common fine and me-dium and few coarse pores; appreciable amount ofcoarse sand; few very fine gravel; few thin brokencutans; smooth gradual boundary; (sample 6.35),

B21 43 - 69 cm reddish brown (5YR4/3) moist, brown (7.5YR5/4) dry,sandy clay; structureless massive; very hard dry, friablemoist very sticky and plastic when wet; common fineand medium roots; common faint small (but more dis-tinct at lower part of horizon) dark red (2.5YR3/6)mottles; common fine and medium pores; amount ofcoarse .fragments increasing with depth; clear smoothboundary; (sample 6.36),

B2g 69 — 105 cm greyish brown (10YR5/2) moist, light brownish grey(10YR6/2) dry, sandy clay loam; structureless massive;many prominent medium and coarse dark red(2.5YR3/6) mottles; very hard dry, slightly friablemoist, slightly sticky and plastic wet; few small promi-nent bluish mottles; few fine medium roots; few smallmedium cutans; common coarse fragments increasingwith depth; smooth gradual boundary; (sample 6.37),

B2g 105 — 150 cm grey (10YR5/1) moist, ,slightly gravelly coarse sandyloam; massive; very few fine roots; many prominentcoarse black mottles; increasing amount of coarse frag-ments, few small prominent dark red mottles; abruptsmooth boundary; (sample 6.38),

C 150+ cm extremely hard mottled granite.

192

Laboratory data of profile No. 6

SampleNo.

6.346.356.366.376.38

SampleNo.

6.346.356.366.376.38

SampleNo.

6.346.356.366.376.38

SampleNo.

6.346.356.366.37

Depth Hori-in cm. zon

0- 23 Al23- 43 BI43- 69 B2169-105 B2g

105-150 B2g

Particle size distribution (M) in %Sand

%>2 2000- 500- 200- 100-mm. 500 200 100 50

3.4 32.0.5 27.2.2 27,3.3 25,

13.2 47

.0 17.5 17,

.4 15.1 13,

.2 12.1 10

.8 11.3 14

.4 12.0 8

Exchangeable cations in meq/100 g

Ca Mg

1.31 0.392.54 1.143.15 1.472.75 1.873.68 2.39

K Na Sum

0.38 tr0.45 tr0.50 tr0.57 tr0.68 tr

Elemental composition of the

SiO2 A12O3 Fe2O3 TiO2

89.1 9.4 2.683.6 13.0 3.580.7 16.2 4.178.0 17.6 4.880.3 14.8 8.0

0.590.630.710.770.71

Trace element content of the 1

Mn Cu Zn

2500 21 15108 24 18228 11 19128 34 25

Cr

595060

110

2.084.135.125.196.75

.1 4.1

.3 3.5

.4 4.8

.8 1.7

.2 4.2

weightSilt

50-20

6.7.5.6.5.

C.E.C. Basesat

5.27 407.51 559.30 559.30 569.77 69

fine earth (% by weight)

CaO MgO

0.19 0.430.19 0.450.18 0.440.19 0.670.22 0.45

fine earth (in |

Co Ni

15 1712 2012 1727 31

K2O Na2O

2.77 0.002.46 0.152.25 0.002.34 0.002.38 0.16

ppm)

Ba Sr

800 45800 22380 32380 40

41

,99,2

%)

P,O,

0.070.040.040.050.05

SiC

Al ;

16.10.

8,7.

Clay

20- < 22

6.0 16.93.5 30.13.7 35.94.5 35.03.7 19.3

pH 1:5H

6.6.6.6.7.

Organic matter% C % !Ni

0.51 0.050.48 0.050.410.220.18

; Sum

105.1104.0104.6104.4107.1

)2 SiO2

O3 R2O3

1 13.679 9.32,5 7.285 6.40

2 O KC1

2 4.62 4.84 4.76 5.10 5.3

P2O5

ppm

1trtr1tr

Loss onignition %

4.67,76

.18

.00

.42

.02

.69

A12O3

Fe2O3

5.85.96.25.7

6.38 2508 41 21 114 51 58 <100 28 9.2 6.84 2.9

Sample

6.346.366.38

Elemental

SiO2

53.052.149.4

composition of the

A12O3

31.935.431.8

Fe2O3

8.18.4

12.0

clay fraction (%

TiO2

1.701.281.02

CaO

1.520.081.32

by weight)

MgO

0.890.811.76

K 2 O

1.591.241.92

Na2O

0.160.000.00

P2OS

1.200.630.77

Mineralogy of the clay fraction Molar ratios

Sample K 1 Mt Mi V Chi Q SiO2 SiO2

No.

6.34 ++ +6.35 ++ +6.36 ++ + tr6.37 ++ + tr6.38 ++ + tr

XX

X

X

X

X

A12O3

2.8

2.5

2.5

R

2.

2.

2.

: O 3

43

17

10

Fe2O3

6.2

6.6

5.0

193

DESCRIPTION OF PROFILE NO. 7

Al 0 - 15 cm Dark brown (10YR3/3) moist and brown (10YR5/3)dry, loamy coarse sand; structureless, single grained; drysoft, moist friable, wet non-sticky and non-plastic; com-mon fine and a few medium and thick roots; smoothgradual boundary; (sample 7.1),

Bl 15 - 34 cm dark brown (7.5YR3/4) moist, brown (7.5YR 5/4) dry,loamy coarse sand; structureless, single grained; dryslightly hard, moist friable, wet non-sticky and non-plastic; common fine, medium and thick roots; few finetabular pores; smooth gradual boundary; (sample 7.2),

B21 34 - 55 cm brown (7.5YR4/4) moist, lightbrown (7.5YR6/4) dry,coarse sandy loam; structureless, single grained; dryhard, moist firm, wet slightly plastic; few fine, mediumand thick roots; few fine and medium tabular pores; fewsmall (1 cm diam.) greyish more consolidated soil frag-ments; smooth gradual boundary; (sample 7.3),

B3 55 - 69 cm brown (7.5YR5/4) moist, pinkish grey (7.5YR6/2) dry,slightly gravelly coarse sandy loam; structureless, singlegrained; dry very hard, moist firm, wet very slightlysticky and slightly plastic; few fine and medium roots;few fine pores; abrupt smooth boundary; (sample 7.4),

C 69+ cm weathered granitic rock with an appreciable amount ofreddish and bluish mottling and of a very gravelly loamycoarse sand texture; (sample 7.5).

194

Laboratory data of profile No. 7

SampleNo.

7.17.27.37.47.5

SampleNo.

7.17.27.37.47.5

SampleNo.

7.17.27.37.47.5

SampleNo.

Depthin cm.

0-1515-3434-5555-6969+

Hori-zon

AlBlB21B3C

Particle size distribution (/i) in % weightSand

% > 2 2000- 500-mm. 500 200

0.9 27.1.1 27.1.7 26.9.4 32.

75.0 33.

.8 24.0

.7 21.3

.3 22.3

.4 25.5

.3 , 24.3

Exchangeable cations in meq/100 g

Ca

1.511.011.501.261.26

Mg

0.260.380.760.810.80

K

0.720.490.360.260.36

Elemental composition of the

SiO2*

85.285.984.885.878.9

Trace

Mn

A12O3 Fe2<

8.4 1.58.9 1.59.4 1.89.0 1.7

11.0 5.9

O3 TiO2

0.420.290.360.360.39

element content of the :

Cu Zn Cr Co

Na

0.26trtrtrtr

200-100

18.19.15.12.13.

Sum

2.751.882.622.332.42

fine earth (%

CaO MgO

0.22 1.0.20 0.0.17 0.0.17 0.0.17 0

fine earth

Ni

.16

.49

.35

.54

.89

(in ]

Ra

82902

by

K

4.

100-50

13.113.713.0

9.19.5

C.E.C.

3.073.284.793.732.42

weight)

2 O Na2O

24 0.144.62 0.564.3.4.

17 1.5981 0.4733 0.82

ppm)

Sr

Silt

50- 20-20 2

6.8 2.95.6 3.35.7 3.15.2 2.46.2 3.5

Basesat. (%)

9057556361

P 2 O S

0.050.070.090.080.08

SiO,

A1 ;O 3

Clay

< 2

6.69.2

13.713.410.0

Organic

% C

0.460.300.200.200.16

Sum

101.3102.5102.7101.8102.5

SiO,

R , O 3

pH 1:5

H 2 O

6.26.06.06.06.4

matter

% N

0.040.030.040.030.02

KC1

5.14.94.74.75.3

P,O,ppm

31tr1tr

Loss onignition %

2.42.42.72.93.8

A12O3

Fe 2 O 3

7.17.27.37.47.5

SampleNo.

7.17.27.37.47.5

188 9 3120 21 522 24 4

4 15 22830 31 3

19 19 6 800 4946 15 <2 1260 5563 <2 <2 2600 4240 < 2 3 800 4250 56 35 1600 52

17.116.415.316.312.1

15.3514.8013.6514.499.05

8.79.68.28.12.9

SampleNo.

7.17.37.5

Elemental

SiO2

54.854.453.9

composition of the clay fraction (%

A1,O3

29.931.831.1

Fe2O3 TiO2

8.1. 2.077.6 1.619.1 1.56

CaO

0.330.090.83

by weight)

MgO

1.191.141.31

K

CM

C

M

•—<

-.o

322081

Na2O

0.920.460.00

P j

po

p

os

356751

Mineralogy of the clay fraction

K I Mt Mi V

Molar ratios

Chi Q SiO2 SiO2

A12O3 R,O 3

trtrtr

xX

XXXX

3.1

2.9

2.5

2.65

2.52

2.48

A12O3

Fe2O3

5.8

6.5

5.4

195

DESCRIPTION OF PROFILE NO. 8

Ap 0 - 2 1 cm Dark brown (7.5YR3/2) moist, brown (7.5YR4/4) dry,coarse sandy loam; weak fine and medium subangularclods; in the upper part of the horizon slight horizontalbedding is visible; very hard dry, friable moist, slightlysticky, slightly plastic wet; common fine medium tubu-lar pores; common very fine, fine and few medium roots;few termite nests 2 cm in diameter; smooth wavy bound-ary; (sample 8.53),

B21 21 — 47 cm dark reddish brown (5YR3/3) moist, reddish brown(5YR4/4) dry, sandy clay loam; structureless; slightlyhard dry, friable moist, sticky and plastic wet; many fineand medium tubular pores; common coarse fragments;common fine termite channels; common fine and me-dium roots; thin cutans in root and termite channels;smooth gradual boundary; (sample 8.54),

B2(g) 47 — 76 cm dark reddish brown (5YR3/3) moist, yellowish red(5YR5/6) dry, sandy clay loam; structure and consis-tence as horizon above; coarse fragments increasing withdepth; common faint dark grey (10YR4/1) and yel-lowish red (5YR4/8) mottles; few fine thin brokencutans; common fine termite nests and root channels;wavy gradual boundary; (sample 8.55),

B2g 76 — 104 cm dark brown (7.5YR3/2) moist, sandy clay loam withcommon distinct very dark grey (10YR3/1) and reddishbrown (5YR/4/4) mottles; structureless; very hard dry,firm moist, sticky and plastic wet; common fine andmedium tubular pores; appreciable amount of coarsefragments; common medium thin broken cutans; fewsmall prominent reddish concretions in the lower part ofthe horizon; clear wavy boundary; (sample 8.56),

B3 1 0 4 - 1 2 9 cm brown (10YR5/3) moist, pale brown (10YR6/3) dry,slightly gravelly coarse sandy loam; common iron(?) concretions up to 1 cm in diameter; occasional angu-lar rock fragments; soft dry, friable moist, sticky andplastic wet; common faint reddish and yellowish mot-tling; occasional very fine roots; clear wavy boundary;(sample 8.57),

C 129 - 144 cm greyish brown (10YR5/2) moist, light yellowish brown(2.5Y6/4) dry, weathered rock of coarse sandy loamtexture; common distinct bluish black fine concretions.

196

Laboratory data of profile No. 8

SampleNo.

8.538.548.558.568.57

SampleNo.

8.538.548.558.568.57

SampleNo.

8.538.548.558.568.57

SampleNo.

8.538.548.558.568.57

Depthin cm.

0-21-47-

i Hori-zon

21 Ap47 B2176 B2(g)

76-104 B2g104-129 B3fl

% > 2mm.

3.27.75.55.9

18.6

Particle size distribution (M) inSand

2000- 500- 200- 100-500 200 100

28.1 17.0 16.7 9.24.7 13.25.4 12.22.9 11.27.6 12.

Exchangeable cations in meq/100 g

Ca

3.244.855.898.47

10.07

Mg

0.671.281.311.652.04

K

0.570.630.690.740.85

Na

0.170.170.170.170.18

0 11.1 80 10.4 6.8 9.9 78 10.5 8.

Sum

4.656.938.06

11.0313.14

50

.4

.4

.9

.8

.5

C.E.C.

7.149.65

10.3311.9716.22

Elemental composition of the fine earth (% by weight)

SiO2

85.679.877.778.783.7

Trace

Mn

184176274336

1600

A12O3 Fe2(

9.4 3.012.7 4.014.2 4.114.7 4.715.1 5.9

3 3 TiO2 CaO

0.0.

49 0.2453 0.27

0.59 0.310. 68 0.390.69 0.44

MgO K2O

0.44 3.060.69 2.360.88 2.480.62 2.771.08 2.31

element content of the fine earth (in ppm)

Cu Zn

12 1512 1721 3334 2434 19

Cr

12586

127120

89

Co Ni

15 2419 2723 3519 3835 43

Ba

<100<100

8001260

<100

Na2O

0.320.260.510.260.00

Sr

< 213121926

% weight

50-20

6.35.05.56.33.2

P

0

Silt

20-2

3.73.74.56.59.6

Basesat.

6572789295

.110.06000

SiC

Al2

15.10.

9.

.12

.06

.08

>2

o3

571

9.38.3

Clay

< 2

18.834.135.334.827.8

pH 1:5

H

6.6.6.7.7.

Organic; %c

0.520.300.230.170.16

Sum

102.6100.6100.8103.099.3

SiO2

R2O3

12.908.917.547.846.63

2 O KC1

5 4.6 4.,8 5,.3 5..6 5.

matter

% N

0.040.04

Loss onignition

46788

.6

.4

.0

.2

.3

A12O3

Fe2O3

55454

.0

.0

.8

.5

.0

99.1.5.5

%

1

P2O5

ppm

812

465

SampleNo.

8.538.548.57

Elemental

SiO2

51.652.153.7

composition of the

A12O3

30.532.530.3

Fe,O,

10.210.39.5

clay fraction (%

TiO;

1.421.311.21

CaO

2.200.081.61

by weight)

MgO

0.661.281.26

K

1.1.1.

,o

577345

Na2O

0.000.080.00

P2OS

1.900.650.99

SampleNo.

8.538.548.558.568.57

Mineralogy of the clay fraction Molar ratios

K 1 Mt Mi Chi SiO, SiO2

A12O3 R2O3

trtr

xxXX

2.92.7

3.0

2.372.27

2.50

4.75.0

5.0

197

DESCRIPTION OF PROFILE NO. 9

Al 0 — 1 6 cm Very dark greyish brown (10YR3/1.6) moist, dark grey-ish brown (10YR4/1.7) dry, coarse sandy loam; mod-erate fine angular blocky structure; hard dry, firm moist,slightly sticky and non-plastic wet; many fine and me-dium roots; common coarse sand grains on the surface;few fine tubular pores; smooth gradual boundary; (sam-ple 9.113),

Bl 16 — 34 cm very dark greyish brown (10YR3/2) moist, dark greyishbrown (10YR4/2) dry, sandy clay loam; weak fine sub-angular blocky structure; slightly hard dry, friable moist,sligthly sticky non-plastic wet; common fine and me-dium roots; common coarse sand grains; common ter-mite nests; smooth gradual boundary; (sample 9.114),

B22 34 - 56 cm dark brown (7.5YR3/1.7) moist, dark greyish brown(10YR4/2) dry, sandy clay; strong fine to medium angu-lar blocky structure; very hard dry, firm moist, stickyand plastic wet; common medium broken cutans; com-mon coarse sand grains; a little very fine gravel; few fineand medium roots; common termite nests up to 5^ cmin diameter; very few fine tubular pores; smooth gradualboundary; (sample 9.115),

B2g 56 - 86 cm very dark grey (10YR3/1) moist, dark grey (10YR4/1.4)dry, sandy clay; weak fine angular blocky structure;common fine distinct strong brown (7.5YR5/6-moist)mottles; few fine roots; few termite nests; commoncoarse sand; common moderately thick broken cutans;smooth gradual boundary; (sample 9.116),

B2g 86 — 112 cm dark grey (10YR4/1) moist, sandy clay; massive; veryhard dry, firm moist, very sticky and plastic wet; com-mon thin broken cutans; many medium and coarseprominent red mottles (5YR5/8-dry and 2.5YR4/8-moist); few medium prominent bluish mottles; (sample9.117),

C 112 —140 cm dark grey (10YR4/1) moist and strong brown(7.5YR5/8) moist, ,slightly gravelly sandy clay; (sam-ple 9.118),

R 140+ cm hard granitic rock.

198

Laboratory data of profile No. 9

Sample

No.

9.1139.1149.1159.1169.1179.118

Sample

9.1139.1149.1159.1169.1179.118

SampleNo.

9.1 139.1149.1159.1169.1179.118

Sample

No.

9.1139.1149.1159.1169.1179.118

SampleNo.

9.1139.1159.118

Sample

No.

9.1139.1149.1159.1169.1179.118

Depth

in err

0-16-34-56-

.

16345686

86-112112-140

Hori-

zon

AlBlB22B2gB2gC

Particle size distribution (JU) inSand

%>2 2000- 500- 200- 100-

mm. 500 200 100 50

0.8 22.8 20.4 18.6 11.30.9 24.9 15.3 15.0 6.72.6 18.9 12.8 9.7 6.80.9 21.7 10.0 7.0 5.31.0 17.9 9.0 7.1 5.85.7 22.6 8.0 7.2 6.0 /

Exchangeable cations in meq/100 g /

Ca

1.724.065.116.666.675.63

Elementa

SiO,

84.279.674.169.162.371.7

Al

( 91216202018

Mg

0.671.371.952.462.532.43

K Na S u m / C.E.C.

/

0.70 tr / 3.09 5.270.63 t r / 6.06 9.00

0.89 tr' 7.95 d u " ! !1.21 / tr 10.33 14.271.36/ tr 10.56 1J..64

1.2-1' 0.17 9.44 ClZOß

composition of the fine earth (% by weight)

O, ,F

/

0) 2"4 39 41 42 58 6

•,Oj TiO, CaO MgO K,0 Na,O

1 0.42 0.25 1.67 3.33 0.331 0.45 0.23 0.42 3.19 0.000 0.61 0.29 1.88 2.63*0.008 0.69 0.39 2.15 2.73 0.219 0.71 0.34 1.12 2.3.5 0.423 0.66 0.29 0.18 (000^)0.00

% weightSilt

50- 20-

6.7 3.45.0 2.75.1 2<\4.3/4.3y.% 8.76.8 7.4

Base

sat. (%)

5967

3 59739179

P,OS

0.130.040.190.170.070.13

Clay

< 2

16.8^ 3 0 . 4

0X2-47.443.7

Organi

% C

0.500.380.390.270.160.05

Sum

101.499.5

100.6100.493.398.6

Trace element content of the fine earth (in ppm) SiO; SiO,

Mn

100130114114212280

Cu

243636304654

Zn

73645373734

Cr Co Ni Ba Sr A I,O3 R

288 6 20 640 28 15.8 13.78228 32 40 460 24 10.9240 32 40 320 24230 32 29 440 28300 26 54 540 28230 32 44 480 22

Elemental composition of the clay fraction (% bv weight)

SiO,

51.552.852.0

A1,O,

31.435.034.1

Fe,O3 TiOj CaO MgO

7.9 1.36 2.79 2.858.3 1.12 0.92 1.057.7 1.14 1.73 0.68

Mineralogy of the clay fraction Molar

K

++++++++++++

I

++

tr+++

Mt Mi V Chi Q SiO,

Al ,0

x 2.8X

tr x 2.6X

X

tr x 2.6

7.55.85.26.5

K , 0

1.510.001.16

ratios

SiO,

R,O,

2.40

2.22

2.26

9.396.475.064.405.34

pH 1

H , O

6.76.5

>6.67.27.9

>8.0

5

KC1

5.04.85.05.76.16.2

c matter P,O<

% N

0.060.06

ppm

5

tr >—.221

Loss onignition %

3.96.18.39.09.58.3

A1,O,

Fe,O3

6.86.26.56.55.34.7

Na,O P

0.000.000.00

AljC

2

os

850.811

Fe,O3

6.3

6.6

6.9

52

~ 7

d

JIJ) X,

DESCRIPTION OF PROFILE NO. 10

Al 0 - 1 0 cm Dark reddish brown (5YR3.4/4) dry and (5YR3/2)moist, coarse sandy loam; moderate fine angular blockystructure; slightly hard dry, friable moist, non-stickynon-plastic wet; common very fine and fine (mainlygrass) roots; few fine and medium tubular pores; smoothgradual boundary; (sample 10.6),

Bl 10 - 33 cm dusky red (2.5YR3/2) moist, dark reddish brown(5YR3.4/4) dry, coarse sandy loam; very ,weak fineangular blocky structure; slightly hard dry, friable moist,non-sticky very slightly plastic wet; few fine and me-dium roots; few fine and medium tubular pores; littletermite activity; very fine and fine gravel increasing withdepth; smooth gradual boundary; (sample 10.7),

B21 33 - 53 cm dark reddish brown (2.5YR2/4) moist, (2.5YR3/4) dry,sandy clay loam, structureless massive; hard dry, friablemoist, slightly sticky and slightly plastic wet; very fewfine and medium roots; few termite channels; smoothgradual boundary; (sample 10.8),

B22 53 - 72 cm dark reddish brown (2.5YR2/4) moist, (2.5YR3/4) dry,slightly gravelly sandy clay loam; structureless massive;hard dry, friable moist, slightly sticky and slightly plasticwet; very few fine roots; few fine tubular pores; fewtermite channels; clear wavy boundary; (sample 10.9),

B3 72 - 91 cm dark reddish brown (2.5YR2/4) moist, (2.5YR3/4) dry,gravelly sandy clay loam; structureless massive; hard dry,friable moist, very slightly sticky non-plastic wet; veryfew fine roots and a few medium roots; very few smalltermite nests; few fine soft CaCÛ3 concretions; clearwavy boundary; (sample 10.10),

(II)C 91+ cm dark reddish brown (5YR3/4) dry and pinkish white(5YR8/2) dry, very gravelly sandy clay loam; appearanceof weathered calcrete rock with granitic matrix; veryhard dry but in some parts soft; (sample 10.11).

200

Laboratory data of profile No. 10

SampleNo.

10.610.710.810.910.1010.11

Sample

Sample

10.610.710.810.910.1010.11

SampleNo.

10.610.710.810.910.1010.11

SampleNo.

10.610.710.810.910.1010.11

Depthin cm.

0-1010-3333-5353-7272-9191+

Hori-zon

AlBlB21B22B3(II)C

Particle size distribution 1Sand

%>2 2000- 500- 200- 100-mm. 500 200 100 50

0.5 261.0 268.8 258.7 29

37.2 2957.6 21

7 20.7 15.8 12.61 18 9 16.6 9.42 17.6 13.4 11.03 152 153 12

Exchangeable cations in meq/100 g

Ca

4.815.346.668.73

12.8117.10

Mg

0.821.001.211.311.471.09

K Na

0.57 tr0.50 tr0.42 tr0.37 tr0.42 tr0.53 tr

Elemental composition of the

SiO,

83.882.880.577.677.164.2

Trace

Mn

4283 9 33443663 3 4238

A1,O, Fe,

8.7 2.410.6 3.612.4 3.912.6 4.313.4 4.011.5 3.9

0 , TiO,

0.390.600.570.590.520.58

element content of the

Cu Zn

8 1218 2418 2810 2117 2824 18

Cr Co

40 1536 2036 1540 1235 833 51

Elemental composition of the

7 12.1 9.23 10.5 8.41 10.7 10.5

Sum C.E.C.

668

101418

20 8.1584 7.9629 9.3641 10.9170 15.3472 17.77

fl) in % weightSil

50-20

6.97.15.95.95.36.8

Base

sat. (%)

7686899697

100

fine earth (% by weight)

CaO

0.440.450.460.500.70

13.24

MgO K,O N

0.33 3.49 10.75 3.30 00.79 3.36 1

a , 0 P,O

73 0.1000 0.0614 0.11

0.32 3.21 0.00 0.030.62 4.00 0.81 0.040.68 2.71 0.33 0.10

"ine earth (in ppm)

Ni

203531172073

Ba Sr

1720 42380 49

1560 421560 12800 42800 80

SiO,

A1,O,

16.213.211.010.5

9.89.5

:lay fraction (% by weight)

Clay

20- < 22

5.2 12.13.9 18.03.3 23.63.5 24.34.6 26.74.5 34.1

pH 1-

H : O

6.96.76.97.17.78.3

Organic matter P,

%C % N

0.68 0.070.37 0.050.39 0.050.36 0.050.37 0.050.33 0.06

Sum

101.4101.599.999.1

101.197.2

SiO,

R,O 3

13.8410.81

9.178.608.217.84

5

KC1

5.75.85.55.66.67.1

ppm

5trtr211

Loss onignition %

4.34.95.66.17.0

16.1

Al ,0

Fe,O

5.64.64.94.85.34.7

3

SampleNo.

10.610.710.810.910.1010.11

SiO, ALO, Fe,O, TiO, CaO MgO K,O Na,O P,O,

10.610.810.11

54.452.453.8

27.029.324.0

11.012.4

9.6

111

.61

.46

.08

1.050.086.52

112

.63

.58

.79

2.072.171.68

0.000.020.00

0.450.620.59

Mineralogy of the clay fraction Molar ratios

Sample KNo.

Mt Mi Chi SiO, SiO, A1,O3

Ai,O3 R ,O , Fe ,O,

trtr

3.4

3.0

3.8

2.67

2.39

3.04

3.6

3.7

3.9

201

DESCRIPTION OF PROFILE NO. 11

Al 0 - 1 6 cm Very dark greyish brown (10YR3/2) moist, brown(10YR5/3) dry, coarse sand; structureless, single grained;soft dry, slightly friable moist, non-sticky and non-plastic when wet; thin broken cutans; common finetubular pores; frequent very fine and few medium andcoarse roots; smooth gradual boundary; (sample 11.45),

Bl 16 - 45 cm dark brown (7.5YR4/4) moist, brown (7.5YR5/4) dry,loamy coarse sand; structureless, single grained; soft dry,slightly friable moist, non-sticky and non-plastic whenwet; rapidly permeable; very few thin broken cutans;common fine, medium and coarse roots; common finetubular pores; coarse fragments increasing with depth;smooth gradual boundary; (sample 11.46),

I WB21 45 - 82 cm yellowish red (5YR4/6) moist, reddish yello'(7.5YR6/6) dry, slightly gravelly loamy coarse sand;structureless, single grained; soft dry, slightly friablemoist, non-sticky and non-plastic when wet; rapidly per-meable; few thin broken cutans; common fine and me-dium roots; common fine tubular pores; smooth gradualboundary; (sample 11.47),

B22 82 - 108 cm yellowish red (5YR4/6) moist, reddish yellow (5YR6/6)dry, slightly gravelly coarse sandy loam; structurelessmassive; soft dry, friable moist, non-sticky and non-plastic when wet; rapidly permeable; few thin brokencutans; common fine tabular pores; ,smooth gradualboundary; (sample 11.48),

B3 108 - 140 cm brown (7.5YR5/4) moist, light brown (7.5YR6/4) dry,slightly gravelly coarse sandy loam; structureless massive;soft dry, friable moist, non-sticky and non-plastic whenwet; rapidly permeable; few thin broken cutans; few fineand medium roots; common medium distinct yellowishred (5YR4/6-moist) mottles, common fine tubularpores; smooth gradual boundary; (sample 11.49),

C 140 - 173 cm brown (10YR5/3) moist, pale brown (10YR6/3) dry,slightly gravelly coarse sandy loam; structureless massive;slightly hard dry, friable moist, slightly sticky and non-plastic when wet; rapidly permeable; few thin brokencutans; few fine tubular pores; common fine faint andsharp yellowish brown (10YR5/6-moist) mottles; fewfine and medium roots; (sample 11.50),

(II)C 173+ cm coarse and very coarse angular gravel.

202

Laboratory data of profile No. 11

SampleNo.

11.4511.4611.4711.4811.4911.50

SampleNo.

11.4511.4611.4711.4811.4911.50

SampleNo.

11.4511.4611.4711.4811.4911.50

SampleNo.11.4511.4611.4711.4811.4911.50

SampleNo.

Depthin cm.

0- 1616- 4545- 8282-108

108-140140-173

Hori-zon

AlBlB21B22B3C

% > 2mm.

2.44.8

10.18.39.0

10.4

Particle size

2000-500

38.929.332.333.633.929.0

S

500-200

21.719.218.819.821.920.7

Exchangeable cations in meq/100 g

Ca

0.900.500.500.500.810.91

Elementa

SiO2 Al2

91.6 5.591.7 4.689.6 6.690.5 6.290.0 6.187.7 8.6

Mg

0.10trtrtr0.100.20

K

0.130.130.130.210.310.31

Na

trtrtr

distribution (M) in % weightand

200-100

27.532.125.418.617.519.9

Sum

1.130.630.63

0.17 0trtr

11

882242

composition of the fine earth (% by

O3 Fe2

1.11.11.41.41.31.7

O 3 TiO,

0.350.310.330.380.350.54

CaO

0.130.100.110.110.110.12

Trace element content of the fine earth

Mn Cu

212 9108 9

8 1276 580 1544 21

Zn

2115

68

1618

Cr

3418242521

4

Co

231919191919

Ni

201714

62418

MgO

0.760.700.610.000.490.42

100-50

1.86.68.19.77.08.3

C.E.C

2.191.752.192.632.633.08

weight

K i O

2.422.102.332.321.982.31

(in ppm)

Ba

800<100<100<100<100<100

Sr

343212

26

20

Silt

50-20

4.04.54.24.75.55.8

Basesat.

523629344646

Na2O

0.231.601.121.550.580.25

SiOj

202

0.91.01.01.41.83.1

%)

P ,O,

0.080.040.060.090.070.04

A12O3

28.433.824.722.925.117.4

Elemental composition of the clay fraction (% by weight)

SiO, A12O3 Fe,O 3 TiO2 CaO MgO K

Clay

< 2 pH 1

H 2 O

5.2 7.07.3 6.7

10.2 5.812.2 5.512.4 5.613.2 5.6

Organic matter

% C % N

0.23 0.030.22 0.020.190.180.240.16

:5

KCI

5.04.64.24.04.24.2

P2OS

ppm

415152

Sum Loss on

'g

102.1 1102.2 1102.6 2102.2 2100.9 2101.6 2

SiOj A12O

R 2 O 3 Fe2C

25.10 7.629.52 6.921.63 7.020.14 7.222.07 7.315.44 7.8

Na :O P

nition %

770555

3

3

111111

.45

.47

.50

515553

.4

.1

.1

29.3 1 .3 1 .

729

6.97.27.2

1.461.841.36

3.770.131.83

0.590.78.0.78

1.822.611.59

0.000.680.35

4.280.451.85

SampleNo.

Mineralogy of the clay fraction

K I Mt Mi V

Molar ratios

Chi SiO, ALO,

11.4511.4611.4711.4811.4911.50

2.9

3.0

2.8

2.56

2.62

2.47

6.7

6.8

7.0

203

DESCRIPTION OF PROFILE NO' 12

Ap 0 - 1 5 cm Dark brown (7.5YR3/2) moist, brown (7.5YR4/4) dry,slightly gravelly coarse sandy loam; moderate fine andmedium angular clods; hard dry, friable moist, veryslightly sticky non-plastic wet; common fine and a fewmedium roots; common medium root channels; few ter-mite holes; clear smooth boundary; (sample 12.39),

B21 15 - 32 cm dark reddish brown (5YR3/2) moist and (5YR3/4) dry,fine gravelly sandy clay loam; massive; very hard dry,friable moist, sticky and slightly plastic wet; few fineand medium roots; appreciable amount of coarse sand;few small termite channels; smooth clear boundary;(sample 12.40),

B22 32 - 59 cm dark reddish brown (2.5YR2/4) moist and (2.5YR3/4)dry, sandy clay; massive; very hard dry, friable moist,sticky and plastic wet; few fine and medium roots; fewtermite channels greyish in colour; common mediumbroken cutans clearly visible on peds and around sandgrains; appreciable amount of coarse sand and a littlevery fine gravel; clear smooth boundary; (sample 12.41),

B2t 59 - 87 cm dark reddish brown (5YR3/3) moist and (5YR3/4) drywith yellowish red (5YR4/6-dry) clay; massive; veryhard dry, friable moist, sticky and plastic wet; very fewfine, medium and coarse roots; common termite activity;appreciable amount of coarse sand and a little very fineangular gravel; occasionally medium and coarse angularquartz gravel is encountered on the boundary to thehorizon below; smooth clear boundary; (sample 12.42),

B3 87 — 102 cm dark reddish brown (5YR3/4) moist, gravelly sandy clay;structureless; slightly hard dry, friable moist, very slight-ly sticky non-plastic wet; common very fine and fineangular gravel; increasing amount of decomposed rockfragments with depth; gradual wavy boundary; (sam-ple 12.43),

C 102 — 125 cm gravelly clay with vivid bluish black and yellowish brown(10YR5/8) moist mottling; (sample 12.44),

R 125+ cm hard granitic rock of coarse grained texture.

204

Laboratory data of profile No. 1 2

SampleNo.

12.3912.4012.4112.4212.4312.44

SampleNo.

12.3912.4012.4112.4212.4312.44

SampleNo.

12.3912.4012.4112.4212.4312.44

SampleNo.

12.3912.4012.4112.4212.4312.44

SampleNo.

Deptrin cm

0-15-32-59-

1

15325987

87-102102-125

Hori-zon

ApB21B22B2tB3C

Particle size distribution (/i) inSand

%>2 2000- 500- 200- 100-mm. 500 200 100 50

5.2 30.7 19.9 15.1 4.16.6 25.5 14.9 12.0 4.36.8 23.2 11.4 8.8 4.27.7 23.0 9.8 7.5 4.5

24.1 21.2 9.8 8.3 4.445.0 19.1 6.1 4.4 2.9

Exchangeable cations in meq/100 g

Ca

4.465.927.928.97

11.2613.48

Elemental

SiO,

76.974.068.268.568.761.3

AU

Mg

1.031.882.672.803.064.19

K Na Sum C.E.C.

0.68 tr 6.17 8.810.68 tr 8.48 12.420.50 tr 11.12 15.660.32 tr 12.09 16.600.42 tr 14.74 17.080.51 tr 18.18 20.00

composition of the fine earth (% by weight)

O3 Fe2Oj TiO, CaO MgO K, 0 Na,O

12.8 3.15.16.18.18.24.

1 4.2 5.5 6.

4 0.52 0.62 0.57 3.47 2.219 0.63 0.54 0.31 3.25 0.324 0.64 0.54 1.06 3.03 2.541 0.68 0.57 1.02 3.21 0.00

9 6.6 0.67 0.66 0.97 3.34 0.175 7.9 0.58 1.40 1.65 2.93 0.13

Trace element content of the fine earth (in ppm)

Mn

3884224 4 04 5 44 6 0292

Cu

281812152124

Elemental

SiO,

Zn

282823333586

Cr Co Ni Ba Sr

43 31 27 <100 52110 35 40 380 32

52 8 24 1000 1240 15 45 1260 16

110 27 50 1280 2014 51 85 380 19

composition of the clay fraction (% by weight)

Fe,O3 TiO, CaO MgO

% weightSilt

50-20

7.47.16.65.56.76.9

Base

sat. %

706871738691

P,O,

0.080.070.080.070.100.05

SiO,

A1,O3

10.28.37.26.36.24.2

20-2

5.

Clay

< 2

5 17.36.6 29.65.6 40.26.7.

10.

O

.7 43.0,8 41.88 49.8

Organic

% N

0.640.610.450.400.370.25

Sum

100.699.197.798.6

100.1100.4

SiO,

R,O3

8.726.885.895.205.053.52

Na 2

pH 1:5

H , O

6.96.86.66.87.37.3

matter

% C

0.070.07

KCI

5.75.25.05.35.85.9

P : O

ppm

1024142

Loss onignition %

10.46.78.39.19.2

11.9

A1,O3

F ^ O ,

5.94.94.74.84.54.9

O P,O,

12.3912.4112.44

49.350.049.9

3 1 .32.33.

1

6

12.012.210.5

1.091.070.84

1.530.071.35

1.1.1.

825444

2.202.311.62

0.000.000.00

0.990.640.75

SampleNo.

12.3912.4012.4112.4212.4312.44

Mineralogy of the clay fraction Molar ratios

Mt Mi Chi SiO; SiO, A1,O,

Al,Oj R,O,

2.7

2.6

2.5

2.16

2.12

2.10

4.1

4.2

5.0

205

DESCRIPTION OF PROFILE NO. 13

Al 0 - 2 1 cm Dark reddish brown (5YR3/3) moist, brown (7.5YR4/4)dry, slightly gravelly coarse sandy loam; very weak finesubangular blocky structure; soft dry, slightly friablemoist, non-sticky, non-plastic wet; many fine and me-dium thick roots; many fine and medium tubular pores;smooth gradual boundary; (sample 13.59),

B21 21 - 44 cm dark reddish brown (5YR3/3) moist, reddish brown(5YR4/4) dry, gravelly coarse sandy loam; structureless,massive; hard dry, friable moist, slightly sticky and plas-tic wet; common fine and medium and occasional thickroots; many medium and coarse pores; coarse fragmentsincreasing with depth; clear smooth boundary; (sample13.60),

B22 44 - 76 cm dark reddish brown (2.5YR3/4) moist, yellowish red(5YR4/6) dry, gravelly coarse sandy loam; structureless,single grained; loose, dry and moist, non-sticky, non-plastic wet; many very fine and fine roots; coarse black,red and yellowish mottling in respectively 5YR and7.5YR hues in some parts of the horizon; wavy gradualboundary; (sample 13.61),

B3 76 —118 cm reddish brown (5YR4/4) moist, strong brown(7.5YR5/6) dry, gravelly sandy clay loam; few fine andvery fine roots; common fragments of partly decom-posed granitic rock; gradual wavy boundary; (sam-ple 13.62),

C 118 — 175+cm partly decomposed very hard granite with a high amountof quartz, weathering products have clayey texture, noroots; (sample 13.63).

206

Laboratory data of profile No. 13

SampleNo.

Depthin cm.

Hori-zon

Particle size distribution (ju) in '.Sand

2000- 500-500 200

200-100

100-50

> weightSilt

50-20

20-2

Clay

<2 pH 1:5

H,O KC1

13.5913.6013.6113.6213.63

0- 2121- 4444- 7676-118

118-175

AlB21B22B3C

16.724.258.160.270.1

35.536.938.830.628.4

19.5 15.7 8.317.8 13.0 7.716.1 8.6 6.2

9.8 9.8 7.68.6 4.8 2.7

8.0 4.3 8.7 6.6 5.15.6 4.1 14.9 6.2 4.54.7 4.3 21.3 6.0 4.47.5 5.8 23.7 6.2 4.52.3 4.7 48.5 6.5 4.3

SampleNo.

Exchangeable cations in meq/100 g

Ca Mg K Na Sum C.E.C. Basesat. %

Organic matter P2OS

%C % N

13.5913.6013.6113.6213.63

SampleNo.

2.272.533.053.569.71

0.491.161.582.15

10.45

0.260.260.210.320.43

0.170.170.090.170.55

3.194.124.936.20

21.14

15.696.708.289.38

26.59

2062606679

0.430.410.410.260.24

0.050.05

Elemental composition of the fine earth (% by weight)

SiO2 A12O3 Fe2O3 TiO2 CaO MgO K2O Na2O P2OS Sum Loss onignition '

13.5913.6013.6113.6213.63

81.0 9.781.0 10.279.0 13.373.6 15.077.7 16.0

2.22.84.34.84.8

0.350.380.390.580.56

0.520.330.330.340.33

0.430.140.880.850.70

3.853.533.883.113.07

0.750.680.910.000.88

0.100.050.100.020.07

98.999.2

103.198.3

104.1

3.04.05.05.2

10.2

No.

Trace element content of the fine earth (in ppm) SiO2 SiO2

Mn Cu Zn Cr Co Ni Ba Sr

A12O3

Al2Oj R2O3 Fe2O3

13.5913.6013.6113.6213.63

SampleNo.

13.5913.6013.6113.6213.63

20014220223884

921281218

1118211727

35403560140

1227311927

1724452740

1260 161260 321260 31<100 381260 35

14.213.410.18.38.2

12.3911.428.366.916.91

7.05.74.94.95.3

SampleNo.

13.5913.6113.63

Elemental

SiO2

49.453.252.3

composition

A12O3

28.029.731.2

Fe

121110

of the

,o3

.2

.8

.9

clay fraction (%

TiO2

1.331.040.93

CaO

2.810.661.64

by weight)

MgO

1.410.921.58

K

2.1.0.

085075

Na2O

0.000.490.00

P

200

.47

.72

.69

Mineralogy of the clay fraction Molar ratios

K 1 Mt Mi V Chi SiO2 SiO, A12O3

A12O3 R2O3 Fe2O3

trtr

xxXX

X

3.0

3.0

2.9

2.36

2.43

2.33

3.6

4.0

4.5

207

DESCRIPTION OF PROFILE NO. 14

Al 0 - 18 cm Dark brown (7.5YR3/2) moist, brown (7.5YR4/4) dry,slightly gravelly loamy coarse sand; structureless, singlegrained; slightly hard dry, slightly friable moist, non-sticky non-plastic wet; common fine and medium roots;many fine tubular pores; few termite channels; smoothgradual boundary; (sample 14.64),

B2 18 - 56 cm reddish brown (5YR4/5) moist, yellowish red (5YR4/8)dry, slightly gravelly coarse sandy loam; structureless,single grained; slightly hard dry, friable moist, slightlysticky, slightly plastic wet; common very fine and fineand few medium and coarse roots; common fine tubularpores; clear wavy boundary; (sample 14.65),

B3 56 - 90 cm yellowish red (5YR4/6) moist and (5YR5/6) dry, gravel-ly coarse sandy loam; loose dry and moist, non-sticky,non-plastic wet; common fragments of partly decom-posed rock; common distinct black and reddish mot-tling; clear in some parts gradual wavy boundary; (sam-ple 14.66),

C 90 — 110+cm hard, partly decomposed pink granite of gravelly coarsesandy loam texture; very few fine roots; (sample 14.67).

208

Laboratory data of profile No. 14

SampleNo.

14.6414.6514.6614.76

Sample

14.6414.6514.6614.67

SampleNo.

14.6414.6514.6614.67

SampleNo.

14.6414.6514.6614.67

SampleNo.

Depthin cm.

0- 1818- 5656- 9090-110

Exchange;

Ca

1.261.361.772.78

Elemental

SiO2 Al2

Hori-zon

AlB2B3C

Particle size distribution (p) inSand

%>2 2000- 500-mm. 500 200

6.1 30,12.6 30,40.0 41,48.4 34,

.1 19.3

.7 20.4,6 17.8,1 17.5

ible cations in meq/100 g

Mg

0.330.300.831.03

K

0.260.260.150.21

composition of the

O3 Fe2O3 TiO2

77.0 15.5 4.72.9 14.4 4,77.7 16.1 8.75.1 15.6 7,

8 0.60,6 0.95.4 1.00,9 0.93

Trace element content of the I

Mn Zn

310 7160 14328 22864 8

Elemental

SiO2

Cu

51528

9

Cr Co

25 1917 27

5 1937 15

composition of the

A12O3 Fe2O3

Na

0.170.170.170.17

200-100

21.418.3

9.710.8

Sum

2.022.092.924.19

fine earth (% by

CaO MgO K

0.42 1.1.83 1.1.90 1.1.94 2.

fine earth

Ni

106

1810

100-50

13.39.67.39.9

C.E.C.

3.593.774.925.81

weight)

2O Na2O

01 3.12 0.0066 1.44 0.0082 1.64 0.0028 1.55 0.00

(in ppm)

Ba

380100100380

clay fraction (%

TiO2 CaO

Sr

32305120

by weight)

MgO

% weightSilt

50- 2020 2

6.4 2.76.5 2.95.7 4.39.0 6.6

Base

sat. %

56555972

P2O5

0.050.120.060.09

SiOj

A12O3

8.48.68.28.2

K 2 O

Clay

< 2

6.811.613.612.1

Organic

% C

0.290.280.200.16

Sum

102.597.8

108.6105.3

SiO2

R 2 o 3

7.047.166.166.19

PH

H2(

6.36.16.06.4

1:5

3 KC1

4.84.54.44.9

matter P2O5

% N

0.030.03

ppm

8587

Loss onignition %

2.22.83.93.9

Al2

Fe2

5.14.93.03.1

Na2O

O 3

o3

P2O5

141414

.64

.65

.67

505151

.3

.3

.4

273029

.6

.9

.1

11.12.11.

027

1.271.331.13

3.200.091.52

111

.11

.12

.08

2.262.222.00

0.000.020.94

301

.28

.86

.16

SampleNo.

14.6414.6514.6614.67

Mineralogy of the clay fraction Molar ratios

K Mt Mi Chi SiO, SiO2 A12O3

A12O3 R2O3 Fe2O3

trtr

trtrtr

XXX

3.12.8

3.0

2.472.25

2.39

3.94.0

3.9

209

DESCRIPTION OF PROFILE NO. 15

A l l 0 - 2 cm Black (10YR2/1) moist, greyish brown (10YR5/2) dry,clay loam; weak medium platy structure; very hard dry,firm moist, slightly sticky and plastic wet; commondistinct medium brown (7.5YR4/4) mottles, commonvery fine pores; abrupt wavy boundary; (sample 15.68),

A12 2 — 4 3 cm very dark grey (7.5YR3/1) moist, dark greyish brown(10YR4/2) dry, coarse sandy loam; structureless, mas-sive; hard dry, friable moist, slightly sticky and plasticwet; little fine gravel; common medium tubular pores;common fine medium and few thick roots decreasing innumber with depth; clear wavy boundary; (sample15.69),

B2g 43 — 69 cm very dark grey (10YR3/1) moist, greyish brown(10YR5/2) dry, sandy clay loam; moderate coarse co-lumnar structure; top of the columns have a light brown-ish grey (10YR6/2) colour; many fine prominent darkred (2.5YR3/6) mottles and a few faint brown(10YR5/3) mottles; common coarse sand grains; fewfine pores; common very fine, fine and few mediumroots; common thin broken cutans; the boundary withunderlying horizon is clear and wavy; (sample 15.70),

B2g 69 - 91 cm dark grey (10YR4/1) dry and moist, slightly gravellysandy clay with an appreciable amount of coarse sand;consistence as horizon above; structureless, massive; fewfine and medium roots; common fine distinct dark red(2.5YR3/6) and strong brown (7.5YR5/6) mottles; com-mon thick large cutans; slightly wavy gradual boundary;(sample 15.71),

B2g 91 - 127 cm grey (10YR5/1) moist and (10YR6/1) dry, slightly grav-elly sandy clay, common coarse sand; very hard dry,firm moist, slightly sticky and plastic wet; prominentbrownish yellow (10YR6/6) and yellowish brown(10YR5/6) medium and coarse mottles, and few smallprominent bluish black mottles; very few very fine roots;common medium and large cutans and a few smallslickensides in the lower part of the horizon; wavygradual boundary; (sample 15.72),

B3 127 - 158 cm greyish brown (10YR5/2) moist and brown (10YR5/3)moist, slightly gravelly sandy clay loam; some yellowishand greyish mottling; calcareous; structureless, massive;common small cutans in the upper part of the horizon,and occasional fine roots; gradual wavy boundary; (sam-ple 15.73),

C 158 — 180+cm greyish brown (10YR5/2) moist, strongly decomposedgranite rock of sandy clay texture with prominent smalldark bluish mottles; (sample 15.74).

210

Laboratory data of profile No. 15

SampleNo.

15.6815.6915.7015.7115.7215.7315.74

SampleNo.

15.6815.6915.7015.7115.7215.7315.74

SampleNo.

15.6815.6915.7015.7115.7215.7315.74

No.

15.6815.6915.7015.7115.7215.7315.74

SampleNo.

15.6815.7015.74

SampleNo.

15.6815.6915.7015.7115.7215.7315.74

Depthin cm

0-2-

43-69-

2436991

91-127127-158158-180

Hori-zon

A l lA12B2gB2gB2gB3C

% > 2mm.

1.14.46.1

10.010.410.132.8

Particle size distribution {ß) inSand

2000- 500- 200-500 200 100

19.5 6.6 6.339.5 1742.4 1534.7 1131.2 1231.1 1327.7 11

Exchangeable cations in meq/1 00 g

Ca

7.722.793.826.957.277.307.03

Mg

2.420.962.214.574.244.775.82

K

1.260.420.210.420.470.480.58

Na

0.170.170.683.143.684.245.16

4 8.33 6.94 6.71 7.31 7.49 7.0

Sum

11.574.346.92

15.0815.6616.7918.59

Elemental composition of the fine earth (% by

SiO,

75.174.276.686.381.873.086.1

Trace

Mn

4 4 03 3 0230176120176320

Al,

16

Oj Fe,

5 8.317.0 8.916.0 7.2121521

7

3 4.73 4.50 5.85 1.5

Oj TiO, CaO

11

00 1.8903 2.02

0.68 1.550.70 1.1000

58 0.4149 0.74

0.43 0.24

element content of the fine ear

Cu

15212821

5<512

Zn

25212634161918

Cr

704060

127322527

Co Ni

23 1435 1915 1019 14

<2 37 10

19 18

MgO K,

100-50

7.15.94.83.85.15.24.7

C.E.C.

15.657.158.96

15.3315.7816.1318.81

weight)

O Na,O

2.81 1.65 1.012.37 1.73 0.164.14 2.31 0.120.48 0.00 0.000.60 4. 8 0.891.92 2.64 0.880.87 3.62 0.34

th (in ppm)

Ba

380<100

800380

<100<100<100

Elemental composition of the clay fraction (%

SiO,

54.454.455.6

A1,O3

29.327.827.3

Fe,C

9.410.6

9.9

3j TiO,

1.441.831.06

Mineralogy of the clay fraction

K

+ +

++++

+++++ +

I

+

+++++

+++

Mt

+++++

Mi

CaO

1.310.091.85

V Chi Q

trtr trtr tr

XX

XX

X

X

X

XX

XX

Sr

323020

666

12

ay weight)

MgO

1.361.731.82

Molar

SiO,

Al,O

3.2

3.3

3.5

% weightSilt

50- 2020 2

11.7 194.4 83.1 54.4 4

Clay

< 2

4 29.49 15.69 21.60 35.0

4.6 4.2 35.54.8 4.4 34.05.9 3.5 39.2

Basesat. %•

7461779899

10099

P,O,

0.120.040.070.050.050.080.14

SiO,

Al.Oj

7.77.48.1

11.99.15.9

19.4

K , O

2.212.951.73

ratios

SiO,

R,O 3

2.61

2.68

2.81

pH 1:

H : O

6.56.27.28.58.88.78.8

Organic matter

% C % N

1.84 0 170.80 0.050.350.170.170.110.15

Sum

108.4107.4108.7105.6108.3106.6100.8

SiO,

R,O 3

5.855.566.319.607.675.02

17.18

Na ,0

0.000.000.00

AI,O

Fc,O

4.9

4.1

4.3

5

KCI

5.24.65.36.66.86.86.6

P ; O ,

16751133

Loss onignition %

9.44.35.5

10.37.44.27.7

A1,O

Fe,O

3.13.03.54.05.35.77.7

0

3

3

o.

570.720

i

74

X

1

I1

i

J

jI

J1

211

DESCRIPTION OF PROFILE NO. 16

Al 0 - 1 2 cm Dark brown (7.5YR3/2) moist, pinkish grey (7.5YR6/2)dry, slightly gravelly coarse sand; structureless, singlegrained; soft dry, loose moist, non-sticky non-plasticwet; many very fine and fine roots; smooth gradualboundary; (sample 16.131),

B21 1 2 - 46 cm brown (7.5YRR4/4) moist, light brown (7.5YR6/4) dry,gravelly loamy coarse sand; consistence as horizonabove; common fine medium and thick roots; commonfine pores; coarse fragments increasing with depth; grad-ual wavy boundary; (sample 16.132),

B22 46 - 67 cm brown (7.5YR5/4) moist, light brown (7.5YR6/4) dry,gravelly loamy coarse to very coarse sand; structureless,single grained; slightly hard dry, loose moist, non-sticky,non-plastic wet; common fine and medium roots; strongwavy boundary varying between 60 and 95 cm depth;(sample 16.133),

B3 67 - 98 cm brown (7.5YR5/3) moist, pinkish grey (7.5YR7/2) dry,gravelly loamy coarse sand; with common prominentmedium and coarse dark red (2.5YR3/6) and black(2.5YR2/0) concretions; few fine medium roots; gradualwavy boundary; (sample 16.134),

C 98 — 130+cm partly decomposed very hard, coarse grained graniterock of a very gravelly loamy coarse sand texture;(sample 16.135).

212

Laboratory data of profile No. 16

SampleNo.

16.13116.13216.13316.13416.135

SampleNo.

16.13116.13216.13316.13416.135

SampleNo.

16.13116.13216.13316.13416.135

SampleNo.

16.13116.13216.13316.13416.135

SampleNo.

16.135

SampleNo.

16.13116.13216.13316.13416.135

Particle size distribution (ß) inSand

Depth Hori- %>2 2000- 500-in cm zon mm. 500 200

0- 12 Al 1412- 46 B21 1746- 67 B22 1867- 98 B3 2598130 C 86

.1 38.8 24.3

.2 43.6 21.7

.9 39.8 25.7

.0 28.9 35.4

.7 52.7 23.8

Exchangeable cations in meq/100 g

Ca Mg K

tr 0.12 0.08tr 0.13 0.08tr 0.20 0.08tr 0.20 0.08tr 0.26 0.13

Elemental composition

SiO2 A12O3 Fe2O3

86.5 8.4 0.885.7 8.4 0.986.0 9.1 0.983.6 9.0 1.279.9 11.5 2.7

Trace element content

Mn Cu Zn Cr

66 < 5 <5 7630 < 5 < 5 <5

<10 < 5 < 5 60276 < 5 < 5 52280 20 8 20

Elemental composition

SiO2 A12O3 Fe

Na Sum

tr 0.20tr 0.21tr 0.28tr 0.28tr 0.39

of the fine eartr

200-100

15.114.313.112.64.7

C.

1.1.1.1.1.

l (% by

TiO2 CaO MgO K

0.18 0.14 0.0.20 0.13 0.0.19 0.15 0.0.27 0.12 0.0.31 0.23 0.

of the fine earth

Co Ni

<2 1218 3420 3816 <250 14

22 4.29 4.46 4.00 5.,42 5.

100-50

97763

.1

.3

.0

.7

.8

% weightSilt Clay

50-20

5.54.84.55.14.3

E.C. Base

7575313175

sat.

1 112212122

weight)

,o

9493561622

(in ppm)

Ba

760720720720720

of the clay fraction (%

2 O 3 TiO2

47.0 26.5 6.8 1.30

Mineralogy of the clay

K I Mt Mi

+ ++ tr

fraction

V Chi

CaO

6.65

Q

XX

XX

XX

XX

XX

by

Na2O

1.530.000.000.662.96

Sr

316

<21019

weight)

MgO

0.53

Molar

SiO2

A12O3

3.0

. 7o

P 2 O :

0.050.050.100.030.02

SiO2

A12O3

17.517.316.115.711.7

K,

20- < 22

2.4 4.83.1 5.24.1 5.84.1 7.24.3 6.4

pH 1:

H2O

5.75.65.75.75.7

Organic matter

%C %

0.29 0.10.22 0.10.140.170.11

; Sum

102.8100.6101.2100.1102.4

SiO2

R : O 3

16.5416.2515.1914.5010.15

O Na2O

2.44 0.29

ratios

SiO, A12O

R2

2.!

O3 Fe2O

i9 6.2

N

3432

5

KC1

4.64.44.54.44.4

P , O S

ppm

1tr1trtr

Loss onignition %

1.61.51.62.22.0

A12O

Fe,O

16.515.316.411.96.8

P,

8.

3

3

3

3

os

50

213

DESCRIPTION OF PROFILE NO. 17

Al l 0 - 15 cm Very dark greyish brown (10YR3/2) moist, greyishbrown (10YR5/2) dry, loamy coarse sand; structureless,single grained; soft dry, friable moist, non-sticky andnon-plastic when wet; common fine pores; many veryfine and fine and medium roots; common coarsefragments of gravel; smooth gradual boundary; (sam-ple 17.151),

A12 15 — 34 cm very dark greyish brown (10YR3/2) moist, brown(10YR5/3) dry, slightly gravelly loamy coarse sand;structureless, single grained; non-sticky and non-plasticwhen wet; common fine pores; many very fine, fine andfew medium and coarse roots; abrupt wavy boundary;(sample 17.152),

(II)C 34— 45+cm coarse fragments of granite and quartz gravel.

214

Laboratory data of profile No. 17

SampleNo.

17.5117.52

SampleNo.

17.5117.52

SampleNo.

17.5117.52

SampleNo.

17.5117.52

SampleNo.

17.52

SampleNo.

17.5117.52

Depthin cm

0-1515-34

Hori-zon

Al lA12

Particle size distribution (M) inSand

%>2 2000- 500- 200- 100-mm. 500 200 100 50

5.4 34.9 23.4 18.5 9,11.8 28.5 25.0 21.1 6

Exchangeable cations in meq/100 g

Ca

1.051.26

Mg

0.070.12

K Na Sum

0.13 0.17 1.420.21 0.17 1.76

,8.6

C.E.C.

3.143.14

Elemental composition of the fine earth (% by weight)

SiO2

84.985.3

Trace

Mn

10824

A12O, Fe2

8.5 1.010.7 1.2

O3 TiO, CaO MgO K2O

0.25 0.20 0.66 4.710.40 0.24 0.04 5.23

element content of the fine earth (in ppm)

Cu Zn

< 5 29 6

Elemental composil

SiO2

51.4

A12O3

26.9

Cr Co Ni Ba

18 15 6 <10040 15 24 <100

tion of the clay fraction (% by

F C J O J TiO2 CaO

7.3 1.31 4.48

Mineralogy of the clay fraction

K

++++

I Mt

++

Mi V Chi Q

XX

XX

Na2O

0.490.89

Sr

66

weight)

MgO

0.58

Molar

SiO2

A12O,

3.2

% weightSilt

50- 20-20 2

5.9 2.58.4 3.3

Basesat. %

4556

P2OS

0.080.10

SiO2

A12O3

16.913.5

K 2 O

2.71

ratios

SiO2

, R 2 O 3

2.76

Clay

< 2

5.07.1

Organic

%C

0.340.29

Sum

100.8104.1

SiO2

R2O3

15.7812.60

Na2(

0.54

pH 1:

H 2 O

6.1.6.3

matter

% N

0.040.03

5

KC1

4.85.1

P2O5

ppm

97

Loss onignition %

1.72.0

A12O

Fe2C

14.114.2

3 P.

4.

A!2O3

Fe,(

5.7

3

P3

.78

215

DESCRIPTION OF PROFILE NO. 18

Ap 0 - 1 5 cm Dark reddish brown (7.5YR3/2) moist, greyish brown(10YR5/3) dry, coarse sandy loam; structureless, mas-sive; very hard dry, slightly friable moist, non-sticky,nonplastic wet; common fine tubular pores; many veryfine and fine roots; common ant activity; smoothgradual boundary; (sample 18.136),

B21 1 5 - 36 cm dark brown (7.5YR3/2) moist, brown (7.5YR4.5/2) dry,coarse sandy loam; structureless massive; very hard dry,friable moist, slightly sticky and slightly plastic wet;common fine and medium roots; common fine andmedium tubular pores; common ant activity; smoothclear boundary; (sample 18.137),

B22 36 — 55 cm brown (7.5YR4.5/4) moist and dry sandy clay loam;structureless, massive; very hard dry, firm moist, slightlysticky and plastic wet; common distinct fine andmedium dark red (2.5YR3/6) mottling in the lower partof the horizon and a few bluish black mottles; commoncoarse fragments; common pores; few fine and mediumroots; clear wavy boundary; (sample 18.138),

B2g 55 — 104 cm greyish brown (10YR5/2) moist, light brownish grey(10YR6/2) dry, sandy clay; moderate to weak mediumcolumnar structure; extremely hard dry, firm moist,slightly sticky and plastic wet; common fine distinctdark red (2.5YR3/6), medium yellowish red (5YR5/6)mottles; many fine and medium strong brown(7.5YR5/6) mottles; few fine bluish black mottles;common large thin cutans; very few fine roots; gradualin some parts clear boundary; (sample 18.139),

B3 104 - J43 cm brown (10YR5/3) moist and grey (10YR5/1) dry,slightly gravelly sandy clay; common distinct mediumyellowish red (5YR4/6) and strong brown (7.5YR5/6)mottles; occasionally bluish black mottling; commonmedium and large thick cutans; few pores; clear in someparts gradual wavy boundary; (sample 18.140),

C 143 — 160+cm partly decomposed hard, coarse grained granitic rock ofvery gravelly sandy clay loam texture; (sample 18.141).

216

Laboratory data of profile No. 18

Sample

18.13618.13718.13818.13918.14018.141

SampleNo.

18.13618.13718.13818.13918.14018.141

SampleNo.

18.13618.13718.13818.13918.14018.141

SampleNo.

18.13618.13718.13818.13918.14018.141

Depth Hori-

0-15-36-55-

15 An36 B2155 B22

104 B2g104-143 B3143- 160 C

% > 2

1.40.63.64.13.2

57.8

Particle size distribution (M) inSand

2000- 500- 200 100-

33.2 20.7 13.3 8.333.7 18.2 10.6 6.030.8 16.4 11.0 6.223.0 12.4 8.9 5.922.5 11.9 8.5 5.026.4 12.2 7.3 5.4

Exchangeablc cations in ineq/1 00 g

Ca

1.411.662.446.917.438.50

Mg

0.671.672.214.755.986.69

K

0.440.250.240.450.450.48

Na Sum C.E.C.

tr 2.52 5.26tr 3.58 6.62tr 4.89 7.530.14 12.25 14.810.18 14.04 16.620.18 15.85 16.70

Elemental composition of the fine earth (% by weight)

SiO,

86.981.581.973.474.474.8

A12O3 Fc,<

7.7 1.911.1 3.111.8 3.715.1 4.815.6 5.014.7 4.9

0, T iO; CaO MgO K,O Na,O

0.39 0.25 1.79 3.59 0.000. 57 0.30 2.09 3.55 0.000.55 0.25 0.45 3.27 0.530. 59 0.36 2.32 3.05 0.470.62 0.46 2.68 3.00 0.330.

Trace element content of

Mn

216150124

86194280

Cu Zn

<5 <528 638 2126 2320 2026 11

Cr

4696

352228

52120

56 0.46 1.78 3.04 0.00

the fine earth {in ppm)

Co Ni Ba Sr

20 4 560 1126 10 600 618 104 600 1326 64 520 1318 36 400 <216 40 560 11

Elemental composition of the clay fraction (% by weight)

% weight)Silt

50-

6.25.22.54.04.03.2

Base

sat. "/.

485465838595

P,O,

0.100.130.060.100.130.03

SiO,

R,O3

19.212.411.8

8.28.18.6

20-

'

7.6.7.5.6.6.

b

062971

Clay

< 2

11.319.725.939.941.440.4

Organic

% C

0.580.380.130.300.170.12

Sum

102.6102.4102.4100.2102.399.7

SiO,

R2O3

16.5610.52

9.866.866.717.13

pH 1:

H , 0

6.26.16.26.97.57.3

matter

% N

0.050.04

5

KC1

5.24.94.75.46.46.3

P,O5

ppm

1trtrtrtrtr

Loss onignition %

3.44.24.97.57.97.7

AljO

Fe,0

6.35.55.14.94.94.8

3

3

Sample SiO2

No.

18.13618.13718.13818.13918.14018.141

A1,O3 Fe,O3 TiO, CaO MgO Na,O P,OS

18.18.18.

136138141

51.452.654.5

24.829.028.0

9.510.610.1

1.451.341.03

4.731.552.25

0.941.481.61

2.291.971.45

0.000.000.03

4.931.450.97

Mineralogy of the ctay fraction Molar ratios

Sample K 1No.

Mt Mi Chi SiO,

AI,O3

SiO AI,O3

Fe,O3

3.5

3.1

3.3

2.83

2.50

2.68

4.1

4.3

4.3

217

DESCRIPTION OF PROFILE NO. 19

Al 0 - 15 cm Dark reddish brown (5YR3/4) moist, brown (7.5YR5/4) •dry, clay loam; apedal; soft dry, friable moist, sticky andplastic wet; common fine roots; (sample 19.109),

R 15+ cm blocked by dolerite.

DESCRIPTION OF PROFILE NO. 20

Al l 0 - 30 cm Dark reddish brown (5YR3/4) moist and dry, ;finegravelly coarse sandy loam; loose, soft, non-stickynon-plastic; common fine roots; gradual boundary;(sample 20.107),

A12 30 - 65 cm dark reddish brown (5YR3/3) moist and (5YR3/4) dry,gravelly coarse sandy loam; consistence as above; in-crease of fine angular gravel with depth; (sample20.108),

(II)C 65+ cm blocked by gravel.

218

Laboratory data of profile No. 19 and No. 20

Particle size distribution (ß) in % weightSand Silt Clay

Sample Depth Hori-No. in cm. zon

%>2 2000- 500- 200- 100-mm. 500 200 100 50

50-20

20-2

< 7 pH 1:5

H2O KC1

19.20.20.

109107108

0-150-30

30-68

A lA l lA12

t r

34.743.8

23543

.4

.3

.0

6.521.519.5

1313

9

.0

.1

.5

7.6.4.

953

10.3.3.

524

19.85.15.0

39.915.315.3

6.77.27.1

5.45.65.3

Exchangeable cations in meq/100 g

Sample Ca Mg K Na Sum C.E.C. Base Organic matterNo. sa t .% ^ ^ J - ppm

19.10920.10720.108

13.005.575.07

2.741.531.70

2.930.410.31

tr

tr

tr

18.677.517.08

27.139.648.10

697887

1.280.450.34

0.220.060.04

23525

7

Elemental composition of the fine earth (% by weight)

Sample SiO2 A12O3 Fe2O3 TiO2 CaO MgO K2O Na2O P2O5 SumNo.

Los onignition '

19.10920.10720.108

SampleNo.

19:10920.10720.108

SampleNo.

19.10920.107

63.068.073.9

20.15.17.

147

10.8.8.

.9 1.

.1 0.

.2 1.

.24

.94

.06

Trace element content of the

Mn

1580560450

Cu

613120

Elemental

SiOj

51.046.8

Zn

1254834

Cr

100112156

composition ol

Al.

28.26.

t O 3

.4

.7

Co

1818

4

• t h e

Fe2O3

14.715.9

1.32 3.1.97 2.2.02 1.

fine earth

Ni

1642038

.38 2.04

.32 3.86

.32 4.29

i (in ppm)

Ba

200760760

clay fraction (% by

TiO2

0.771.79'

CaO

1.652.36

0.281.550.58

Sr

10250155

weight)

MgO

1.453.14

0.0.0.

SiO

Al2

5.37.57.1

422012

K 2 O

1.172.24

102.7102.4109.1

SiO2

R2O3

3.945.605.48

Na2O

0.000.00

12.94.44.6

A12O3

Fe2O3

2.93.03.4

P2OS

0.892.21

Mineralogy of the clay fraction Molar ratios

Sample KNo.

1 Mt Mi Chi Q SiO2 SiO2

A12O3 R2O3 Fe,O3

19.109 ++ +20.107 + + +20.108 + + +

tr

tr

tr

X

X

X

3.13.0

2.292.16

3.02.6

219

DESCRIPTION OF PROFILE NO. 21

Al 0 - 3 3 cm Reddish brown (5YR4/3) moist and (5YR4/4) dry,coarse sand; structureless, single grained; soft dry, loosemoist, non-sticky, non-plastic wet; common very fine,fine and few medium roots; few termite holes; smoothgradual boundary; (sample 21.96),

B21 33 — 80 cm yellowish red (5YR4/6) moist and dry, loamy coarsesand; structureless, single ,grained; common fine andmedium tubular pores; consistence as horizon above; fewvery fine, fine and occasional medium roots; smoothgradual boundary; (sample 21.97),

B22 80 - 129 cm dark reddish brown (2.5YR3/4) moist, yellowish red(5YR4/8) dry, coarse sandy loam; structureless, massive;slightly hard dry, friable moist, non-sticky, non-plasticwet; common very fine, fine and few medium roots;common fine tubular pores; smooth gradual boundary;(sample 21.98),

B3 129 - 170 cm reddish brown (2.5YR3/6) moist, yellowish red(5YR5/6) dry, coarse sandy loam; structure and consis-tence as horizon above; coarse fragments increasing withdepth; very few fine roots; common fine and mediumtubular pores; abrupt smooth boundary; (sample 21.99),

(II)C 170+ cm extremely hard mottled granite.

220

Laboratory data of Profile No. 21

SampleNo.

21.9621.9721.9821.99

Sample

No.

21.9621.9721.9821.99

SampleNo.

21.9621.9721.9821.99

SampleNo.

21.9621.9721.9821.99

SampleNo.

21.9621.9721.99

Sample

No.

21.9621.9721.9821.99

Depthin cm

0- 3333- 8090-129

129-170

Hori-zon

AlB21B22B3

Particle size distribution (M) in % weightSand Silt Clay

%>2 2000- 500- 200-mm. 500 200 100

1.1 36.3 22.1.1 33.4 26.2.2 29.8 22.3.3 33.6 20.

Exchangeable cations in meq/100 g

Ca

1.261.261.261.26

Elemental

SiO2 Al :

90.8 5.87.9 9.82.2 10.82.7 9.

Mg

0.070.030.230.20

K Na

0.26 tr0.31 tr0.31 0.180.25 tr

5 20.89 16.83 16.40 15.6

Sum

1.591.601.981.71

composition of the fine earth (% by

O3 Fe2<

9 1.91 2.81 2.02 2.9

0, TiO2 CaO

0-25 0.130.56 0.140.51 0.160.57 0.17

MgO K

100-50

9.87.09.28.7

C.E.C.

3.914.354.784.78

weight)

50-20

2.52.43.33.9

Base

sat. "A

41374136

2O Na2O P

0.19 3.29 0.960.55 3.31 0.001.12 3.53 0.950.65 2.83 0.64

Trace element content of the fine earth (in ppm)

Mn Cu

212 24184 30156 37166 30

Elemental

SiO2

48.549.548.4

Zn

61337

9

Cr Co Ni

150 6 22156 6 29136 14 24150 < 2 4

Ba

900720540540

composition of the clay fraction (%

A12O3

30.732.932.5

Mineralogy of the c

K I

H :

Mt

Fe2O3 TiO2

10.5 1.4211.0 1.3610.1 1.25

lay fraction

CaO

3.061.232.61

Mi V Chi Q

tr

X

X

X

X

Sr

141113

<2

20- < 22

0.6 7.51.2 12.32.1 16.91.9 16.3

pH 1:5

H2O KC1

6.36.36.36.4

Organic matter

° % C % N

0.16 0.030.18 0.030.050.15

jO s Sum

0.02 103.50.00 104.30.10 100.70.08 99.7

SiC

Al2

26.16.13.15.

by weight)

MgO

0.740.801.10

)2 SiO2

O3 R 2 O 3

6 21.665 13.818 12.253 12.79

K2O Na2O

1.81 0.001.92 0.001.39 0.00

Molar ratios

SiO2

A12O3

2.72.6

2.5

SiO2 A12O;

R 2 O 3 Fe2O

2.21 4.62.10 4.7

2.11 5.0

4.74.74.74.8

P2OS

ppm

6312

Loss onignition %

2.13.13.53.2

Al,

Fe2

4.85.17.85.0

>

3

o3

o3

P2OS

3.321.212.66

221

DESCRIPTION OF PROFILE NO. 22

Al l 0 — 2 cm Very dark greyish brown (10YR3/2) moist, sandy clay;thin brittle crust; the material has a weak fine crumbstructure; soft dry, friable moist, sticky and plastic wet;abrupt wavy boundary; (sample 22.90),

A12 2 - 28 cm very dark grey (10YR3/1) moist and dry, clay with somecoarse fragments; largest structural elements are weakcoarse prisms which break down into weak coarsesubangular blocky; very hard dry, firm moist, slightlysticky, slightly plastic wet; common fine and mediumroots; few tubular pores; common vertical cracks up to2.5 cm wide; cracks take 12% of the volume; few finecalcium carbonate concretions; wavy gradual boundary;(sample 22.91),

A13 28 — 64 cm very dark grey (10YR3/1) moist and dry, clay with anappreciable amount of coarse sand and fine grit; struc-tureless, massive; in some parts however, fine andmedium wedge shaped elements; very hard dry, firmmoist, slightly sticky, slightly plastic wet; commoncracks, some are 1 cm wide at 50 cm depth; commonfine calcium carbonate concretions; common very fine,fine and few medium and thick roots; gradual smoothboundary; (sample 22.92),

Aca 64 — 97 cm very dark grey (10YR3/1) moist and dry, clay; commonfine and medium pressure faces; consistence and coarsefragments as horizon above; few fine and very fine roots;common fine cracks up to 0.5 cm wide; common finecalcium carbonate concretions; very few fine pores;smooth gradual boundary; (sample 22.93),

Aca 97 — 127 cm very dark grey (10YR3/1) moist and dry, clay; massive,structureless; very hard dry, firm moist, slightly sticky,slightly plastic wet; very few fine roots; calcareous;common pressure faces; few small slickensides; smoothgradual boundary; (sample 22.94),

IIC 127 — 170 cm dark reddish brown (5YR3/4) moist, clay; structureless,maassive; hard dry, friable moist, slightly sticky andplastic wet; highly calcareous, many medium soft cal-cium carbonate accumulations; common fine distinctbluish mottling; common shiny pressure faces and coarsefragments; occasional fine roots; (sample 22.95),

170+ cm blocked by coarse gravel.

222

Laboratory data of profile No. 22

Sample

No.

22.9022.9122.9222.9322.9422.95

Sample

22.9022.9122.9222.9322.9422.95

SampleNo.

22.9022.9122.9222.9322.9422.95

SampleNo.

22.9022.9122.9222.9322.9422.95

Depth

in cm

0-2-

28-64-

2286497

97-127127-170

Hori-

zon

A l lA12A13AcaAcaIIC

Particle size distribution (ß) inSand

% > 2 2000- 500- 200- 100-

mm. 500

0.9 14.51.6 9.1.9 11.2.0 14.2.6 9.

3013

3.2 10.0

ZUU 1UO

7.3 9.6 67.8 11.2 67.8 8.8 68.3 8.6 57.4 10.5 6

5U

.0

.1

.7

.9

.27.0 9.7 4.3

Exchangeable cations in meq/100 g

Ca

22.2423.5119.8517.5015.3716.15

Mg

9.11.12.14.15.17.

303789184108

K Na

1.34 0.090.81 0.180.65 1.110.60 1.290.66 2.220.66 2.22

Elemental composition of the

SiO,

70.071.271.571.069.068.4

Trace

Mn

540590650670786640

AI ,O 3 A1,O3 TiO,

15.216.515.415.015.015.9

6,6

.3 0.78

.8 0.776.6 0.726,.3 0.706.6 0.716,.8 0.73

Sum C.E.C.

32.97 35.4835.87 39.4734.50 37.3233.57 35.9133.66 32.8736.11 38.50

Base

sat.%

93919294

100• 94

fine earth (% by weight)

C;aO MgO K,O

1.60 1.70 2.901.1.1.2.1.

70 1.91 2.8758 2.48 2.8151 1.67 2.6937 2.92 2.5996 2.58 2.82

element content of the fine earth (in ppm)

Cu

315542423858

Zn

405260404152

Cr Co

50 2760 35

180 58180 44136 20224 30

Elemental composition of the <

Ni Ba

47 80050 < 1 0 040 54044 72055 54068 720

clay fraction (% by

N a , O

1.370.001.941.021.251.42

Sr

353223242436

weight)

% weightSilt

50-

20

8.57.67.66.68.06.6

20-

Clay

< 2

10.6 43.10. 5 47.10.9 47.10.14.11.

1

5.5.2

,5 46.0,3 44.,2 51

Organic matter

% C

0.600.500.550.530.160.41

P ,O,

0.080.050.050.030.090.11

SiO,

A1,O,

7.87.57.98.07.87.3

% N

0.090.09

Sum

100.0101.5103.1100.0100.5100.7

SiO,

R,O 3

6.185.906.216.336.105.74

.3

.2

pH 1:

H , 0

8.28.28.48.28.58.3

P2OS

17343tr2

5

KC1

6.66.86.96.97.27.1

CaCOj

%

0.640.520.690.832.950.99

Loss onignition %

10.711.711.811.412.112.3

Al,0

Fe,O

3.83.73.73.73.63.7

3

3

Sample SiO,No.

SampleNo.

22.9022.9122.9222.9322.9422.95

AI,O, Fe,O, CaO MgO K , O Na,O P,O,

22.9022.9222.95

55.555.756.1

242423

.32

.4

12.012.212.0

0.760.780.75

2.652.392.68

2.762.482.98

1.231.631.27

0.000.000.00

0.730.660.70

Mineralogy of the clay fraction

K 1 Mt Mi V

Molar ratios

Chi SiO, SiO,

A1,O, R , O ,

AI,O,

Fe,O,

3.9

3.9

4.1

2.94

2.95

3.06

3.2

3.1

3.1

223

DESCRIPTION OF PROFILE NO. 23

Al 0 - 1 2 cm Dusky red (2.5YR3/2) moist, dark reddish brown(5YR3/3) dry, sandy clay; weak medium subangularblocky structure; hard dry, friable, slightly sticky andplastic wet; common fine and medium roots; few finepores; common very fine cracks 1 mm wide; wavygradual boundary; (sample 23.85),

B21 1 2 - 40 cm dark reddish brown (2.5YR2/4) moist and dry, clay;apedal; extremely hard dry, firm moist, slightly stickyand plastic wet; common fine, medium and few thickroots; few very fine cracks; common small brokencutans; very few fine tubular pores; appreciable amountof coarse fragments; smooth gradual boundary; (sample23.86),

B2t 40 - 76 cm dusky red (10R3/4) moist, dark reddish brown(2.5YR3/4) dry, clay with common coarse fragments;structureless, massive; few fine medium and thick roots;consistence as horizon above; very few very fine cracks;few medium tubular pores; common fine thin brokencutans; smooth gradual boundary; (sample 23.87),

B23 76 - 122 cm dusky red (10R3/4) moist, dark reddish brown(2.5YR3/4) dry, clay; amount of very fine gravel andcoarse sand increasing with depth; structureless, massive;hard dry, slightly friable moist, very slightly sticky andslightly plastic wet; few fine and medium roots; few fineand medium tubular pores; smooth clear boundary;(sample 23.88),

B3 122 — 160 cm dark red (2.5YR3/6) moist, fine gravelly sandy clay;structureless, massive; hard dry, firm moist, slightlysticky and slightly plastic wet; very few fine, mediumand thick roots; (sample 23.89),

B3 160 — 175 cm as horizon above,

175+ cm blockage by extremely hard calcrete.

224

Laboratory data of profile No. 23

SampleNo.

23.8523.8623.8723.8823.89

SampleNo.

23.8523.8623.8723.8823.89

SampleNo.

23.8523.8623.8723.8823.89

SampleNo.

23.8523.8623.8723.8823.89

Depthin cm.

0-12-4a

i

124076

76-122122-160

Hori-zon

A lB21B2tB23B3

Particle size distribution (»i) in % weightSand

%>2 2000- 500- 200- 100-

mm. 500 200 100 r

0.8 21.3.0 153.0 17.3.8 19.

40.0 22.

.0 9.9

.3 8.7

.6 8.1

.1 7.3

.0 8.5

Exchangeable cations in meq/100 g

Ca

10.8510.06

9.659.13

12.18

Mg

33,322

.81

.77

.47

.95

.73

K

1.411.020.750.700.80

Elemental composition of the

SiO2

70.274.170.175.373.2

Al,

14.18.19.18.16.

,C

931.2.2

»3 ?

6.7.7.6.5.

0349.8

i, TiOj

0.730.790.760.740.66

Trace element content of the

Mn

590490480450472

Cu

1834312438

Elemental

Zn

4043321931

Cr Co

41 3741 4870 1960 1560 31

composition of the

Na

0.090.09trtr0.09

10.9 5.10.6 5.

3U

.4,6

9.4 5.610.3 5.10.1 5.

Sum

16.1614.9413.8712.7815.80

81

C.E.C.

21.1819.5019.4018.1118.94

fine earth (% by weight)

CaO

0.610.500.480.460.54

fine ear

Ni

6773503554

MgO K,O

1.41 2.910.00 2.910.87 2.800.50 3.040.84 3.31

th (in ppm)

Ba

<100<100<100<100

380

clay fraction (% by

Na,O

0.271.482.080.000.00

Sr

121219

616

weight)

Silt

50-

20

4.95.35.16.16.2

Basesat.9!

7677717183

P,O,

0.120.080.030.060.03

SiOj

A1,O3

8.46.56.27.07.7

20-2

8.15.44.64.55.0

t

Clay

< 2

39.849.149.646.943.1

Organic

% C

1.260.640.490.350.37

Sum

101.0101.5103.7105.1100.6

SiO,

R3O3

6.715.204.995.676.23

pH 1:

H 2 O

6.96.46.46.67.0

matter

% N

0.140.10

5

KCL

5.24.94.84.95.3

P2O5

1112trtr

Loss onignition %

10.310.911.010.0

8.5

A12O

Fe,O

3.93.94.04.24.4

3

3

Sample SiO2 A12O3 Fe,O3 TiO, CaO MgO K,O Na,O P2CNo.

23.23.23.

858789

51.349.550.4

28.531.030.2

13.813.513.4

1.200.930.96

111

.48

.19

.64

111

.51

.63

.35

111

.58

.48

.17

0.000.000.00

0.620.800.90

Mineralogy of the clay fraction Molar ratios

Sample K I Mt Mi V Chi Q SiO2 SiO, A12O3N o- ÄÜÖ", R^Ö"3 Fe,O3

23.85 ++ + + x 3.1 2.33 3.223.86 ++ + + x23.87 ++ + + x 2.7 2.12 3.623.88 ++ + + x23.89 ++ + + x 2.8 2.20 3.5

225

DESCRIPTION OF PROFILE NO. 24

Al l 0 — 19 cm Dark brown (7.5YR3/2) moist and dry, sandy clayloam: very weak subangular blocky structure; slightlyhard dry, firm moist, slightly sticky and slightly plasticwet; many fine, medium and a few very coarse horizon-tal roots at the base of the horizon; common very finecracks; common fine pores; clear wavy boundary;(sample 24.81),

A12 19 — 45 cm dark brown (7.5YR3/2) moist and dry, sandy clay loamwith an appreciable amount of coarse sand and a littlefine gravel; medium coarse prismatic structure; very harddry, firm moist, slightly sticky and slightly plastic wet;common cutans on the vertical prismatic faces; commoncracks often 1 cm wide; common fine hard calciumcarbonate concretions; common very fine and few finemedium roots; few coarse tubular pores; clear wavyboundary; (sample 24.82),

ACca 45 — 76 cm very dark greyish brown (10YR3/2) moist and dry,slightly gravelly sandy clay loam; weak coarse angularblocky structure; common medium broken cutans onthe ped faces; extremely hard dry, firm moist, slightlysticky and slightly plastic wet; very few very fine roots;very few medium pores; few cracks, some 1 cm wide at50 cm depth; many fine and medium calcium carbonateconcretions; abrupt clear boundary; (sample 24.83),

ACca 76 — 170 cm brown (7.5YR5/4) moist, slightly gravelly coarse sandyloam; highly calcareous; fairly hard dry, friable moist,non-sticky, non-plastic wet; few fine roots; abundantmedium and coarse subangular calcrete fragments; (sam-ple 24.84),

170+ cm blockage by extremely hard calcrete.

226

Laboratory data of profile No. 24

SampleNo.

24.8124.8224.8324.84

SampleNo.

24.8124.8224.8324.84

SampleNo.

24.8124.8224.8324.84

SampleNo.

24.8124.8224.8324.84

Particle size distribution (p) inSand

Depth Hori- %>2 2000- 500- 200- 100-in cm. zon mm. 500 200 100 50

0- 19 Al l 2.6 28.5 10.4 11.2 8.019- 45 A12 3.9 28.2 8.0 10.8 6.545- 76 ACca 4.4 22.2 7.8 10.4 5.976-170 ACca 8.3 30.0 17.4 12.8 5.6

Exchangeable cations in meq/100 g

Ca Mg K Na Sum C.E.C. Base

sat. %

20.72 7.25 0.75 0.18 28.90 32.94 8821.40 11.12 0.38 0.18 33.08 33.11 10021.41 14.21 0.43 0.18 36.23 35.88 10012.85 13.11 0.43 0.37 26.76 25.10 100

Elemental composition of the fine earth {% by weight)

SiO2 A12O3 Fe2O3 TiO2 CaO MgO K2O Na2O

79.3 .11.7 3.8 0.57 0.26 0.60 3.24 0.0077.0 11.5 4.6 0.66 2.32 1.66 2.07 0.7875.9 l t .8 4.9 0.71 4.13 1.31 2.01 0.0069.3 11.6 4.0 0.52 8.93 1.89 2.68 2.29

Trace element content of the fine earth (in ppm)

Mn Cu Zn Cr Co Ni Ba Sr

432 18 25 120 31 18 <100 18458 9 12 32 12 20 <100 3464 24 23 29 31 54 <100 26400 31 27 50 17 40 1720 53

% weightSilt

50- 20-20 2

Clay

<2

6.7 7.0 28.26.9 7.1 32.57.6 11.8 34.39.5 8.7 16.0

Organic

%C

0.580.430.350.16

P2O5

0.050.090.030.12

SiOj

A12O3

11.511.410.910.2

matter

% N

0.110.07

Sum

99.5100.6100.8101.3

SiO,

R2O3

9.539.098.608.31

pH 1:5

H2O KC1

8.2 6.88.6 7.28.7 7.39.0 7.5

P2O, CaCo3

ppm

23 2.70tr 5.102 14.07

Loss onignition %

10.111.112.714.1

A12O3

Fe2O3

4.83.93.84.5

SampleNo.

24.8124.8224.84

Elemental

SiO2

57.158.456.8

composition of the

A1,O3

23.823.119.5

Fe2O3

11.110.510.0

clay fraction (%

TiO,

0.841.020.68

CaO

3.012.544.52

by weight)

MgO

2.852.504.24

K j O

1.021.160.95

Na2O

0.000.220.82

0.0.2.

o5

395752

SampleNo.

24.8124.8224.8324.84

Mineralogy of the clay fraction Molar ratios

I Mt Mi Chi SiO2

A12O3 Fe2O3

trtrtrtr

trtrtrtr

xxXXXX

4.14.3

4.9

3.143.32

3.72

3.43.4

. 3.1

227

DESCRIPTION OF PROFILE NO. 25

A l l 0 - 3 cm Very dark grey (10YR3/1) moist, clay; soft dry, friablemoist, sticky and plastic wet; very thin brittle crustwhich overlies fine granular mulch; common coarse sandgrains; wavy abrupt boundary; (sample 25.75),

A12 3 — 33 cm very dark grey (10YR3/1) moist and dry, clay loam;largest structural elements are coarse prisms determinedby the cracks; cracks are 1 cm wide at the base of thehorizon; moderate medium angular blocky structure,with a few wedged shaped elements in the lower part ofthe horizon; very hard dry, firm moist, sticky and plasticwet; common fine and medium roots; very few finepores; cracks take 10% of the volume; smooth gradualboundary; (sample 25.76),

A13 33 — 67 cm very dark grey (10YR3/1) moist and dry, clay; commoncoarse sand grains; very weak fine wedged shapedstructure; few fine pressure faces; consistence as horizonabove; few very fine and fine roots; few medium hardcalcium carbonate nodules; common fine cracks of 0.5 cmwide; smooth gradual boundary; (sample 25.77),

ACca 67 — 96 cm black (10YR2/1) moist and dry, clay loam; with somecoarse fragments in some parts of the horizon; massive ,apedal structure; few very fine roots; common finecracks less than 0.5 cm wide; common small pressurefaces; very few small and medium calcium carbonateconcretions; smooth gradual boundary; (sample 25.78),

ACca 96 — 142 cm very dark grey (10YR3/1) moist and dry, silty clay loamwith occasional coarse fragments; massive structure;common pressure faces; few small intersecting slicken-sides; very few very fine roots; common very fine cracks;coarse fragments of calcium carbonate in lower part ofthe horizon; smooth gradual boundary; (sample 25.79),

(II)C 142 - 190 cm red (2.5YR4/6) moist and very dark grey (2.5YR3/0)moist, weathered calcrete of very gravelly coarse sandyloam texture; common coarse and very coarse subangu-lar fragments of calcrete of a pinkish white colour(7.5YR8/2) moist; the red material is non to slightlycalcareous; blocked by very hard calcrete at the base ofthe horizon; (sample 25.80).

228

Laboratory data of profile No. 25

Sample

25.7525.7625.7725.7825.7925.80

Sample

25.7525.7625.7725.7825.7925.80

SampleNo.

25.7525.7625.7725.7825.7925.80

Sample

25.7525.7625.7725.7825.7925.80

SampleNo.

25.7525.7725.80

SampleNo.

25.7525.7.625.7725.7825.7925.80

Deptr

0-3

33-67-

33 36796

96-142142-190

Hori-

A l lA 1 2A 1 3ACACca(II)C

% > 2

1.60.70.81.41.3

69.0

Particle size distribution (M) inSand

2000- 500- 200- 100-

9.3 3.2 6.7 7.47.8 2.9 5.5 4.7

10.1 3.4 5.2 5.77.4 3.2 5.0 5.34.9 2.2 4.0 5.4

41.8 12.0 6.2 3.9

Exchangeable cations in meq/100 g

Ca

31.7735.6333.4532.4835.8324.71

Mg

1011111113

9

644545893129

K

1.970.950.950.841.010.61

% weight;Silt

50- 20

9.3 1213.0 27

Clay

< 2

5 51.60 39.1

7.3 12.9 55.415.5 30.6 33.017.7 36 1 29.74.7 12.0 19.4

Na Sum C.E.C. Base Organic mattersat

0.19 44.57 52.28 851.51 49.54 60.61 823.69 49.54 58.18 855.42 50.63 60.79 835.62 55.77 77.63 724.35 38.96 45.38 86

Elemental composition of the fine earth (% by weight)

SiO,

85.879.578.078.583.778.6

Trace

Mn

8 3 01160102412101250

6 6 6

A1,C

8.611.612.013.3

9.614.4

j Fe,

2.03.03.83.92.84.6

D3 TiO, CaO MgO K.,0 Na,0

0.47 0.24 0.51 3.43 1.640.40 0.19 0.57 4.89 0.000 58 0.28 1.07 4.12 1.460.75 0.47 0.53 3.68 0.3800

47 0.29 0.15 3.66 2.2859 0.35 0.86 3.11 0.33

element content of the fine earth (in ppm)

Cu

281828151524

Zn

352917283229

Cr

86140147152140

86

Co Ni Ba Sr

35 10 <100 5935 28 <100 3212 20 <100 3819 38 <100 2627 45 100 3119 54 100 19

Elemental composition of the clay fraction (% by weight)

SiO,

56.457.157.9

A1,O,

23.422.621.4

Mineralogy

K

+

+++++

I

+

+++++

Fe,O3 TiO, CaO MgO

12.212.612.2

of the clay fra

Mt

++

++++++++

Mi

0.80 2.96 2.750.88 2.60 2.130.68 2.43 4.11

ction Molar

V Chi Q SiO,

A1,O3

xx 4.1X X

xx 4.3X X

X

x 4.6

'° %C

0.830.780.440.740.650.07

P,O,

0.070.080.120.050.040.06

SiO,

AI,O3

17.011.611.010.014.9

9.3

K , O

1.011.361.00

ratios

SiO,

R,O3

3.07

3.17

3.37

% N

0.120.10

Sum

102.8100.3101.4101.5102.9102.8

SiO,

R,O,

14.799.969.188.44

12.587.74

Na,O

0.000.000.00

A1,O3

Fe,O3

3.0

2.8

2.8

pH 1

H , O

8.28.78.88.68.58.6

P,O,

25 '10

91110

1

5

KC1

6.56.76.86.86.77.0

CaCo3

Ó.910.450.640.820.621.13

Loss onignition %

13.115.215.115.315.914.2

Al,0

Fe ;O

6.66.05.05.45.44.9

P

3

3

o,

0.480.810 36

229

DESCRIPTION OF PROFILE NO. 26

Al l 0 — 2 cm Coarse and medium granitic sand, loose dry, on hardsmooth surface, abrupt boundary; (sample 26.01),

A12 2 — 24 cm dark reddish brown (5YR3/2) moist, sandy loam; weakmedium subangular blocky structure; hard, firm, slightlysticky and plastic; common fine and medium roots;common coarse sand; gradual boundary; (sample 26.02),

B21 24 - 48 cm dark reddish brown (5YR3/3) moist (5YR3/4) dry,sandy clay loam; weak medium subangular blockystructure; hard, firm, slightly sticky and slightly plastic;roots decreasing with depth; coarse sand grains andcoarse fragments increasing with depth; gradual bound-ary; (sample 26.03),

B22 48 - 80 cm dark reddish brown (5YR3/3) moist, reddish brown(5YR4/4) dry, slightly gravelly sandy clay loam; apedal;common coarse sand; common fine calcium carbonateand some hard concretions; soft, friable, sticky andplastic; (sample 26.04),

B2ca 80 - 120 cm dark reddish brown (5YR3/4) moist, yellowish red(5YR4/6) dry, gravelly sandy clay loam; apedal; highlycalcareous; common hard fine calcium carbonate concre-tions; soft, friable, slightly sticky and plastic; (sample26.05),

(II)C 120 cm blockage by coarse gravel.

230

Laboratory data of profile No. 26

SampleNo.

26.0126.0226.0326.0426.05

SampleNo.

26.0126.0226.0326.0426.05

SampleNo.

26.0126.0226.0326.0426.05

SampleNo.

26.0126.0226.0326.0426.05

SampleNo.

Depthin cm

0- 22- 24

24- 4848- 8080-120

Hori-zon

Al lA12B21B22B2ca

%>2mm.

12.33.93.06.9

20.1

Particle size distribution (ji) inSand

2000- 500-50 200

43.7 26.719.5 18.417.9 15.417.8 14.519.2 13.5

Exchangeable cations in meq/100 g

Ca

1.269.74

14.5017.1018.79

Mg

0.532.353.742.933.45

K

0.230.840.520.480.55

Na

trtrtr0.18tr

200-100

16.219.516.716.714.8

Sum

2.0:12.9^18.7f20.6<22.7<

Elemental composition of the fine earth (% by

SiO2 Al3:O 3 Fe2(

74.4 10.6 5.371.4 12.8 7.170.8 14.0 7.370.0 14.3 7.870.6 13.6 7.0

0, TiO2

0.841.281.141.231.11

Trace element content of the :

Mn Cu

420 34886 22880 24670 42670 42

Elemental

SiO2

Zn

2636464644

Cr

150184180

92136

Co

3224262620

1 composition of the

A12O3 Fe2O3

CaO MgO K

2.52 2.2.31 2.2.33 1.2.46 3.3.22 1,

fine earth

Ni

4048552610

.22 3.

.06 2.68 2..35 2..65 2.

100-50

6.911.512.810.610.4

C.E.C.

! 2.14^ 12.55i 17.72> 18.06) 20.94

weight)

2O Na2O

53 1.8665 2.1564 0.2851 0.7678 0.00

i (in ppm)

Ba

700460400360400

clay fraction (%

TiO2 CaO

Sr

3532324848

by weight)

MgO

% weightSilt Clay

50-20

4.56.65.77.05.8

Basesat. 9!

94100100100100

P2O ;

0.120.220.140.130.05

SiO2

A12O3

11.99.58.68.38.8

20- <22

! pH 1:

H 2 O

0.0 2.0 7.25.6 18.9 7.25.7 25.8 7.76.0 27.4 8.57.6 28.7 8.6

Organic

' %C

0.080.760.450.440.39

, Sum

101.4101.9100.3102.599.9

SiO2

R2O3

9.017.006.436.176.63

O Na

matter

% N

0.010.080.07

5

KC1

6.26.06.27.27.3

P 2 O

168041

69

Loss onignition %

1.36.37.17.68.2

A12O,

Fe2O

3.22.83.02.93.0

2O P2

)

3

26.0126.0326.05

52.351.651.0

19.425.524.4

9.15.14.

611

0.620.700.63

7.2.4.

150097

3.2.2.

563713

0.810.730.43

0.080.990.00

6.1.2.

541548

Mineralogy of the clay fraction

Mt Mi V

Molar ratios

SampleNo.

26.0126.0226.0326.0426.05

K

+++++

1

++trtrrr

Chi SiO2 SiO2 A12O3

R2O3 Fe2O3

4.6

3.4

3.6

3.48 3.2

2.49 2.6

2.60 2.7

231

DESCRIPTION OF PROFILE NO. 27

Al 0 — 2 0 cm Dark reddish brown (5YR3/3) moist and yellowish red(5YR4/6) dry, clay; structureless; soft dry, friable moist,sticky and plastic wet; common very fine roots; (sample27.106),

20 cm blocked by dolerite.

DESCRIPTION OF PROFILE NO. 28

Al 0 — 38 cm Black (5YR2/1) moist and dry, clay; structureless; veryhard dry, slightly friable moist, sticky and plastic wet;common very fine and fine calcium carbonate concre-tions; (sample 28.105),

38 cm blocked by dolerite.

232

Laboratory data of profile No. 27 and No. 28

Particle size distribution (ß) in % weightSand Silt

Sample Depth Hori- %>2 2000- 500- 200- 100- 50- 20-No. in cm. zon mm. 500 200 100 50 20 2

Clay

<2 p H l : 5

H2O KC1

17.28.

107105

0-200-38

AlA l

trtr

27.4.4

32

.0

.86.4.

92

8.76.4

8.5.

92

21 .15.

55

48.658.5

7.18.9

5.7.

82

Exchangeable cations in meq/100 g

Sample CaNo.

Mg K Na Sum C.E.C. Basesat.%

Organic matter P2O5

%C %N p p m

27.10728.105

SampleNo.

SampleNo.

18.437.6

6.2318.80

1.600.39

tr1.13

26.2357.92

33.15 7946.27 100

1.200.94

0.250.10

276

Elemental composition of the fine earth (% by weight)

SiO2 A12O3 Fe3O3 TiO2 CaO MgO K2O Na,O P2O, Sum Loss onignition %

27.10728.105

SamplNo.

27.10628.105

SampleNo.

27.10628.105

65.663.8

Trace

Mn

754680

18.17.

28

10.10.

87

element content

Cu

2626

Elemental

SiO2

50.954.4

Zn

6734

Cr

1686

composition

Al,

26.23,

: O 3

35

Fe

1513

1.061.07

of the

Co

3850

of the

2o3

.1

.8

1.80 2.5.31 3.

fine earth

Ni

2262

75 1.23 0.

1348

(in ppm)

Ba

300200

clay fraction (%

TiO2

0.720.71

CaO

3.063.89

by

0.000.00

Sr

1532

weight)

MgO

1.352.93

00,

SiC

Al,

6.16.1

.54

.04

l2

o3

K 2 O

0.460.00

101.9102.5

SiO2

R2O3

4.444.40

Na, O

0.000.00

13.16.

Al,

Fe,

2.62.6

98

o3;O3

P2OS

2.210.83

Mineralogy of the clay fraction Molar ratios

K I M t Mi Chi SiO2 SiO2

Fe2O3

27.106 ++28.105 +

3.33.9

2.402.86

2.72.7

233

DESCRIPTION OF PROFILE NO. 29

Al 0 - 2 2 cm Brown (7.5YR4/3) moist, light brown (7.5YR6/4) dry,loamy sand; weak fine subangular blocky structure;slightly hard dry, loose moist, non-sticky, non-plasticwet; common fine and medium tubular pores; many veryfine, fine and medium roots; smooth gradual boundary;(sample 29.119),

B21 22 — 36 cm reddish brown (5YR4/4) moist; reddish yellow(7.5YR6/6) dry, sandy loam; structureless; slightly harddry, friable moist, slightly sticky and slightly plastic wet;common fine pores; common very fine, fine and mediumroots; faint reddish mottling in the lowest part of thehorizon; smooth gradual boundary; (sample 29.120),

B2 36 - 50 cm yellowish red (5YR4/6) moist (5YR5/6) dry, sandy clayloam; structureless; hard dry, friable moist, sticky andplastic wet; common fine tubular pores; common veryfine, fine and medium roots; few fine thin reddishbroken cutans increasing in size and number with depth;occasional coarse fragments; smooth gradual boundary;(sample 29.121),

B2t 50 - 80 cm yellowish red (5YR4/8) moist (5YR 5/8) dry, sandy clayloam; structure and consistence as horizon above;common fine and medium pores, rapidly permeable;common medium fairly thick broken cutans of a darkred (2.5YR3/6) coulor; occasional coarse fragments; fewvery fine and fine roots; smooth gradual boundary;(sample 29.122),

B3 80 - 95 cm yellowish red (5YR4/6) moist, reddish yellow (5YR6/6)dry, gravelly sandy clay loam; structureless; very harddry, friable moist, sticky and plastic wet; pores andpermeability as horizon above; common fine prominentdark red (2.5YR3/6) mottles; few fine pinkish grey(7.5YR6/2) mottles; few fine and medium roots; fewtermite channels filled with grey material; abrupt wavyboundary; (sample 29.123),

C 95 — 113 cm partly weathered schist of very gravelly sandy clay loamtexture in horizontal bedding with occasional coarsefragments of finely grained quartzite; black and reddishand yellowish colouring common; occasional fine roots;(sample 29.124).

234

Laboratory data of profile No. 29

Sample DepthNo. i

29.11929.120

n cm.

0- 22!2- 36

29.121 36- 5029.122 )0- 8029.123 80- 9529.124 95-113

Exchange^

Sample Ca

No.

29.11929.12029.12129.12229.12329.124

.25

.26

.51

.26

.26

.26

Elemental

Sample SiO, Al,No.

29.119 95.2 4.29.120 87.4 9.29.121 85.3 12.29.122 82.5 13.29.123 82.2 13.

Horizon

AB21B2B2tB3C

Particle size distribution (ß) inSand

%>2 2000- 500- 200-mm. 500 200 100

tr 12.3 31.4 36tr 9.1 39.0 22

2.1 8.3 27.5 22

427

2.5 9.7 23.4 22.918.2 8.5 23.7 2465.5 20.3 23.1 19

ble cations in meq/100 g

Mg

0.200.230.640.570.430.43

K Na Sum

0.13 tr 1.580.21 tr 1.700.21 tr 2.360.13 tr 1.960.13 tr 1.820.15 tr 1.84

composition of the fine earth (%

o,

58567

Fe ,O, TiO, CaO MgO

1.0 0.41 0.16 0.982.0 0.59 0.11 0.062.3 0.70 0.14 0.582.6 0.74 0.15 1.092.6 0.76 0.12 0.21

45

by

K

000

100-50

7.53.55.05.77.85.8

C.E.C.

3.233.245.445.234.344.78

weight)

,O Na,O

51 0.0055 0.0050 0.30

0.59 0.000.66 0.27

% weightSil

50-20

0.40.81.41.22.13.7

Base

sat. %

495343384239

P,O

0.060.010.110.150.00

Clay

20- < 22

3.7 8.33.8 21.64.1 31.04.5 32.65.7 27.84.8 22.8

pH 1:5

H,O KC1

7.16.26.26.05.75.6

Organic matter

% C % N

0.27 0.040.25 0.040.270.130.200.10

Sum

102.9100.5102.4101.4100.6

6.34.44.64.44.34.4

P,O,ppm

trtrtrtr

Loss onlgn

2.44.55.75.95.6

tion %

29.124 73.8 21.6 2.1 0.77 0.13 1.28 0.34 0.00 0.10 100.1 8.2

Trace element content of the fine earth (in ppm) SiO, SiO, A1,O,

No.

29.11929.12029.12129.12229.12329.124

SampleNo.

Mn

6624

< 1 0146060

Cu

244224242226

Zn

< 517

96

< 54

Cr

< 511212696

172146

Co

1816684

18

Ni

4< 214302030

Ba

<100<100<100

240140200

Elemental composition of the clay fraction (% by

SiO, Al Fe,O,i TiO, CaO

Sr

7<2

78

< 27

weight)

MgO

A1,O,

35.915.211.610.310.2

5.8

K,O

R,O,

31.3113.4310.349.209.095.46

Na,

Fe,Os

6.97.78.68.28.2

15.8

0 P,OS

29.11929.12129.124

49.650.649.1

32.7 5.637.9 7.136.7 6.7

1.39 4.101.50 0.821.54 2.20

0.480.622.65

0.610.620.53

0.74 4.790.00 0.840.39 2.65

SampleNo.

29.11929.12029.12129.12229.12329.124

Mineralogy of the clay fraction Molar ratios

K I Mt Mi Chi SiO,

A1,O,

Al.O,Fe,O,

2.6

2.3

2.3

2.32

2.03

2.03

9.2

8.4

8.6

235

DESCRIPTION OF PROFILE NO. 30

Al l 0 - 23 cm Very dark greyish brown (10YR3/1.6) moist, greyishbrown (10YR4/2) dry, sand; very fine subangularblocky; soft dry, loose moist, non-sticky, non-plasticwet; many fine medium and occasional coarse roots; fewant holes; smooth gradual boundary; (sample 30.125),

A12 23 - 52 cm brown (7.5YR4/3) moist and (1OYR5/3) dry, sand;structureless; slightly hard dry, loose moist, non-sticky,non-plastic wet; common fine, medium and occasionalcoarse roots; few ant holes; smooth gradual boundary;(sample 30.126),

A13 52 - 80 cm brown (7.5YR4/4) moist, light brown (7.5YR5.6/4) dry,sand; structure, consistence and permeability as horizonabove; common fine, medium and occasional coarseroots; smooth gradual boundary; (sample 30.127),

A14 8 0 - 120 cm brown (7.5YR4/4) moist, light brown (7.5YR6/4) dry,sand; structure, consistence and permeability as horizonabove; few fine and medium roots; smooth gradualboundary; (sample 30.128), (

A3 120 — 165 cm strong brown (7.5YR5/5.6) moist, reddish yellow(7.5YR6/6) dry, sand; structureless; hard dry, loosemoist, non-sticky non-plastic wet; common medium •sand grains; very few fine medium and coarse roots; jsmooth gradual boundary; (sample 30.129), ,

C 165 — 220 cm strong brown (7.5YR5/6) moist, reddish yellow j(7.5YR6/6) dry, sand; structure, consistence and perme-ability as horizon above; occasional fine and mediumroots; few fine yellowish red (5YR5/8) mottles; coarsefragments increasing with depth; (sample 30.130).

236

A

Laboratory data of profile No. 30

SampleNo.

30.12530.12630.12730.12830.12930.130

Sample

30.12530.12630.12730.12830.12930.130

SampleNo.

30.12530.12630.12730.12830.12930.130

SampleNo.

30.12530.12630.12730.12830.12930.130

SampleNo.

30.12630.130

SampleNo.

30.12530.12630.12730.12830.12930.130

Depthin cm.

0- 2323- 5252- 8080-120

120-165165-220

~~"—

Hori-zon

A l lA12A 1 3A14A3C

Particle size distribution (M) inSand

%>2 2000- 500- 200- 100-mm. 50 200 100 50

tr 7tr 7tr 7tr 9tr 11tr 11

9 34.3 42.1 11.13 29.1 49.8 9.57 29.5 49.9 8.42 29.1 48.6 7.93 37.2 41.3 5.36 34.6 37.2 9.9

Exchangeable cations in meq/1 00 g

Ca

1.200.20trtrtrtr

Mg

0.260.130.070.070.070.10

K

0.130.080.080.050.050.05

Elemental composition of the

SiO, A1,O3 Fe,

98.9101.7

97.897.798.495.0

2-6 1.0!-7 0.81.1 0.51 5 0.6!-3 0.61 6 0.7

O3 TiO,

0.310.310.200.250.170.22

Trace element content of the

Mn Cu Zn

212 <5 <534 < 5 < 510 10 <520 <5 <524 <5 <520 <5 <5

Cr Co

50 18<5 420 1646 1876 1676 <2

Elemental composition of the

SiOj

42.144.2

A1:O3

22.325.5

Fe,O3

7.39.0

% weightSilt

50- 20-20 2

0.2 0.40.4 0.60.4 0.61.2 0.60.6 1.40.8 0.8

Na Sum C.E.C. Base Orfsat

tr 1.59 2.19 73tr 0.41 1.31 31tr 0.15 0.87 17tr 0.12 0.87 14tr 0.12 0.87 14tr 0.15 0.87 17

fine earth (% by weight)

CaO MgO K,O Na2O

0.15 0.43 0.49 0.020.10 0.35 0.39 0.000.10 1.81 0.27 0.220.11 1.96 0.35 0.000.09 0.05 0.36 1.430.09 0.85 0.23 0.27

fine earth (in ppm)

Ni Ba Sr

10 <100 <2< 2 <100 < 2

4 140 <24 <100 < 2

10 <100 <24 <100 <2

clay fraction (% by weight)

TiO, CaO MgO

1.31 11.68 "" 0.061.44 8.28 0.51

Mineralogy of the clay fraction Molar

K 1

++ +++ +++ +++ +++ +++ +

Mt Mi V Chi Q SiOj

A1,O3

XX

xx 3.2XX

XX

XX

xx 2.9

'° %C

Clay

< 2

4.03.33.53.52.95.1

pH 1:

H , O

6.56.36.16.55.85.4

anic matter

: %N

0.35 0.040.15 0.020.100.060.050.05

P,O,

0.060.00 /0.080.100.020.08 /

SiOj

A1,OS

65.39103.8

>100>100>100

100.7

K : O

0.750.69

ratios

SiO,

R2O3

2.65

2.40

Sum

103.9105.4102.0102.5102.3

99.1

SiO:

R,O 3

52.7580.03

>10089.77

>10077.82

Na3O

0.000.00

A1,O3

Fe,O3

4.8

4.5

C/N

98

5

KC1

6.05.85.25.35.34.6

P,OS

ppm

1212233

Loss onignition %

1.91.20.90.80.81.1

A12O

Fe,O

4.23.43.23.83.53.4

P,

14

3

o5

.47 '10.31

DESCRIPTION OF PROFILE NO. 31

Al l 0 - 15

Allca 1 5 - 39 cm

A12ca 39 - 67

A13ca 67 - 1 1 6 cm

ACca 116 - 176 cm

(II)C 176 cm

Dark greyish brown (10YR4/2) moist and dry, sandyloam; moderate medium subangular blocky structure;hard dry, friable moist, slightly sticky and slightly plasticwet; many fine, medium and a few thick roots; commonfine cracks up to 0.5 cm wide; highly calcareous; wavygradual boundary; (sample 31.142),

dark greyish brown (10YR4/2) moist, greyish brown(10YR5/2) dry, sandy clay; moderate medium prismsbreaking into coarse angular blocky; very hard dry,slightly friable moist, slightly sticky and slightly plasticwet; cracks up to 1,5 cm wide; few pores; faint thinbroken cutans; many fine and medium roots; highlycalcereous; a little termite activity; few coarse sandgrains and fine gravel; smooth gradual boundary; (sample31.143),

greyish brown (10YR5/2) moist, light brownish grey(10YR6/2) dry, slightly gravelly sandy clay loam;moderate medium and coarse prisms breaking intomoderate coarse angular blocky structure; extremelyhard dry, firm friable, slightly sticky and slightly plasticwet; common thick cutans along the ped surfaces; fewfine and medium tubular pores; CaCO3 as horizonabove; few very fine and fine roots; some cracks 1 cmwide at 50 cm depth; smooth gradual boundary; (sample31.144),

greyish brown (10YR5/2) moist and dry, slightly gravel-ly sandy clay loam; weak coarse blocky structure;extremely hard dry, friable moist, slightly sticky andslightly plastic wet; amount of soft and hard CaCO3concretions increasing with depth; highly calcereous; fewthin broken cutans; very few fine and occasionalmedium roots; coarse fragments increasing with depth;few fine cracks up to 0.5 cm wide; smooth gradualboundary; (sample 31.145),

brown (1OYR5/3) moist, pale brown (10YR6/3) dry,slightly gravelly sandy loam; structureless, massive; harddry, friable moist, non-sticky, non-plastic wet; no roots;(sample 31.146),

blockage by calcrete.

238

Laboratory data of profile No. 31

SampleNo.

Particle size distribution (M) in % weightSand Silt

Depthin cm.

Hori-zon

2000- 500-500 200

200-100

100-50

50-20

20-2

Clay

<2 pH 1:5

H,O KC1

31.142 0- 15 Al l 2.3 7.0 10.7 31.0 6.7 7.2 22.2 15.2 7.9 7.531.143 15- 39 Allca 4.6 8.0 12.0 25.2 8.3 7.7 1.4 37.4 7.8 7.531.144 39- 67 A12ca 5.1 9.3 10.4 24.8 9.0 7.2 6.4 32.9 7.8 7.531.145 67-116 A13ca 7.7 8.0 11.8 24.2 6.0 7.2 19.2 23.6 7.8 7.531.146 116-176 ACca 7.1 7.2 9.3 23.8 8.6 5.9 23.6 21.6 7.8 7.4

Exchangeable cations in meq/100 g

SampleNo.

Ca Mg K Na Sum C.E.C. Basesat.%

Organic matter

% C % N C/N

P,O, CaCOjppm %

31.14231.14331.14431.14531.146

SampleNo.

19.8325.6422.0723.6820.74

1.161.511.752.232.84

0.480.270.270.290.35

tr0.090.050.050.05

21.4727.5124.1426.2523.98

20.6723.0323.5525.4123.12

100100100100100

0.800.510.500.320.23

0.110.08

Loss onignition l.

10.2813.0614.6313.2313.62

Elemental composition of the fine earth (% by weight)

SiO, A1,O3 FeaO3 TiO, CaO MgO K,O Na2O P,O, Sum

31.14231.14331.14431.14531.146

80.677.577.576.677.1

7.28.08.28.28.6

2.83.13.43.43.3

0.590.570.570.600.58

7.509.499.669.478.76

0.00 \0.860.00 \0.670.00 0.850.00 io.680.00 /0.97

0.001.060.720.000.03

0.100.120.110.110.12

99.7100.5101.199.099.5

11.713.713.413.713.1

SampleNo.

31.14231.14331.14431.14531.146

Trace element content of the fine earth (in ppm)

Mn Cu Zn Cr Co Ni Ba Sr

SiO, SiO,

Al,Oj RjO3

A1,O3

Fe,O3

376460468554660

10<5<54229

<591313

<526522040

1630444418

10203240

300260220540680

6102116

18.916.516.015.915.2

15.1713.1912.6812.5912.20

4.04.03.83.84.0

SampleNo.

31.14231.14431.146

Elemental composition of the

SiO2

56.062.060.0

Al,

20.21.20.

o3

390

Fe,O3

7.88.98.0

clay fraction (% by

TiO,

0.820.950.82

CaO

11.29 •'2.35'6.61

weight)

MgO

I.SS^2.11 ;1.90

K 3 O

0.710.860.72

Na,O

0.280.300.00

P2OS

1.190.651.94

Mineralogy of the clay fraction Molar ratios

SampleNo.

31.14231.14331.14431.14531.146

K

+

++++

1

+

+trtrrr

Mt Mi Chi SiO, SiO, A1,O3

A1,O3 R,O 3 Fe,O3

xxXXXXXXXX

4.7

4.8

5.1

3.76

3.81

4.06

4.1

3.9

4.0

239

DESCRIPTION OF PROFILE NO. 32

A 0 - 30 cm Dark brown (10YR3/3) moist, brown (10YR5/3) dry,coarse sandy loam; structureless, massive; hard dry,friable moist, non sticky, non plastic wet; common fineand medium pores; horizon looks grey in appearancewhen compared with the underlying redder horizons;smooth gradual boundary; (sample 32.147),

B21 30 - 61 cm reddish brown (5YR4/4) moist, yellowish red (5YR5/6)dry, clay, structureless, massive; hard dry, friable moist,slightly sticky and slightly plastic wet; many distinctmedium red (2.5YR4/6) mottles increasing in promi-nence with depth; common fine and medium thincutans, slightly darker than the soil matrix; little coarsefragments and some coarse sand; common fine roots; alittle biological activity; smooth gradual boundary;(sample 32.148),

B22 61 - 88 cm yellowish red (5YR4/6) moist, brown (7.5YR5/4) dry,slightly gravelly clay; structure and consistence as above;common coarse and medium dark red (2.5YR3/6)mottles; common coarse fragments; very few fine andmedium roots; few tubular pores; smooth gradualboundary; (sample 32.149),

B3 88 - 1 1 0 cm strong brown (7.5YR4/6) moist and (7.5YR5/6) dry,slightly gravelly clay ; structureless, massive; hard dry,friable moist, slightly sticky and plastic wet; commonprominent medium and coarse dark red (2.5YR3/6)mottles; few fine and coarse bluish black mottles;occasional fine roots; common fine tubular pores; coarsefragments increasing with depth; clear, in some partswavy boundary; (sample 32.150),

C 110 — 125 cm strong brown (7.5YR4/6) moist matrix, weatheredgranitic rock of a gravelly clay texture; prominent red,black and some yellowish mottling; hardness increasingwith depth; no roots; (sample 32.151),

R 125+ cm hard granite.

240

Laboratory data of profile No. 32

SampleNo.

32.14732.14832.14932.15032.151

Sample

No.

32.14732.14832.14932.15032.151

SampleNo.

32.14732.14832.14932.15032.151

SampleNo.

32.14732.14832.14932.15032.151

Deptrin cm

l

0- 3030- 6161- 8888-110

110-125

Hori-zon

AB21B22B3C

mm.

1.21.68.39.7

24.3

Particle size distribution (ß) iSand

2000- 500- 200- 100-500 200 100 50

26.0 19.9 23.4 6.015.6 9.2 10.7 6.014.5 9.1 11.3 3.913.3 8.3 10.9 6.116.2 9.0 10.5 6.1

Exchangeable cations in meq/100 g

Ca

2.113.263.073.692.75

Mg

0.391.441.581.691.51

K

0.180.180.180.210.24

Na Sum C.E.C.

tr 2.68 3.94tr 4.88 9.32tr 4.83 9.34tr 5.59 9.81tr 4.50 8.44

Elemental composition of the fine earth (% by weight)

SiOj

86.973.671.663.270.8

Trace

Mn

2205254

100150

Al,

8.619.621.724.420.6

>3 Fe2

2.44.85.46.55.9

O3 TiO2 CaO MgO K2O NajC

0.0.0.

Q.0.

53 0.32 0.27 1.04 0.0064 0.24 0.62 1.10 0.0069 0.26 1.03 1.21 0.7978 0.30 0.42 1.25 2.8074 0.27 0.56 1.21 0.79

element content of the fine earth (in ppm)

Cu

3434384242

Zn

76

103

<5

Cr

52128

6612866

Co Ni Ba Sr

34 12 120 < 2<2 <2 120 3< 2 36 <100 < 2< 2 34 <100 < 2< 2 34 <100 5

n % weightSilt Clay

50-20

4.34.23.53.95.1

Base

sat.%

6852525753

) P2O,

0.000.110.120.120.06

SiO2

A12O3

17.16.45.64.45.8

20- < 22

6.0 14.45.0 49.35.5 52.27.6 49.99.0 44.1

pH 1:

H , O

6.15.85.86.15.9

Organic matter

% C % N

0.51 0.040.39 0.060.360.230.31

, Sum

100.1100.7102.8

99.8100.9

SiO2

R2O3

14.505.504.833.764.94

C/N

137

5

KC1

5.04.64.54.84.9

P2O5

ppm

1trtrtr1

Loss onignition %

3.99.29.9

10.28.6

A12O

Fe2O

5.66.46.35.95.5

3

SampleNo.

32.14732.14832.151

Elemental

SiO2

49.650.249.7

composition of the

A12O3

32.936.836.5

Fe2O3

7.48.79.0

clay fraction (% by

TiO2

0.970.890.89

CaO

3.520.811.20

weight)

MgO

0.690.690.70

K 2 O

0.931.030.90

Na2O

0.000.000.00

P2O,

3.990.751.13

Mineralogy of the clay fraction

SampleNo.

32.14732.14832.14932.15032.151

K.

+ +

++++++++

1

trtrtr

Mt Mi V Chi

Molar

SiO2

A12O3

2.6

ratios

SiO2

R2O3

2.242.01

Al,

Fe2

7.06.6

o,

X

trtrtr 2.3 2.00 6.4

241

DESCRIPTION OF PROFILE NO. 33

A 0 - 30 cm Dark brown (10YR3/3) moist, greyish brown(10YR5/2) dry, coarse sandy loam; structureless, singlegrained; slightly hard dry, loose moist, non-sticky,non-plastic wet; common fine pores; common very fine,fine and medium roots; smooth gradual boundary;(sample 33.152),

B21 30 - 58 cm dark brown (10YR4/3) moist, brown (10YR5/3) dry,coarse sandy loam; structure as above; hard dry, friablemoist, slightly sticky and very slightly plastic wet; littletermite activity; common fine and medium pores; fewfine and medium roots; very faint brownish mottling inthe lower part of the horizon; smooth gradual boundary;(sample 33.153),

B22 58 - 99 cm dark brown (10YR4/3) moist, light brownish grey(10YR6/2) dry, slightly gravelly coarse sandy loam;amount of coarse fragments increasing with depth;apedal; very hard dry, friable moist and slightly stickyand plastic wet; common prominent small and mediumreddish brown (5YR4/4) mottles in which few blackprominent mottles; few faint thin broken cutans; com-mon fine and medium tubular pores; occasional coarseroots; smooth clear boundary; (sample 33.154),

C 99 — 130 cm hard, partly weathered granular granitic rock withcommon red and black mottling of very gravelly sandyclay loam texture; in some parts the orginal coarsegranitic rock structure is still visible; occasional pinkfeldspars; no roots; (sample 33.155).

242

Laboratory data of profile No. 33

SampleNo.

33.15233.15333.15433.155

Sample

No.

33.15233.15333.15433.155

SampleNo.

33.15233.15333.15433.155

SampleNo.

33.15233.15333.15433.155

SampleNo.

Depthin cm.

0- 3030- 5858- 9999-130

Hori-zon

ABBC

Particle size distribution ()i) inSand

%>2 2000- 500- 200-mm. 500 200 100

2.1 28.5.9 29,

13.2 31.77.0 30.

.7 22.,2 16..2 17..3 13.

Exchangeable cations in meq/100 g

Ca

1.211.722.232.43

Elemental

SiO2 Al,

Mg

0.410.871.341.97

K

0.230.260.290.39

composition of the

O3 Fe,

84.1 9.1 1.780.6 11.5 2.680.4 12.4 2.986.1 14.0 3.7

,O3 TiO2

0.420.580.540.65

Trace element content of the :

Mn Cu

150 34120 34380 46114 50

Elemental

SiO2

Zn

<5<5

73

Cr Co

56 16< 5 1652 38

100 38

composition of the

A12O3 Fe2O3

Na

trtrtrtr

.8 19.3

.7 16.9

.0 15.0,7 13.5

Sum

1.852.853.864.79

fine earth (% by

CaO

0.470.450.470.34

MgO K

0.82 4.1.59 4.0.15 3.0.47 4.

100-50

7.7.8.5.

C

4.4.5.7.

9408

.E.C.

38.39.28.06

weight)

"°00149819

fine earth (in ppm)

Ni

24102872

Ba

760820860760

clay fraction (%

TiO2 CaO

by

Na2O

4.491.251.140.57

Sr

27273631

weight)

MgO

% weightSilt Clay

50-20

6.28.95.66.7

Base

sat. %

42657368

P2O,

0.150.130.110.04

SiO2

A12O3

15.811.911.010.5

K2

20- < 22

6.0 9.45.4 15.54.5 18.75.9 24.1

pH 1:5

H2 O KC1

6.1 5.16.1 4.96.1 4.96.3 5.1

Organic matter P2OS

% C % N

0.48 0.050.29 0.030.220.15

[ Sum

105.2102.8102.1109.9

SiO2

R2O3

14.0910.409.588.95

O Na2O

C/N p p m

10 210 1

tr1

Loss onignition %

2.83.53.44.3

A12O3

Fe2O3

8.57.16.76.0

P2OS

33.15233.15333.15433.155

33.15233.15333.155

49.952.151.2

25.429.530.5

8.39.39.6

1.391.421.39

5.511.982.28

1.1.0.

083799

2.292.261.84

0.000.000.00

6.112.062.20

Mineralogy of the clay fraction Molar ratios

Sample KNo.

Mt Mi Chi SiO2 SiO2

A12O3 R2O3 Fe2O3

trtr

3.33.0

2.9

2.762.49

2.37

4.85.0

5.0

243

SUMMARY

This research deals with the development of soils in the semi-arid environ-ment of central East Botswana, Africa.The material for this study was collected from the catchment areas of twotributaries of the Limpopo rivier, i.e. the Mahalapshwe and Bonwapitse.The physical environment reflects the variations in climate, the contras-ting parent material, the soil-vegetation relationships and the landscapeson which the soils are encountered.The soil temperature and moisture regimes, notably the alternating wetand dry seasons, proved significant to the development of the soils.Granite and dolerite are the main suppliers for the parent material of thesoils. The soil texture has a major influence on the formation of vegeta-tion patterns of which three were recognized. Slight differences in topo-graphy are responsible for remarkable variations in soil profile develop-ment, especially in the granitic region.Base exchange phenomena are almost entirely dependent on the clayfraction, as the amount of organic matter is very low.Coarse sand was found to be a major component of the sand fraction,whereas the distribution of clay in the soils gives rise to the recognition offour clay curves, which were correlated with various soil-forming pro-cesses. Good correlations were found between a number of elements(SiO2, Al2O3 and Fe2O3) and some grain size fractions. The amountof trace elements is low. Zinc showed a high correlation with a group ofbivalent trace elements (Co, Cu and Ni); their occurrence is mainlyconfined to the finer soil fractions.Mineralogical studies point to the advanced stage of weathering in thesoils, disclosing that only the more persistent mineral species are retained.In addition the mineral composition of the sand fraction indicates thevariation in origin of the parent materials. Five heavy mineral suites wererecognized, the light mineral composition being closely associated withthe heavy mineral composition.Micromorphological observations indicated the presence of "pedorelicts"as well as the limited occurrence of more recent clay translocation andaccumulation in most soils.Plasmic fabrics could partly be correlated with the various soils.No uniform pattern emerged relative to the B-horizon in Alfisols.The Vertisols are similar in their development as is known from otherparts of the world, although the distinct animal activity is a remarkable

245

feature. The sandy soil of the Kalahari shows little profile development.Enrichment of iron is observed in the C-horizons of the granitic soils onthe peneplain. This phenomenon is considered a paleo-feature. Redistribu-tion of iron takes place in some hydromorphic Alfisols.The soils were classified according to the USDA 7th Approximation(1970) and correlated with the FAO/UNESCO system (1968, 1970). Fourorders are recognized: Entisols, Vertisols, Inceptisols and Alfisols. Thepresence of the large number of Haplustalfs is typical in the transitionzone from the dry to the wet tropics.Land use is aimed at beef production, relatively small areas are cropped.There is a tendency that, especially around population centres, less suit-able soils are taken into cultivation.

246

SAMENVATTING

Onderwerp van deze studie vormt de bodemgenese in het semi-aride,oostelijk deel van Centraal Botswana, Afrika.Het materiaal voor dit onderzoek werd verzameld uit de stroomgebiedenvan twee zijrivieren van de Limpopo, t.w. de Mahalapshwe en de Bonwa-pitse.De plaatsgehadhebbende klimaatsveranderingen, de sterke verschillen inmoedergesteente, de relatie tussen bodem en vegetatie en de landschaps-eenheden waarin de verschillende bodems werden aangetroffen, zien zichweerspiegeld in het totale fysische milieu.De bodemtemperatuur en het vochtigheidsregime, in het bijzonder deafwisselende natte en droge seizoenen, zijn belangrijke factoren in debodemontwikkeling.Graniet en doleriet vormen de voornaamste gesteenten waarvan hetmoedermateriaal afkomstig is.De bodemtextuur heeft een grote invloed op de vorming van vegetatie-patronen, waarvan er drie werden onderscheiden.Speciaal in het granietgebied blijken kleine verschillen in de topografieverantwoordelijk te zijn voor opmerkelijke variaties in profielontwikke-ling.Grof zand is het voornaamste bestanddeel van de zandfractie. De variatiein kleigehalten was zodanig, dat vier kleicurven konden worden aange-toond, die gecorreleerd werden met verschillende bodemvormende proces-sen.De uitwisseling van basen geschiedt voornamelijk via de kleifractie aange-zien het organische stofgehalte bijzonder laag is.Goede correlaties werden gevonden tussen een aantal elementen (SiO2,Al2O3 en Fe2 O3 ) en enige korrelgrootten. De hoeveelheid sporenelemen-ten is laag.Zink vertoonde een goede correlatie met een groep tweewaardige sporen-elementen (Co, Cu en Ni); hun voorkomen is voornamelijk beperkt tot deklei- en siltfractie.Dé hoge mate van verwering in de meeste bodems werd mineralogischaangetoond. Hierbij bleek dat slechts de meest weerstandskrachtige mine-ralen behouden bleven.Tevens duidde de mineralogische samenstelling van de zandfractie op deverscheidenheid van oorsprong van het moedermateriaal.Vijf zware-mineraalgroepen werden onderscheiden. De samenstelling der

247

lichte mineralen is nauw verbonden mee die der zware.Micromorfologische waarnemingen toonden de aanwezigheid van „pedore-licten" aan, alsmede het beperkt voorkomen van recent kleitransport en-accumulatie.Plasmic fabrics konden slechts gedeeltelijk worden gecorreleerd met deverschillen in bodemvorming. Het fabric van de B-horizonten van deAlfisolen bleek niet uniform te zijn.De ontwikkeling van de Vertisolen onderscheidt zich van die in anderegelijksoortige gebieden van de wereld door de opmerkelijke dierlijkeactiviteit.Aan de zandbodem van de Kalahari kon geen noemenswaardige profielont-wikkeling worden onderkend. Aanrijking van ijzer werd waargenomen inC-horizonten van granietbodems op de peneplain, waarvan het ontstaan alseen paleoverschijnsel wordt opgevat. In sommige hydromorfe Alfisolsvindt een redistributie van ijzer plaats.De bodems werden geclassificeerd volgens de USDA 7th Approximation(1970) en gecorreleerd met het FAO/UNESCO systeem (1968, 1970).Vier orders werden onderscheiden: Entisols, Vertisols, Inceptisols en Al-fisols.Typisch voor de overgangszone van droge naar natte tropen vormt deaanwezigheid van een groot aantal Haplustalfs.Landgebruik is voornamelijk gericht op veeteelt (vleesproductie), betrek-kelijk kleine arealen worden voor de verbouw van gewassen gebruikt. Erbestaat een neiging om speciaal rond de bevolkingscentra minder geschiktegronden in cultuur te brengen.

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259

CURRICULUM VITAE

De auteur werd geboren in 1940 te Baarn en behaalde het H.B.S.-b diplo-ma in 1957 aan het Baarns Lyceum aldaar.In datzelfde jaar ving hij aan met de studie fysische geografie aan deRijksuniversiteit te Utrecht. Hij specialiseerde zich in de bodemkunde enlegde in 1964 het doctoraal examen af.Gedurende meer dan drie jaar (1965—1968) was hij werkzaam als bodem-kundige voor de FAO in het kader van een trainings- en karteringsprojectte Wad-Medani, in de Sudan.In de loop van 1968 volgde overplaatsing naar Mahalapye en vervolgensGaborone in Botswana, waar hij als bodemkundige verbonden was aan eenFAO/UNDP-SF multidisciplinair project voor landontwikkeling.In september 1971 werd aangevangen met een voortgezet onderzoekaangaande de bodemgenese in Oost Botswana, aan het BodemkundigInstituut der Rijksuniversiteit te Utrecht.

260

W. Siderius - Soil Transitions in Central East Botswana (Africa)

ERRATA

pp. 5, 107 and 251p. 6, line 2 from bottomp. 7, line 5 from topp. 10, on small scale map

and p. 11p. 14, line 8 from topp. 17, caption of Table 2p. 20, line 8 from topp. 22, bottom linep. 28, delete: line 20 from toppp. 28, 30 and 39p. 35, Table 12, (column 2)p. 36, bottom line

and line 3 from bottomp. 38, line 13 from bottomp. 43, line 15 from topp. 45, in Key to Fig. 7 and p. 28

and line 13 from top61, Table 18 (column 6)

and (column 5)p. 81, line 21 from topp. 85, line 10 from top etc.p. 91, line 2 from bottomp. 96, line 2 from top

and line 4 from bottomp. 99, Table 29, line 6 from topp. 102, line 7 from bottomp. 105, legend of Fig. 32p. 107, line 4 from bottomp. 118, line 5 from topp. 1 19, Table 33p. 122, caption of Fig. 35p. 130, line 3 from topp. 135, line 18 from topp 136, caption of Fig. 42p. 137, line 18 from topp. 141, line 4 from bottomp. 147, line 2 from bottom

read:read:read:read:read:read:read:read:read:

read:read:read:read:read:read:read:read:read:read:read:read:read:read:read:read:read:read:read:read:read:read:read:read:

DoeglasBotswana1530Bonwapitseiscals57Mueller

Boocock

LanthanumNa20vertical

Si02

CArchaeanaremetadoleritechannel39CECmargaliticphosphorusferric0.915SrtourmalinePettijohnHarmsen, 1959E Ce Acrossed

148, caption of Table 35and line 11 from bottom

149, line 7 from bottom153, line 7 from bottom

extinctionRhoads

read: circumferenceread: situread: widensread: Mohr and Van

Barenratio radiusread:

read:read:read:

toooutlined

and line 5 from bottom, delete: oilier, 1959155, line 4 from top read: Burridge157, Table 40, insert headings as in Table 39159, Table 42 read: Al 20 3

Fe2°3

instead of:instead of:instead of:instead of:instead of:instead of:instead of:instead of:instead of:

instead of:instead of:instead of:instead of:instead of:instead of:instead of:instead of:instead of:instead of:instead of:instead of:instead of:instead of:instead of:instead of:instead of:instead of:instead of:instead of:instead of:instead of:

Douglas or DouglesBoswana1615Bonwapisteincalc47Mueeler

BoockockLathanumMgOcerticalSio2

EArchaenismeta-doeritechannal36C.E.C.margalithicphosphateferri-0.915StoermalinePettijJohnHarmse, 1961"•ecrosed

instead of: distinctioninstead of: Roadsinstead of: circumpherenceinstead of: sity

widessinstead of:instead of:

instead of:instead of:instead of:

Mohr et al.

radius ratioSiO2

to

162, caption of Fig. 48 read: AI2O3

instead of ourlined

instead of Barridge

instead of: AI2O3

instead of: Ae 20j163, Table 45, insert:fraction lines in Molar ratios167, line 13 from bottom read: criticism instead of: critisism169, line 9 from top read: integrated instead of: intergrated

and line 10 from top read: FAO/UNESCO instead of: UNESCO175, bottom line, delete: 500

insert: subordinated) , dominant(++) ; moderate amount(x), large amount(xx)183,249.,249,256,

insert: Barshad, I., 1969. Chemistry of soil development. In: Bear (ed.), pp.1-70insert: Bawden, M.G., 1965. Some soils of Northern Bechuanaland. D.O.S. England,line 4 from bottom read: Rooij, N.M. de instead of: Rooij, N.M.

p.256, line 11 from bottom read: 1965 instead of: 1963p. 258, insert: Tiurin, I.V., 1965. The system of soil classification in the USSR.

Pédologie, Spec. No. 3, pp. 7-24.

Stellingen

1. In tegenstelling tot de invloed van Kalahari zand op de bodemvormingin Transvaal (Zuid-Afrika), is deze gering te noemen voor wat betreftde huidige bodemgenese in centraal oost Botswana.

(Harmse, H. J. von M., 1967, proef-schrift Utrecht)

2. De aanwezigheid van "pedorelicten" in bodemmateriaal is niet zondermeer een aanwijzing voor policyclische bodemvorming, zoals vaak wordtaangenomen.

(Ruellan, A., 1971 in Paleopedology(ed. Yaalon), ISSS and ïsr. Univ.Press, Jeruzalem)

3. Standarisatie van geomorfologische begrippen kan slechts gerealiseerdworden door de oprichting van een internationaal overlegorgaan, naaranalogie van de thans bijna 50 jaar bestaande ISSS.

(International Society of Soil Science,opgericht 1924)

4. Een theoretische benadering van de bodemsamenstelling uit de totaal-chemische analyse resultaten is dan zinvol, indien de mineralogischecompositie van het bodemmateriaal bekend is.

5. De bestudering van micro-sedimentologische verschijnselen in de bodemverdient meer aandacht.

(dit proefschrift)

6. Bij het waarderen van de invloed van het moedermateriaal op de bodem-vorming is een grove indeling van gesteenten zoals die in "zuur, inter-mediair of basisch" vaak onvoldoende om verschillen in bodems ver-oorzaakt door variaties binnen een bepaald type uitgangsgesteente tete begrijpen.

7. De invloed van het werk van E. W. Hilgard op de moderne begrips-vorming in de pédologie wordt nog te weinig onderkend.

(Jenny, H., 1961, E. W. Hilgard andthe birth of modern soil science,Collana Delia Vista "Agrochemica",Pisa)

8. Enkele richtingen in de aardwetenschappen, waaronder de bodemkunde,zijn bij uitstek geschikt voor toegepast onderwijs in ontwikkelings-landen.

9. Op vele kaarten is de begrenzing van de Kalahari woestijn foutiefaangegeven.

(Meulenhof Atlas, 1959; EdinburghWorld Atlas, 1967; Grote Bosatlas,1968)

10. De klassifikatie van ongekonsolideerd oppervlaktemateriaal van de maanals "lunar soil" (maanbodem of maangrond) is voorbarig gezien dehuidige fragmentarische kennis omtrent de bodemvormende faktorendie op dit materiaal hebben ingewerkt.

(Proceedings Apollo 11 Lunar ScienceConference, 1970, Vol.'s 1-3, PergamonPress, Inc. ; NASA Preliminary ScienceReport Apollo 14,1971, Washington)

11. De tijd is nabij dat het schrijven van een proefschrift korter zal durendan het vormen van een kabinet.

W. SIDERIUS 2 mei 1973

W.Siderius - Soil Transitions in Central East Botswana (Africa)

ERRATA

pp. 5,107 and 251p.6, line 2 from bottom,p.7, line 5 from top,p.10, on small scale map,p.14, line 8 from top,p.17, caption of Table 2,p.20, line 8 from top,p.22, bottom line,p.28, line 16 from top andp.30, line 6 from topp.36, bottom line,p.38, line 13 from bottom,p.43, line 15 from top,p.45, in Key to Fig.7,p.61, Table 18,p.85, line 10 from top etc.p.96, line 2 from top,p.102,line 7 from bottom,p.107,line 4 from bottom,p.118,line 5 from top,p.119,Table 33,

p.122,caption of Fig.35,p.147,line 2 from bottom,p.149,line 7 from bottom,p.153,line 5 from bottom,p.155,line 4 from top,p. 159,Table 42

read: Doeglasread: Botswanaread: 15read: 30read: isread: calsread: 57read: Mueller

read: Boocockread: Na2Oread: SiO2

read : Cread: Archaeanread: metadolerite.read: CECread: phosphorusread: Srread: Pettijohnread: Harmsen,1959read: ECg

read: crossedread: Mohr and Van Barenread: toodelete: oilier,1959read: Burridgeread: AI2O3

Fe2O3

read: A1.0,

insteadinsteadinsteadinsteadinsteadinsteadinsteadinstead

instead

insteadinsteadinsteadinsteadinsteadinsteadinsteadinsteadinsteadinsteadinstead

insteadinsteadinstead

insteadinstead

instead

of:of:of:of:of:of:of:of:

of:of:of:of:of:of:of:of:of:of:of:of:

of:of:of:

of:of:

of:

'• Douglas

Boswana1615incalc47Mueeler

Boockock

MgOSioo

E 2

Archaenmeta-doeriteC.E.C.phosphateSPettij johnHarmse,1961EcecrosedMohr et al.to

BarridgeA12O3

A12°3Ae2°3p.162,caption of Fig.48,

p.163,Table 45, insert: fraction lines in Molar ratiosp.167,line 13 from bottom, read: criticism instead of: critisismp.169,line 10 from top, read: FAO/UNESCO instead of: UNESCOp. 249, insert :Barshad, I., 1969. Chemistry of soil development. In : Bear (ed.) pp.1-70.p.256,line 4 from bottom, read: Rooij.N.M.de instead of: Rooij.N.M.p.258,insert:Tiurin,I.V.,1965.The system of soil classification in the USSR.

Pédologie, Spec.No.3,pp.7-24.

C: