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Chemistry and mineralogy of acid sulfate soils and potentialutilization of green manures as acid soil amendments

Poolpipatana, Sunthorn, Ph.D.

University of Hawaii, 1994

V·M·I300 N.ZeebRd.Ann Arbor, MI 48106

CHEMISTRY AND MINERALOGY OF ACID SULFATE SOILS AND POTENTIAL

UTILIZATION OF GREEN MANURES AS

ACID SOIL AMENDMENTS

A DISSERTATION SUBMITTED TO THE GRADUATE DIVISION OF THEUNIVERSITY OF HAWAII IN PARTIAL FULFILLMENT OF THE

REQUIREMENTS FOR THE DEGREE OF

DOCTOR OF PHILOSOPHY

IN

AGRONOMY AND SOIL SCIENCE

MAY 1994

By

Sunthorn Poolpipatana

Dissertation Committee:

Nguyen V. Hue, ChairpersonJames A. SilvaRollin C. JonesHaruyoshi Ikawa

Charles L. Murdoch

iii

ACKNOWLEDGEMENTS

I express my deepest sense of gratitude to almighty Buddha

who enabled me to complete this manuscript.

The author would like to express his sincere gratitude

towards the many people who have helped to make this

dissertation a reality. I wish to acknowledge my major

professor, Dr. Nguyen V. Hue, for his inspiring guidance, keen

interest and unfailing kindness during the span of the entire

study. The author is also deeply indebted to his advisory

committee : Dr. James A. Silva, Dr. Rollin C. Jones, Dr.

Haruyoshi Ikawa, and Dr. Charles L. Murdoch for their academic

expertise and constructive criticism to improve the research

and this manuscript.

I would also like to thank Dr. Sombat Teekasap for his

helpful discussion, computer assistance and moral support

during my research in Thailand. Special thanks are also

extended to Dr. Arom Sriprichit, Dean of the Faculty of

Agricultural Technology, KMITL, Bangkok, for the service as

emeritus and for the permission to embark on my study leave.

The author would be guilty for a serious omission if he

failed to make special mention of the profound debt of

gratitude he owes Dr. Apisak Phorpan, and other personnel at

the Department of Soil Sciences, KMITL, for providing

laboratory and greenhouse space, technical advice and

assistance in field transportation and soil survey.

iv

The author would also like to extend his sincere

appreciation to the Thai farmers in the Bangkok Plain, who not

only allowed the soil samplings in their rice fields but also

inevitably insisted upon helping him. These people, who make

up the backbone of Thai society, will always have a special

place in my heart.

I am forever in debt to my parents, Sungk and Sakorn, who

provided financial and moral support without which my

education could not have occurred. I would also like to thank

my elder sister for her continual support. Last, but not

least, my wife, Savanit ("Nita"), is deeply appreciated for

her unending patience, love and understanding. In addition,

she has devoted herself to the care of our business and our

horne, and without this I would not have had the peace of mind

necessary to complete my graduate program.

v

ABSTRACT

Potential and actual acid sulfate soils in the Bangkok

Plain are strongly acidic, high in solution AI, Fe, Mn and low

in Ca in the unamended state. For successful intensive crop

production, lime and green manure applications are necessary.

The use of locally available green manures, as "a self-liming

material" is a promising strategy for alleviating Al toxicity

and raising the concentration of basic cations in the soil

solution.

The growth and acidity tolerance of four tropical legumes

(Pigeon pea : Cajanus cajan, Sesbania aculeata, S. rostrata

and S. speciosa) were studied in a greenhouse experiment for

potential green manure sources. Two acid sulfate soils (Typic

Sulfaquents, Bang Pakong (Bg) series; and Sulfic Tropaquepts,

Rangsit (Ra) series) were adjusted to four pH levels: 3.8 or

4.0 (original soil pH), 4.5, 5.5 and 6.5 (amended with lime).

Based on green manuring criteria of high biomass production

and high N content, C. cajan and S. aculeata were better

sui ted to the acid sulfate soils than S. rostrata and S.

speciosa. The legumes responded differently to stresses

imposed by Al toxicity. C. cajan tolerated nearly three times

the level of Al as the Sesbania species Cri tical Al

concentrations in shoots (for 10% dry matter reduction) were

80 mg kg-1 for the former and 30 mg kg- 1 for the latter.

A greenhouse experiment was conducted to quantitatively

compare the effects of two green manures {O, 20, 40, 80 Mg ha"

vi

ground tops of S. aculeata and C. cajan) and lime (0, 4, 8 Mg

CaC03 ha-1) on Al detoxification for upland rice grown for 60

days in two acid sulfate soils. Green manure applications

effectively detoxified AI. C. cajan was better than S.

aculeata in reducing AI+3 activities in soil solution. Green

manure and CaC03 amendments were compared by estimating

amounts of the materials required to decrease Al+3 activi ty

until relative root length of the rice plants were ~ 90%. The

application rates of CaC03, C. cajan and S. aculeata required

to meet this criterion were 5.3, 44.4, 57.5 Mg ha",

respectively.

An incubation study for 90 days was conducted to determine

changes in the solid phase, the solution phase, and

mineralogical properties of representative acid sulfate soils

following lime (CaC03 6 Mg ha-1) and green manure (sesbania 40

Mg ha") applications. In the unamended soils, strong

acidification occurred, resulting in pH decreases to ~ 2.8. By

contrast, liming and green manuring increased soil pH, EC,

total exchangeable bases, and reduced Al saturation

percentage. X-ray diffraction analysis showed no detectable

change in soil minerals by either amendment in spite of strong

acidification occurring in the control. Detailed mineralogical

study of handpicked yellow particles from soils at 90 days

after incubation showed the presence of jarosite. Soil

solution analysis suggested that jarosite might be formed by

precipitation of K+, Fe+3 and S04-2. Based on the ion activity

vii

products and stability diagram, lime and sesbania applications

resulted in the formation of AI-hydroxy sulfate minerals. The

Ar3 activities were pH dependent and apparently controlled by

the solubility of an alunite-like mineral, KAI3(OH)6(S04)2'

having a pKsp of 81.4.

viii

TABLE OF CONTENTS

ACKNOWLEDGEMENTS iii

ABSTRACT......................... v

LIST OF TABLES xiv

LIST OF FIGURES :xv.ii.

CHAPTER 1. Introduction 1

References 5

CHAPTER 2. Problem statement, justification and

obj ectives 7

References 12

CHAPTER 3. Genesis and chemistry of acid sulfate soils

in the Bangkok Plain, Thailand; and

potential problems for rice production 13

3.1 Occurrence of acid sulfate soils 13

3.2 Definition of acid sulfate soils 16

3.3 Classification and taxonomy of acid

sulfate soils 17

3.4 Physiography of acid sulfate land in the

Bangkok Plain 19

3 .5 Genesis of acid sulfate soils 23

3.5.1 Geogenetic process 23

3.5.1.1 Mechanism of pyrite

formation 25

3.5.2 Pedogenetic process 26

3.5.2.1 Pyrite oxidation following

drainage 27

3.5.2.2 Iron sulfide-oxidizing

bacteria 30

3.5.2.3 Neutralization of acidity 31

3.5.2.4 Products from oxidation and

neutralization 35

3.5.2.5 Profile development toward

maturi ty 40

3.6 Problems of acid sulfate soils to rice

production 42

ix

3.6.1 Adverse effect of H+ 42

3.6.2 Aluminum toxicity........... 42

3.6.3 Iron toxicity... 45

3.6.4 Phosphorus deficiency.. 49

3.6.5 Low base status 50

References 51

CHAPTER 4. Aluminum toxicity and tolerance of plants to

acid soils 60

4.1 Al toxicity symptoms and mechanism of

plant tolerance .. 60

4.1.1 Physiological and biochemical

effects of Al toxicity.. 60

4.1.2 Mechanisms of plant adaptation and

Al tolerance to acid soils.. .... 63

4.2 Tolerance of green manure crops to acid

soil conditions 65

4.2.1 Type of green manures for in situ

incorporation... 65

4.2.2 Sesbania tolerance to adverse

condi tions 70

4.2.2.1 Flood tolerance...... 70

4.2.2.2 Salt tolerance 70

4.2.2.3 Acid tolerance 71

References 73

CHAPTER 5. Chemistry of acid sulfate soils and Aluminum

detoxification by organic amendments 78

5.1 Chemistry and mineralogy of aluminum,

iron and sulfur in acid sulfate soils 78

5.1.1 Acid development by oxidation of

pyrite 78

5.1.2 Mineralogy and oxidation products

of pyrite 79

5.1.3 Solution chemistry of iron in acid

soils 82

5.1.3.1 Ferric species .. 82

x

99

97

101

102References

5.1.3.2 Ferrous species 83

5.2 Aluminum chemistry in acid soils... 84

5.2.1 Hydrolytic reactions of AI...... 84

5.2.2 Solubility and precipitation of

Aluminum 85

5.2.3 Al speciation as an index of

phytotoxicity................... 86

5.3 Al detoxification by organic amendments 88

5.3.1 Influences of plant residues, sewage

sludge and animal wastes .... .... 89

5.3.2 Al detoxification by organic

complexation 90

5.4 Green manure and organic management for

acid soils 92

5.5 Influence of soil organic materials on

Al transformation 94

5.5.1 Formation of soluble organic-

aluminum complexes 94

5.5.1.1 Formation and stability of

bonds 94

5.5.1.2 Solubility of organic-AI

complexes 95

5.5.1.3 Significant role of pH.. 96

5.5.2 Adsorption of organic anions on

variable-charged surface of AI, and

iron minerals 97

5.5.2.1 Evidence for ligand

exchange .

5.5.2.2 Effects of adsorption on

soil pH .

5.5.2.3 Effects of adsorption on

surface charge .

xi

CHAPTER 6. Selected physical and chemical properties of

acid sulfate soils used in the present

study 110

6.1 General description of the soils 110

6.2 Site selection and soil sample

preparation 110

6.3 Chemical characteristics of the aerated

soils 112

6.4 Particle size distribution D5

6.5 Lime titration curves 116

References 118

CHAPTER 7. Differential acidity tolerance of tropical

legumes grown for green manure in acid

sulfate soils 119

7.1 Abstract 119

7.2 Introduction 120

7.3 Materials and methods 121

7.3.1 Plant selection from a field

survey..... 121

7 . 3 .2 Soil sampling and analysis 122

7.3.3 Pot experiment 124

7 .4 Results and discussion 125

7.4.1 Differential growth as measured by

dry matter yield and N

accumulation 126

7.4.2 Soil acidity and chemical

composition of the legumes 129

7.5 Summary and conclusions 139

References 142

CHAPTER 8. Ameliorating aluminum toxicity in upland

rice grown on acid sulfate soils, using

green manures 144

8.1 Abstract 144



xii

8.3.1 Soils and organic amendments 147

8.3.2 Plant growth study 147

8.3.3 soil-solution collection and

analysis 149

8.4 Results and discussion 150

8.4.1 Effects of lime and green manure

amendments on chemical properties

of the soil solution 150

8.4.2 Effects of CaC03 and green manure

amendments on rice growth 161

8.4.3 Relationship between rice growth

and soil-solution composition 166

8.5 Summary and conclusions 170

References 171

CHAPTER 9. Mineralogical and chemical properties of

two acid sulfate soils as affected by lime

and green manure applications 174

9 . 1 Abstract 174



9.3.1 Soil selection and properties '" 177

9.3.2 Chemical analysis ., 178

9.3.3 Mineralogical analysis 179

9.3.4 Soil incubation study 180

9.3.5 Statistical analysis 182

9.4 Results and discussion................ 182


on the soil solid phase 182


on soil-solution phase 186

9.4.3 Mineralogical composition of acid

sulfate soils .. , 195

9.4.4 Effects of lime and green manure

on metal solubility and mineral

stability diagram 201

APPENDIX A

APPENDIX B

on metal solubility and mineral

stability diagram .

9.5 Summary and conclusions .

References .

xiii

201

207

208

212

217

xiv

LIST OF TABLES

Table

3.1 Distribution of acid sulfate soils in the Southeastand East Asia 15

6.1 Some important chemical properties of the aeratedacid sulfate soils 114

6.2 Particle size distribution and textures of thetwo acid sulfate soils 116

7.1 Selected physical and chemical properties of theacid sulfate soils used in the greenhouseexperiment 123

7.2 Dry-matter yield of the four green manure legumesgrown on two acid sulfate soils from Thailand,and the associated analysis variance. 127

7.3 Regression equation of dry-matter weight (Y, ing pot-1

) against soil pH (X) and plant Nconcentration (N, in %). There were 24 observationsfor each legume 128

7.4 Nutrient composition of four tropical legumesused as green manures as affected by different pHin two acid sulfate soils.... 132

8.1 Some selected physical and chemical properties ofthe two unamended acid sulfate soils used in thegreenhouse experiment 148

8.2 Chemical properties of the soil solution asaffected by various levels of CaC0 3 (Ca), sesbania(Ses) and pigeon pea (Pea) application to twoacid sulfate soils 151

xv

8.3 Chemical properties of the soil solution asaffected by various levels of CaC03 (Ca), sesbania(Ses) and pigeon pea (Pea) application to twoacid sulfate soils 154

8.4 Calculated activities of monomeric Al species andsulfate as affected by various levels of CaC03

(Ca), sesbania (Ses) and pigeon pea (Pea)application to two acid sulfate soils... ... ..... 157

8.5 Calculated activities of basic cations in thesoil solution as affected by various levels ofCaC03 (Ca), sesbania (Ses) and pigeon pea (Pea)application to two acid sulfate soils. ..... ..... 160

8.6 Plant height, dry weights, Al and Fe concentrationsin shoots and roots, and relative root length ofrice plants as affected by various levels of CaC03

(Ca), sesbania (Sea) and pigeon pea (Pea) applicationto two acid sulfate soils ., 162

8.7 Relationship between growth parameters of riceplants and various Al toxicity indices derivedfrom soil solution attributes in two acid sulfatesoils (all treatments combined) 163

9.1 Chemical properties of the soil solid phase asaffected by CaC03 (6 Mg ha") and sesbania (40 Mgha-1 ) application at various incubation periodsfor two acid sulfate soils 183

9.2 Summary of source, degree of freedom (df), and Fvalue of analysis of variance for soil-solidphase as affected by CaC03 (6 Mg ha") andsesbania (40 Mg ha") application at variousincubation periods for two acid sulfate soils 184

xvi

9.3 pH, EC and cationic concentrations in the soilsolution as affected by CaC03 (6 Mg ha") andsesbania (40 Mg ha-1

) application at variousincubation periods for two acid sulfate soils 188

9.4 Concentrations of Al, Fe, Mn and anions in thesoil solution as affected by CaC03 (6 Mg ha-1 ) andsesbania (40 Mg ha-1 ) application at variousincubation periods for two acid sulfate soils 191

9.5 Effects of CaC03 (6 Mg ha-1) and sesbania (40 Mg

ha-1) application on pH and activities of ions in

the soil solution at various incubation periodsfor two acid sulfate soils...................... 194

9.6 Soil solution ion activities and ion activityproducts of acid sulfate soils as affected byCaC03 (6 Mg ha") and sesbania (40 Mg ha")application at various incubation periods for twoacid sulfate soils 202

xvii

LIST OF FIGURES

Figure

3.1 Physiographic regions of Thailand.... 20

3.2 Areas of different physiographic regions in theSouthern Bangkok Plain. 22

6.1 Generalized soil map of the Southern BangkokPlain showing the sample site as : II. Rangsitvery acid phase, at Nakhon Nayok Province; and IV.Bang Pakong, at Chachoengsao Province 111

6.2 Titration curves of theBg = Bang Pakong, Typicsit, Sulfic Tropaquepts

two acid sulfate soils.Sulfaquents; Ra = Rang-

117

7.1 Total N uptake (A) and plant N concentration (B)of the four green manure legumes grown in acidsulfate soils at different levels. CC : Cajanuscajan, SA : Sesbania aculeata, SR : S. rostrata,SS : S. speciosa. Vertical bars are standarderrors. Unlimed pH is designed as 3.9 which isthe average of 3.8 and 4.0, the actual pHs ofthe two unamended soils......................... 130

7.2 Relationship between dry-matter yield of the fourgreen manure legumes and the Ca concentration inplant tops 134

7.3 Plant Fe and Mn concentrations in the four greenmanure legumes as a function of soil pH. Verticalbars are standard errors........................ 136

7.4 Relationship between relative biomass of the fourgreen manure legumes and the Ca/Al ratio in planttops 140

xviii

8.1 Relationship between Al+3 activity and soilsolution pH as affected by all treatments for twoacid sulfate soils 159

8.2 Effects of CaC03 (a), sesbania (b) and pigeon pea(c) application on relative root length of riceplants at 60 days after planting 165

8.3 Relationship between relative growth and soilsolution composition of two acid sulfate soils 167

8.4 Relationship between relative root length and Alor Fe concentration in rice roots 168

8.5 Relationship between K, Ca, Mg and Sconcentrations in the rice shoots and theirrespective activities in soil solution 169

9.1 Relationship between Al (a) and Mn (b)concentrations and pH in the soil solution of twoacid sulfate soils. Regression equation for a) .y~= 2.5*104(exp-1.2X), r 2 = 0.92**; b). v: = 164 25.6X, r 2 = 0.97**; and c). v: = 193.8 - 29.7X,r 2 = O. 93 *. 192

9.2 X-ray diffraction patterns of the clay fractionof the Bg soils as affected by CaC03 and sesbaniaapplication at 2 and 90 days after incubation.Ca 0 = Control; Ca 6 = 6 Mg ha? CaC03 ; Ses 40 =40 Mg ha'" sesbania 196

9.3 X-ray diffraction patterns of the clay fractionof the Ra soils as affected by CaC03 and sesbaniaapplication at 2 and 90 days after incubation.Ca 0 = Control; Ca 6 = 6 Mg ha" CaC03 ; Ses 40 =40 Mg ha" sesbania 197

xix

9.4 X-ray diffraction patterns of materials separatedfrom yellow mottles from the Bg soil after 90 daysof incubation. Only spacings the distinct jarositepeak (at 3.18 °A) is shown with symbol J 199

9.5 Soil solution activities relative to stabilityline for AI-hydroxy sulfate minerals and gibbsitein two acid sulfate soils. Plotted data pointsabove a line indicated supersaturation, andpoints below a line indicated undersaturation.Assumed pK + pOR = 12.3 ± 1.0. 205

CHAPTER 1

Introduction

Of the total 12.5 million hectares of reported potential

and actual acid sulfate soils in the world (Dent, 1992), 1.5

million hectares occur in Thailand. Among these, 0.9 million

hectares are located in the Bangkok Plain. The remainder are

found in various parts of the eastern coast, and on the

peninsular which makes up the southern region (Parkpian et

al., 1991).

Generally, acid sulfate soils have very low pH, and

originate from saline and brackish marine sediments. These

soils are formed in coastal regions where large quantities of

pyrite (Fe8J have accumulated in intertidal sediments. Pyrite

accumulation is a result of anaerobic conditions, which cause

sulfate (8°4- 2) from sea watez to be reduced to sulfide (8-2

) .

The alkalinity formed during sulfate reduction is leached from

the sediments by tidal action, leading to a potential acidity.

If these soils are aerated, either by natural or artificial

drainage, pyrite will be oxidized to sulfuric acid and the

soils are acidified. As a result, the pH drops below 4. In

this situation, hydrogen ions, aluminum, iron, manganese, and

organic acids can accumulate to levels toxic to plants.

Therefore, plants are unlikely to grow well unless the soils

are ameliorated. An extremely low pH (3.0) at the surface

layer characterizes young acid sulfate soils. This type of

soils (SuLfaquep t s ) has been found in large areas in some

2

countries such as Indonesia (Alkasuma et al., 1990), Vietnam

(Xuan, 1993), and Malaysia (Ting et al., 1993).

As these soils age, characteristic minerals are formed. The

important minerals found in developed acid sulfate soils are

jarosite, alunite, goethite, hematite, ferric hydroxide, and

gypsum. Because these soils contain neutralizable base and the

stepwise potentially reserved acidic substances (e.g.,

jarosite), soil pH's appear to be maintained at 4.0. The more

mature these soils are, the deeper the jarositic and the

pyritic horizons are, resulting in surface soils with higher

pH. At this stage, however, the soil conditions are very

adverse to plant growth, and yields are low.

The Bangkok Plain is the main rice growing area of

Thailand. This plain is essentially a river valley widening to

coastal plain and crossed by various bifurcations of the

rivers and tidal channels. During the rice growing season, the

plain is inundated by river floods and rain water. Under the

prevailing water conditions one rice crop per year is the

rule, and yield production ranges between 0.5 and 2.5 tons

ha", Low productivity (0.5 - 1.5 tons ha") is associated with

a large complex of nearly 0.6 million hectares of acid sulfate

soils in the central part of the Bangkok Plain (Attanandana,

1990) .

Acid sulfate soils have predominantly developed soil

profiles with characteristics of Sulfic Tropaquepts. They

developed in pyritic (1 - 2.5 % S) non-calcareous gray clays,

3

deposited in a brackish tidal environment 3 - 7 thousand years

ago. More recent fluviatile and marine deposits of adjacent

upstream and downstream parts of the Plain gave rise to soils

without acid sulfate conditions, of higher productive

potential, and characteristics of predominantly Typic

Tropaquepts.

The processes involved in soil genesis of acid sulfate

soils of the Bangkok Plain and their behavior on drying and

flooding have been studied by van Breemen (1976) and their

fertility amelioration by Ponnamperuma et al. (1973) and

Attanandana (1982). Fertility studies have shown that acid

sulfate soils are generally well developed physically, but

their high acidity retards microbiological activity. possible

causes of poor plant performance have been identified as Al

and Fe toxicities and deficiencies of N, P, and Ca.

Methods to control acidification during reclamation and to

prevent or improve collateral adverse soil conditions, should

be developed to suit specific agro-ecological situations. In

the Bangkok Plain, this development has been initiated since

1970 (Komes, 1973; Jugsujinda et al., 1978; Charoenchamrat

cheep et al., 1982, Satawathananont, 1986; Moore et al., 1990)

and has focused on improving acid sulfate soils for rice

cultivation. The most commonly used management strategy for

these soils is liming. Since a large quantity of marl has been

found at a shallow depth nearby, liming has been widely

recommended. Because of differences among acid sulfate soils

4

and the variation in land use, both positive results and

negative results with lime have been reported.

Leaching and submergence are also promising management

practices for ameliorating acid sulfate soils. Leaching is

usually manipulated after the soil has been exposed to

oxidation and after subsequent flooding in order to dissolve

toxic substances. Natural leaching by rainfall is preferred to

artificial leaching because of scarcity of water resource and

expenses in the dry season.

Submergence induces reduction processes caused by an

insufficient supply of oxygen. Reduction causes numerous

changes in the chemical, physicochemical, and microbiological

properties of a soil. For instance, reduction decreases redox

potential (Eh) and reduces sulfate, and increases pH,

available iron, manganese, phosphorus, sulfide and organic

acids. In contrast to submergence, drainage brings about

oxidation and the reverse processes take place.

This dissertation aims mainly at developing alternative

practices that prevent or alleviate the toxicity of acid

sulfate soils used for growing rices, with emphasis on (i)

selection of Al tolerant plants to be used as green manures,

(ii) incorporation of green manures into the soil so that soil

acidity and Al phytotoxicity can be reduced, and (iii)

identification of possible chemical and mineralogical changes

of aluminum, iron and sulfate in acid sulfate soils as

affected by green manure applications.

5

References

Alkasuma, Hendro Praseyto, A.K. Bregt, and J.A.M. Janssen.1990. Observation density and map scale for survey ofacid sulfate soils A case study in Pulau Petak,Southern Kalimantan. AARD/LAWOO, ILRI. Wageningen, TheNetherlands.

Attanandana, T. 1982. Fertility problem of acid sulfate soilof Thailand. Ph.D. Thesis. Kyoto Univ., Kyoto, Japan.

Attanandana, 1990. Problems and amelioration of acid sulfatesoils. In. C. Suwannarat (ed.) Soil and Water Analysis forCharacterization of Acid Sulfate Soils. Course Manual.Dept. of Soil Science, Fac. of Agriculture, KasetsartUniv., Bangkok, Thailand.

Charoenchamratcheep, C., B. Tantisira, P. Chitnuson, and v.Sinaiem. 1982. The effects of liming and fertilizerapplications to acid sulfate soils. In H. Dost and N. vanBreemen (eds.) Proc. Bangkok Symp. Acid Sulfate Soils,pp. 157-171. ILRI Publ 31. Wageningen, The Netherlands.

Dent, D.L. 1992. Reclamation of acid sulfate soils. Adv. SoilScience. 17 : 79-122.

Jugsujinda, A., Y. Tadashi, and N. van Breemen. 1978. Aluminumtoxicity and phosphorus deficiency in acid sulfate soilsof Thailand. IRRI Newsl. 3 : 1.

Kames, A. 1973. The reclamation of some problem soils inThailand. Soils of the ASPAC Region Part 5 : Thailand.Tech. Bull. No. 14. FFTC, Taiwan, ROC.

Moore, P.A., T. Attanandana, and W.H. Patrick Jr. 1990.Factors affecting rice growth on acid sulfate soils. SoilSci Soc. Am. J. 54 : 1651-1656.

Parkpian, P., P. Pongsakul, and P. Sangtong. 1991.Characteristics of acid soils in Thailand: A review. InR.J. Wright (ed.) Proc. 2nd Int. Symp. on Plant SoilInteractions at Low pH, pp. 397-405. Beckley, WestVirginia.

Ponnamperuma, F.N., T. Attanandana, and G. Beye. 1973.Amelioration of three acid sulfate soils for lowlandrice. In H. Dost (ed.) Acid Sulfate Soils Proc. Int.Symp., pp. 391-406. ILRI Publ. 18, vol. II. Wageningen,The Netherlands.

6

Satawathananont, S. 1986. Redox, pH and ion chemistry of acidsulfate rice soils in Thailand. Ph.D. Diss., Louisianastate Univ., Baton Route (Diss. Abstr. 87-10589).

Ting, C.C., R. Bt. Saari, W.H. Diemont, and A. Bin Yusoff.1993. The development of acid sulfate area in formermangroves in Merbok, Kedah, Malaysia. In D.L. Dent andM.E.F. van Mensvoort (eds.) Selected Paper of the Ho ChiMinh City Symp. on Acid Sulfate Soils, pp. 95-101. ILRIPubl. 53. Wageningen, The Netherlands.

van Breemen, N. 1976. Genesis and solution chemistry of acidsulfate soils in Thailand. Agric. Res. Rep. No. 848,Center for Agric. Publ. and Document. Wageningen, TheNetherlands. 263 p.

Xuan, Va-Tong. 1993. Recent advances in integrated land useson acid sulfate soils. In D.L. Dent and M.E.F. vanMensvoort (eds.) Selected Paper of the Ho Chi Minh citySymp. on Acid Sulfate Soils, pp. 129-136. ILRI Publ. 53.Wageningen, The Netherlands.

xxxxxxxxx

7

CHAPTER 2

Problem statement, justification, and objectives

Along with soil solution chemistry, mineralogy plays an

important role in acid sulfate soils. Both phases have marked

effects on solubility, stability, and activities of many soil

components, particularly toxic elements, such as Al+3, Fe+2 and

Mn+2• Therefore, effective management of acid sulfate soils

would not be possible unless the chemical and mineralogical

properties of such soils are better understood and modified.

Unfortunately, most research papers presented at the

International symposium on Acid Sulfate Soils, which was first

held in Wageningen, the Netherlands (Dost, 1973), then in

Bangkok, Thailand (Dost and van Breemen, 1982), in Dakar,

Senegal (Dost, 1988), and recently in Ho Chi Minh City,

Vietnam (Dent and Mensvoort, 1993), dealt mainly with

identification, mapping, pH and metal ion chemistry of acid

sulfate soils. No detailed studies have been presented on the

significant relationship between the chemical and

mineralogical properties. The participants in these symposia

did recognize, however, that one of the major limitations in

the utilization and management of acid sulfate soils is a lack

of understanding of chemical and mineralogical processes that

control the release/sorption of the soil chemical

constituents.

8

1. Selection and adaptability of green manures to Al tolerance

in acid sulfate soils

In Thailand, a number of green manure legumes, e.g.

Sesbania, Sunn hemp, Vigna and Cajanus, possess many ecotypes

potentially tolerant to AI. They should be evaluated for their

amendment value to acid sulfate soils. Liming such soils is

not feasible in many cases, and subsoil liming is impractical.

Therefore, the use of AI-tolerant green manure genotypes as

soil amendments appears to be a reasonable approach. Fitting

plants to soil conditions is sometimes more desirable and

economical than adjusting soil conditions to plants.

2. Effects of green manure amendment on Al phytotoxicity in

acid sulfate soils

Aluminum phytotoxicity is an important constraint to

agricultural production. Managing Al toxicity requires the

ability to predict or measure its severity then to control it.

Two approaches will be adopted in this research towards

reclamation of acid sulfate soils to seek AI-tolerant

ecotypes of green manures i and to ameliorate Al toxicity

permitting growth of many important crops.

Amelioration requires that Al toxicity be sufficiently

reduced to permit establishment and sustained growth of

suitable crops by the incorporation of green manure which have

passed the selection process or have shown a great ability to

survive in very acid conditions. The reclamation process

9

consists of successful growing of AI-tolerant green manure

plants followed by their incorporation to improve

physicochemical properties of acid sulfate soils, and to

increase crop yields.

3. Roles of aluminum, iron and sulfate in acid sulfate soils

Moore et al. (1990) indicated that the most important

constraints on acid sulfate soils were related to the combined

effects of low pH and Al toxicity, and Fe stress (high Fe and

low base status). Strategies used in ameliorating acid sulfate

soils include liming, leaching and organic amendments.

Attanandana (1982) reported that both liming and leaching

hastened a drop in the redox potential of submerged acid

sulfate soils in Thailand. The addition of fresh organic

matter has been found to accelerate soil reduction and shorten

the period of high Fe+2 concentrations (IRRI, 1976).

Reduction of Fe compounds is the most important process

that causes pH increases following submergence of acid sulfate

soils. Since these soils contain considerable amounts of S,

the reduction of S04-2 can also significantly increase soil pH

at low redox potential (van Breemen, 1975). Earlier work has

showed that reduced S species accumulate under anaerobic

condi tions, whereas S04-2 concentrations increase under

oxidized conditions (Charoenchamratcheep et al. 1987).

Several researchers have found that the pH's of most acid

sulfate soils increase slowly as compared to normal soils and

10

the pH rarely exceeds 6.0, even after six months of

submergence (van Breemen and Pons, 1978). The slow increase in

pH has been attributed to adverse conditions for microbial

reduction, low contents of metabolizable organic matter, and

low contents of easily reducible ferric oxide (van Breemen,

1976). The high buffer capacity due to dissolved and

exchangeable AI, adsorbed S04-2 and basic sulfate mineral

components would also require large amounts of Fe reduction to

produce a significant increase in pH (van Breemen and

Moormann, 1978).

Moore and Patrick (1991) found that oxidation of acid

sulfate soils results in dramatic changes in metal solubility.

In such dynamic systems, maximum dissolved AI+3 activities

appear to depend directly on pH and the activity of dissolved

sulfate, presumably as a result of near equilibrium with

AIOHS04, according to pAIOHS04 = 17.2. Because pS04 varies

little and is close to 2.3 in most actual and potential acid

sulfate soils, the relation pAl = 0.9 + pH is sufficiently

accurate for most practical purposes.

4. Objectives

This study focused on the chemistry of AI, Al containing

minerals, and the relationship of Al with plant growth in acid

sulfate soils. The goal is to properly manage acid sulfate

soils for increased crop yields.

The specific objectives of this study are to:

11

1. Investigate the potential and adaptability of common

green manure plants to acid sulfate soils;

2. Evaluate the effect of selected green manures on Al

toxicity, growth and mineral nutrition of rice, and identify

possible mechanisms responsible for Al detoxification;

3. Assess chemical and mineralogical changes in some acid

sulfate soils amended with green manures.

12

References


Charoenchamratcheep, C., C.J. Smith, S. Satawathananont, andW.H. Patrick Jr. 1987. Reduction and oxidation of acidsulfate soils of Thailand. Soil Sci. Soc. Am. Proc. 51 :630-634.

Dent, D. L., and M.E.F. van Mensvoort. 1993. Selected Paper ofthe Ho Chi Minh City Symp. on Acid Sulfate Soils. ILRIPubl. 53. Wageningen, The Netherlands.

Dost, H. 1973. Acid Sulfate Soils Proc. of the Int. Symp. ILRIPubl. 18, Vol. I and II. Wageningen, The Netherlands.

Dost, H. 1988. Selected Paper of the Dakar Symp. on AcidSulfate Soils. ILRI Pub!. 44. Wageningen, TheNetherlands.

Dost, H., and N. van Breemen. 1982. Proc. of the Bangkok Symp.on Acid Sulfate Soils. ILRI Publ. 31. Wageningen, TheNetherlands.

IRRI, 1976. Annual Report 1975. International Rice ResearchInstitute. Los Banos, Philippines. 479 p.

Moore, P.A., T. Attanandana, and W.H. Patrick Jr. 1990.Factors affecting rice growth on acid sulfate soils. SoilSci Soc. Am. J. 54 : 1651-1656.

Moore. P.A., and W.H. Patrick Jr. 1991. Aluminum, boron andmolybdenum availability and uptake by rice in acidsulfate soils. Plant Soil 136 : 171-181.

van Breemen, N. 1975. Acidification and deacidification ofcoastal plain soils as a result of periodic flooding.Soil Sci. Soc. Am. Proc. 39 : 1153-1157.


van Breemen, N., and F.R. Moormann. 1978. Iron-toxic Soils. InSoils and Rice, pp. 781-800. International Rice ResearchInstitute. Los Banos, Philippines.

van Breemen, N., and L.J. Pons. 1978. Acid sulfate soils andrice.In Soils and Rice, pp. 739-762. International RiceResearch Institute. Los Banos, Philippines.

13

CHAPTER 3

Genesis and chemistry of acid sulfate soils in the Bangkok

Plain, Thailand; and potential problems for rice production

3.1 Occurrence of acid sulfate soils

Acid sulfate soils occur in all climatic zones with the

majority of them being located in relatively recent coastal

marine sediments. The sulfidic materials which produce acid

sulfates on oxidation are not limited to coastal regions. They

are often associated with inland pyritic material such as

ligni te. When such materials are brought to the surface

through mining or other activities, sulfuric acid will be

produced. Unfortunately, no detailed report on the upland acid

sulfate soil acreage in the world is available. However, the

upland acid sulfate soils have reportedly been found in

Germany (Buurman et al., 1973), the Netherlands (Poleman,

1973; van Wallenburg, 1973), British Columbia and Alberta of

Canada (Pawluk and Dudas, 1978), North Carolina and many other

states in the United States (Carson et al., 1982), and

Australia (Fitzpatrick et al., 1993).

From the FAG Soil Map of the World, Revised Legend (1991),

it is estimated that there is a total area of 12.6 million

hectares of lowland acid sulfate soils. Among these 6.7, 3.7,

2.1, and 0.1 million hectares occur in Asia and the Far East,

Africa, Latin America, and North America, respectively. Dent

(1992) also estimated that in recent coastal plains and tidal

14

swamps world wide, there are some 12 - 14 million ha, mostly

in the tropics, where the topsoil is severely acid, or will

become so if drained. In addition, there may be twice this

area of potentially acid material overlain by shallow peat or

alluvium. Estimates of the extent and distribution of acid

sulfate soils, however suffer more than most from scant field

survey, unreliable laboratory data, and variable definition.

In the Bangkok Plain, for example, van der Kevie and Yenmanas

(1972) estimated an area of 760,000 ha of acid sulfate soils;

but Osborne (1985), defining extreme acidity as base

saturation < 50% and extractable aluminum > 5 cmol , kg- 1 ,

estimated the area of extremely acid soils as only 226,400 ha.

Some acid sulfate soils have developed naturally as a

result of changes in hydrology or relative sea level, for

example, those of the Bangkok Plain. In Senegal, a falling

water table as a result of extended drought since 1971 has

caused new acid sulfate soils on the low estuarine terraces

and intertidal flats (Sadio and van Mensvoort, 1993). But

extensive areas of acid sulfate soils have been developed as

a result of deliberate land drainage.

On a global scale, acid sulfate soils are not extensive.

They often occur in regions of critical population pressure,

notably in the Southeast Asia and West Africa where other land

for subsistence food production is not readily available.

Distribution and classification of acid sulfate soils in the

Southeast and East Asia are depicted in Table 3.1.

Table 3.1. Distribution of acid sulfate soils in the Southeast and East Asia (Langenhoff, 1986).

15

Country Area Reliability oJ Soil Classification(IG' ha) bJ

Bangladesh

Chittagong 200 Sulfaquents, SulfaqueptsKhulna Sunderbans 500 Sulfaquents

Burma 180 SulfaquentsChina

Coastal areas 67 + Sulfaquepts, Sulfic Haplaquepts(South of Rukien)

India

Kerala 110 + highly organicSulfaquepts, partly26,000 ha) affected by salinity

West Bengal 280 SulfaquentsIndonesia

Kalimantan and 2000 mainly organic Sulfaquents,Sumatra Sulfaquepts and Sulfihemists

Cambodia 200 + mainly Sulfaquepts

Japan 4 ++ Sulfaquepts, Sulfic Haplaquepts17 ++ potentially acidshallow seabottom

MalaysiaWest Malaysia 150 + highly organic Sulfaquepts and

SulfaqueptsSarawak 10 mangrove marshes

PhilippinesLuzon, Mindanao 7 SulficTropaquepts,

South Korea 3 + Sulfic Haplaquepts, SulfaquentsThailand

Bangkok Plain 600 ++ SulficTropaquepts(550,000 ha),Sulfaquents (±IO,OOO ha),

Southeast Coast 20 + Sulfaquepts, SulfaquentsPeninsular 50 + Sulfaquepts, partly high organic

matterVietnam

Me Kong Delta 1000 mainly Sulfaquepts, smallerareasof Sulfic Tropaquepts and highlyorganic Sulfaquents

oJ reliability hectarage estimate: - (poor); + (fair); ++ (good);bJ gross estimates

16

3.2 Definition of acid sulfate soils

The definition given to "Acid Sulfate Soils" by Pons (1973)

refers to all materials and soils in which, as a result of

soil formation processes, sulfuric acid either will be

produced, is being produced, or has been produced in amounts

that have a lasting effect on main soil characteristics.

Traditionally, the expression of "acid sulfate soils" is often

used in a narrow sense for soils with "cat clay". The cat clay

phenomenon is a combination of conspicuous straw-yellow

jarositic mottles and a very low pH.

Brinkman and Pons (1973) categorized acid sulfate soils

into three groups and defined soils in each group as follows.

1) A potential acid sulfate soil or material is a soil or

reduced parent material which is expected by the person

identifying it to become an acid sulfate soil or material upon

drainage and oxidation under certain defined field conditions.

2) An actual acid sulfate soil is a soil with one or more

horizons consisting of acid sulfate materials, i. e., materials

containing soluble acid aluminum and ferric sulfates in

concentrations toxic to most common dry-land crops. Such

materials have high proportions of exchangeable aluminum, pH

(in water) below 4, and may have characteristically pale

yellow mottles by basic sulfates of iron, potassium iron or

sodium iron sulfates. Some have white mottles of aluminum

sulfates.

17

3) A pseudo-acid (or para) sulfate soil contains one or

more horizons with characteristic yellow mottling (basic iron

sulfates) that is commonly associated with acid sulfate

conditions. The soil does not have a pH below 4 and does not

contain free acids or more than 60% exchangeable aluminum.

3.3 Classification and taxonomy of acid sulfate soils

with the notable exception of the FAO legend (1991), most

recent morphological classifications distinguish between

(actual) acid sulfate soils and potential acid sulfate soils.

Soil Taxonomy (Soil Survey Staff, 1990) distinguishes the

sulfuric horizon (with a pH in water < 3.5 and j arosi te

mottles) from sulfidic material (reduced material containing

0.75% or more S and less than three times as much CaC03

equivalent) that will become a sulfuric horizon if oxidized.

Potential acid sulfate soils are placed in the Order

Entisols, as Sulfaquents (Aquents with sulfidic material

within 50 em of the mineral soil surface), Sulfic Fluvaquents

(Fluvaquents with sulfidic material between 50- and 100-cm

depth), or Sulfihemists (Histosols with sulfidic material

within the 100-cm depth) .

Actual acid sulfate soils can be classified as Sulfaquepts

(Aquepts with a sulfuric horizon that has its upper boundary

within 50 em of the soil surface), Sulfic Tropaquepts

{Tropaquepts with jarosite mottles and a pH 3.5 to 4 somewhere

within the 50-em depth, or with jarosite mottles and a pH < 4

18

in some part between 50- to 150-cm depth), or Sulfic

Haplaquepts (comparable to Sulfic Tropaquepts but under a more

temperate climate). The distinction between Sulfaquepts and

Sulfic subgroups is very useful agronomically in that the

former are generally unsuitable for agriculture without costly

amendments, whereas the latter can often be made productive

with lesser modifications.

Acid sulfate soils that are dominantly organic may be

Sulfohemists (Histosols with a sulfuric horizon that has its

upper boundary within 50 cm of the surface). Unfortunately,

acid sulfate soils with a peaty top soil, which may well be

different agronomically from those lower in organic matter,

are not separated from the histic subgroups.

Recent proposals to modify Soil Taxonomy regarding acid

sulfate soils (Fanning and Witty, 1993) are:

1) Redefine the sulfuric horizon, taking account of acid

sulfate soils that do not show jarosite mottles, as a layer>

15 cm thick with a pH in water < 3.8 and evidence that this

acidity is caused by oxidation of pyrite (either jarosite

mottles, underlying sulfide material or 0.05% or more soluble

sulfate) ;

2) Redefine sulfidic material simply as material that shows

a drop of at least 0.5 pH units to < 3.8 during incubation for

8 weeks in moist, oxidized conditions;

3) Provide a new Great Soil Group of Sulfochrepts for acid

sulfate soils that are not poorly drained {e.g., on mine

19

spoil) and a new subgroup of Salorthidic Sulfaquepts for very

saline acid sulfate soils.

3.4 Physiography of acid sulfate land in the Bangkok Plain

Thailand is situated between 5° and 21° N Latitude, and

between 97° and 106° E Longitude, and covers area about

5.14x10 5 km2• Physiographically the country can be subdivided

into six regions (Fig. 3.1). The Central Plain, which includes

the Bangkok Plain, is a deep syncline filled with sediments.

It forms the lower Chao Phraya River basin which is

characterized by riverine sediments in its upper reaches and

marine and tidal marsh deposits near the coast. The

Continental Highlands (200 - 500 m high) and part of the

Southeast Coast region belong to the catchment area of the

rivers forming the delta of the Bangkok Plain. The Northeast

Plateau belongs to the watershed of the Me Kong River, which

drains into the South China Sea. Peninsular Thailand and the

Southeast Coast region are generally hilly and mountainous

with altitudes between 200 and 1000 m.

The largest continuous area with acid sulfate soils

covering about 8000 km2 (0.9 million hectares), is found in

the Bangkok Plain between Bangkok and the southern boundary of

the Chao Phraya riverine alluvium. Non-acid marine soils occur

in a 25 - 40 Jan wide zone between the acid soils and the

coast. Most of the smaller coastal plains fringing the

Southeast Coast and Peninsular Thailand have smaller areas

20

LAOS

BURMA Chiang Mal

200 300km! ,

PHYSIOGRAPHIC REGIONSOF THAILAND

KAMPUCHEA

o 100~~._---'--~

o Cental plain

~ Southeast coast

~ Nonheasl plateau

ITllill Central highlands

~ Nonh and wesl continental hIghlands

m Peninsular Thailand

OF

GULF

THAILAND

ANDAMAN

SEA

Figure 3.1. Physiographic regions of Thailand (modified from

Moorrnann and Rojanasoonthon, 1972).

21

with acid sulfate soils, often intimately associated with

potentially acid soils and non-acid marine soils.

Acid sulfate soils occur mostly in the Bangkok Plain;

hereby the physiographic features of this region will be

described briefly. Vacharotayan (1977) divided the Bangkok

Plain into three subregions: Old Delta, New Delta, and Fan

Complex Area (Fig. 3.2).

The Old Delta is the area located from the northern part of

Chainat Province to the Northern part of Ayutthaya Province.

The area of this subregion is 5900 krn2• The altitude gradually

decreases from 15 m above mean sea-level at the apex near

Chainat to 5 m at Ayutthaya and 1.5 m near the coast. The

soils are derived from riverine alluvium but are often

underlain by marine and brackish sediments, some of which are

potential acid sulfate parent materials.

The New Delta is comprised of two types of areas:

(1) Deltaic High. This area occupies about 2000 krn2 • The

elevation is from 3.5 to 5 m above the mean sea-level. The

plains are the location of the Bangkok Metropolitan Area and

some other provinces in the southeastwards. Soils are of non

acid-sulfate marine deposit origin.

(2) Delta Flat. This area has lower elevation than the

Deltaic High. It is about 1 to 2 m above the mean sea-level,

and during the rainy season the land will occasionally be

inundated. The area comprises 11,000 krn2, of which 80% or 8000

krn2 is composed of acid sulfate soils.

22

INDEX TO M,\P 5 II EETSPATHUM TIIANI. NONTHABliRI. BANGKOK-THON BURl. SAMUT SAKIION AND SAMUT PRAKAN PROVINCE

.i 0 III 20 lfl til .ill am~j=!.:_"=Lh4_~ I .,..,,/

I

IOI~JO'

'-', 1

\.

c:

c:It-2ft "'0-

7-.."

../../

.._.-"

13"""

\

<, ~r\

I' )1

III

" I

1"II

I

\ ........I.

I

t'

Figure 3.2. Area of different physiographic regions in the

Southern Bangkok Plain (after Vacharotayan, 1977).

The Fan Complex Area develops, forming piedmont topography,

along with the marginal parts of the Bangkok Plain, where the

Plain merges with the mountain ranges. The total area is

estimated to be 18,000 km2 •

23

3.5 Genesis of acid sulfate soils

Genesis of acid sulfate soils is composed of two main

processes: a geogenetic process and a pedogenetic process. The

geogenetic process deals with the formation of pyrite and is

termed "sulfidization", whereas the pedogenetic process

involves oxidation of pyrite and is termed "sulfuricization"

(Pons and van der Kevie, 1969).

3.5.1 Geogenetic process

In the coastal area closed to the estuarine environment,

the accretion of sediment takes place under the influences of

brackish and marine water. Later, when the sediments are piled

high enough, and when the influence of tidal action decreases

due to the subsidence of the mean sea-level, some

telmatophytic plants such as Rhi zophora , Avicennia, and some

other species are able to grow. With the continued flushing of

the marine or brackish tide which supply dissolved sulfate,

limit aeration, and carry away the carbonate produced, and

with the supplementary organic matter from the plants, the

secondary pyrite forms slowly and gradually accumulates (van

Breemen, 1973b). If the conditions suited for pyrite formation

and accumulation are prolonged, the pyrite content in the soil

might be as high as 10% of the mass of the soil (van Breemen

and Pons, 1978; van Breemen, 1980). van Breemen (1976) also

found 160 to 225 cmol , 8°4-2 kg- 1 of soil from air-exposed

pyrite sample collected from absolutely non-oxidized pyrite

substratum in acid sulfate soils of the Bangkok Plain.

24

The essential factors for the accumulation of pyrite as

summarized by Pons and van Breemen (1982) are:

1) sulfate (e.g., from sea water) is continuously supplied

over an appreciable period;

2) iron containing minerals are present in the sediments;

3) metabolizable organic matter;

4) sulfate reducing bacteria, which are practically always

present;

5) anaerobic environment;

6) limited aeration for oxidation of all sulfide to

disulfide (and/or sulfide to elemental sulfur).

Sulfate is abundant in seawater, thus it will not be a

limiting factor in pyrite accumulation. The surface reducing

bacteria in the genera Desulfovibrio and Desulfotomaculum are

known to be ubiquitous in marine environments. The clayey

sediments of most tidal swamps contain sufficient fine-grained

iron oxide for the formation of 2 - 6% pyrite, but iron could

be limiting where the soil or sediment is peaty or mainly

quartz sand. In tropical seas, suspended sediment is normally

low in organic matter for appreciable formation of sedimentary

pyrite. The dense mangrove vegetation on tropical tidal

marshes can however, supply abundant organic matter. Thus, in

mangrove swamps, most of the ingredients for pyrite formation

are supplied; and limited aeration will take place depending

on tidal influence on soil hydrology.

25

3.5.1.1 Mechanism of pyrite formation

According to Pons and van Breemen (1982), the formation

of pyrite (cubic FeS2 ) requires (1) the reduction of sulfate

to sulfide under the influence of dissimilatory sulfate

reducing bacteria in an anaerobic environment; (2) the partial

oxidation of sulfide to polysulfide or elemental sulfur; and

(3) either formation of Fe(II)-monosulfide (from Fe(III)

oxides or Fe-containing silicates and dissolved sulfide)

followed by the combination of elemental sulfur and Fe(II)

monosulfide to pyrite, or direct precipitation of pyrite from

dissolved Fe(II) iron and polysulfide ion (Roberts et al.,

1969; Goldhaber and Kaplan, 1974; Dent, 1986). Whatever the

mechanism in operation, formation of pyrite with any Fe(III)

oxide as the source of iron will take place according to the

following overall reaction (CH20 stands for organic matter) :

Fe203(s) + 4804-2

( 0'1) + 8CH20 + 1/202Cg) ---> 2Fe82(s) + 8HC03- coCj ) + 4H 20 (l) [1]

This overall reaction includes reduction of all sulfate to

sulfide. Dissolved sulfide reacts rapidly with dissolved

ferrous iron or with ferric oxide to form black ferrous

sulfide, which is amorphous to X-ray diffraction or gives

broad lines of mackinawite (tetragonal FeS). In the presence

of oxidants such as 02 or ferric ion, part of the dissolved or

solid sulfide can be oxidized to elemental sulfur. Elemental

S reacts with dissolved sulfide to form aqueous polysulfide,

which in turn reacts with FeS to form pyri te, FeS2 , ei ther

directly or with greigite (cubic Fe 3S4 ) as an intermediate

26

(Rickard, 1975). The pathway involving greigite, which

apparently requires atmospheric oxygen, yields frarnboidal

pyrite; whereas the other reaction, in the absence of oxygen,

gives non-frarnboidal pyrite (Sweeny and Kaplan, 1973; Rickard,

1975). The carbonate alkalinity formed by this process is

either removed by diffusion or leaching, or is retained by

precipitation of CaC03 (Hardan, 1973). The interstitial water

of marine muds associated with sulfate reduction is often

highly supersaturated with calcite, possibly because dissolved

organic compounds inhibit crystallization (Berner, 1971). The

alkalinity is carried away, leading to a separation of the

potentially acid material (pyritic mud) and actual alkalinity

(HC03- ) , which is mainly absorbed by the vast mass of the

oceans. This process is the crucial step that is responsible

for the formation of acid sulfate soils.

3.5.2 Pedogenetic process

This process involves profile development, and is mainly

implicated in (1) pyrite oxidation; (2) neutralization of

acidity; and (3) formation of the products resulting from

oxidation and neutralization. The pedogenetic process starts

taking place when the geogenetic process is terminated; that

is, when the land elevation is high enough to lessen the

influence of tidal action that brings about the anaerobiosis.

The elevation of the land, relative to the mean sea-level, may

be caused by either an increase aggradation of the sediment,

27

a retreat of the seawater, or a tectonic uplift of the land

(van Breemen, 1973b).

3.5.2.1 Pyrite oxidation following drainage

Appreciable aeration of potential acid sulfate soils

and subsequent acidification starts only after the water table

stays below the upper part of the pyritic zone for several

weeks. A prerequisi te for such drainage is decreased tidal

influence. This is brought about either gradually by natural

processes (coastal accretion or a relative decrease in mean

sea-level or by tectonic uplift) or more abruptly by man-made

activities. Many mangrove areas of the Southeast Asia are

still under the tidal influence, acidification takes place in

material from the subsoil brought to the surface by the mound

building mud lobster, Thalassina anomale (Andriesse et al.,

1973) .

Pyrite is stable only at low redox potential (Eh),

i.e., under reduced conditions, and will be oxidized in

oxygenated environments. Under natural conditions, dissolved

aqueous oxygen attacks pyrite slowly at first, yielding

ferrous iron and elemental sulfur

[2]

Further oxidation of elemental S to sulfate or sulfuric acid

with 02 as oxidant is a slow process

28 + 302 + 2H20 ---> 2804-2 + 4W [3]

It is likely that equation [2] and [3] are strictly chemical

reactions because the pH is near or above neutral, and is not

28

suitable for Thio- and Ferro-organisms. Arkesteyn (1980)

reported that Thiobacillus thiooxidans can oxidize elemental

sulfur to sulfate under these conditions. Thiobacillus

bacteria grow optimally at pH 2 to 3.5 (Nordstrom, 1976)

although they can survive at pH levels as high as 6 and 7

(Nordstrom, 1982). Whether chemical or biochemical processes,

the reaction [3] takes place in acid sulfate soils and causes

an increase in acidity. The ferrous iron in reaction [2] can

be further oxidized chemically or biochemically to ferric

iron. It is noted that Thiobacill us ferrooxidans can grow

optimally between pH 2.5 and 5.8 (Ivarson et al., 1982). If

the pH is high (> 4), ferrous iron will be oxidized and

precipitated to ferric hydroxide as follows

[4]

If the pH is low enough « 4), the Fe+3 ions will still be in

a dissolved form after oxidation :

[5]

Dissolved ferric ions in reaction [5], on the other

hand, react rapidly with pyrite according to :

Fe82 + 2Fe+ 3 ---> 3Fe+2 + 28 [6]

Under most experimental conditions, further oxidation of

elemental S is virtually instantaneous according to

28 + 12Fe+3 + 8H20 ---> 12Fe+2 + 28°4-2 + 16W [7]

The combination of reactions [6] and [7] yields

[8]

29

Because ferric iron is appreciably soluble only at low

pH ( < 4), reactions [6], [7], and [8] are limited to acid

environments. In the presence of oxygen, the dissolved ferrous

iron produced by these reactions can be further oxidized to

ferric iron, and pyrite oxidation can continue. But there is

a kinetic restriction : at low pH the oxidation rate of Fe+2

by O2 is slow. However, T. ferrooxidans and Ferrobacillus

ferrooxidans which are acidiophillic can overcome this

barrier, thereby promoting pyrite oxidation (Singer and Stumm,

1970) .

Pyrite oxidation in situ is usually much slower due to

the rate limiting diffusion of oxygen into the wet pyrite

substratum (van Breemen, 1973b; De Richmond et al., 1975).

Pons (1965) noted that pyrite in sediment riched with calcium

carbonate is oxidized slowly, disappearing over a period of

centuries; but that pyrite in acid sulfate soils is oxidized

rapidly.

Complete oxidation and hydrolysis of iron to ferric

oxide yields 2 moles of sulfuric acid per mole of pyrite:

Fe82 + 15/402 + 7/2H20 ---> Fe(OH)3 + 2804 -2 + 4W (9]

The term sulfuricization has been used to denote these

acidification processes (Fanning, 1978).

In addition to the foregoing reactions, in the presence

of cations such as K+, Na", or H30+, pyrite may be oxidized

directly to jarosite or its counterparts according to van

Breemen (1973b); van Mensvoort and Tri (1989):

30

FeS2 + 15/402 + 5/2H20 + l/3K+ ---> 1I3KFe3(S04)2{OH)6 + 4/3S04-2 + 3W

[10]

3.5.2.2 Iron sulfide-oxidizing bacteria

Microbes catalyze many reactions including those

related to the formation and oxidation of sulfide-ore deposits

(zajic, 1969; Kushner, 1978). Desulfovibrio (e.g., species

desul furi cans) and Desulfotomaculum have been mentioned for

their important roles in pyrite formations. Some oxidation

reactions involving sulfur compounds occur rapidly in vitro at

normal temperatures and pressures. The oxidation of some of

the more reduced compounds will however, proceed only if they

are mediated by certain microorganisms. Most of these

microorganisms belong to the Thiobacilli, a group of

chemoautotrophic organisms capable of utilizing the energy

obtained from the oxidation of H2S, So, S203-2, S406-2, S03-2 to

S04-2 for the assimilation of CO2 (Nor and Tabatabai, 1977).

Three species of Thiobacillus have been isolated from

acid mine wastes: T. ferrooxidans, which oxidizes ferrous iron

and pyrite as well as sulfur; T. thiooxidans, which oxidizes

only sulfur and pyrite; and T. acidophilus, which is a

facultative autotroph (grows on either inorganic or organic

substrates) , oxidizing sulfur but not ferrous iron.

Thiobacillus acidophilus cannot oxidize pyrite unless it is in

a mixed culture (Guay and Silver, 1975). These bacteria are

acidophilic with optimal growth conditions around a pH of 2 to

3 (Kelley, 1985), although they can survive up to pH values as

31

high as 6 or 7 (Nordstrom, 1982). The T. ferrooxidans and

Ferrobacillus ferrooxidans are able to mediate oxidation of

Fe+2 to Fe+3• Members of the Beggiatoaceae, photoautotrophic

bacteria of the Rhodobacteriaceae, and Sphaerotilus can also

catalyze the oxidation of sulfur compounds, but Thiobacilli

are by far the most important group (van Breemen, 1973b).

3.5.2.3 Neutralization of acidity

Three different fractions capable of deactivating

sulfuric acid have been identified (a) carbonate alkalinity in

solution, (b) exchangeable bases, and (c) weatherable

minerals.

a. Alkalinity. Alkalinity mainly refers to the

carbonates of Ca and Mg that have accumulated in the sediments

during aggradation that comes with flood water. Calcium

carbonate contents are low or nil in most marine sediments of

the humid tropics but may be appreciable (frequently higher

than 10%) in sediments of arid and humid temperate regions.

The acidity from 1% (mass fraction) of pyrite-S is

approximately balanced by 3% calcium carbonate. If seawater is

entrapped in a sediment and all dissolved sulfate is reduced

to sulfide, the increase in bicarbonate would lead to

supersaturation with calcium carbonate (Pons et al., 1982).

van Breemen (1973b) has noted that the alkalinity of

interstitial waters in non-alkaline soils rarely exceeds 10

meg L- 1• Hence, at moisture contents up to 100%, dissolved

alkalinity can contribute to the neutralization of 1 meg of

32

acid per 100 g of soil at the most. Seawater has an even lower

alkalinity (2 to 2.5 meq L-1) than most near-neutral ground

waters, and cannot be considered as an effective buffering

agent, even if copious leaching is applied. Nonetheless, in

the eastern part of the Bangkok Plain of Thailand, centuries

of seasonal flooding with moderately alkaline water (2 to 5

meg HC03- L-1i van Breemen, 1973a) have probably increased the

pH of the upper horizons of acid sulfate soils to near

neutrality (van Breemen, 1973b).

b. Exchangeable bases. Active H~ created by the first

stage of pyrite oxidation can be exchanged with exchangeable

bases at the soil colloid surfaces. Between 5 and 10 cmoL, kg-1

of acid in soil is taken up by the exchange complex of typical

acid sulfate soils in Thailand where pH drops from 7.5 or 7 to

about 5. The total amount of acid taken up by the exchange

complex of an acid soil is approximately equivalent to the

difference between the CEC at pH 7 and the amount of

exchangeable bases at the soil pH (van Breemen, 1973a). For

acid sulfate soils of pH 3.5 to 4 from the Bangkok Plain, this

amount is about 10 to 30 cmol , kg-1 of soil (Sombatpanit,

1970) .

c. Weatherable minerals. Essentially all alumino

silicates are attacked by acid and release alkaline metals and

monomeric silica into solution. If incongruent dissolution

takes place, aluminum released from the minerals will be kept

as potential acidity in some minerals such as kaolinite, or

33

basic aluminum sulfate [AI (OH)S04] . If monomeric silica

polymerizes and precipitates as amorphous silica, the reaction

will favor the breakdown of minerals, thereby buffering the

acidity. Generally, the layer silicate minerals are

concentrated in the clay fraction and because of their large

specific surface area, clay minerals can be the important H+

consumers. Like other common clay soils, acid sulfate soils

have been reported containing clay minerals; kaolinite,

illite, smectite (montmorillonite, beidellite, nontronite),

chlorite, glauconite, and charmosite (Satawathananont, 1986).

In most cases, kaolinite is the predominant clay mineral and

is the product of most weathering processes under acid

conditions that occurs widely in acid sulfate soils (van

Breemen, 1973b). The mechanisms of neutralization of acidity

by the minerals can be written as follows:

(i) Congruent dissolution:

M-Al-silicate + (3+a)W + bH20 ---> aM' + AI+3 + cH4Si04o (11)

(ii) Incongruent dissolution

M-Al-silicate + aW + bH20 ---> aM' + cH4Si04° + Al (silicate) residue

(12)

where M = exchangeable bases; a, b, and c are whole integers.

(iii) Incongruent Mg-montmorillonite or smectite

to kaolinite

6Mg-rnontmorillonite + 2W + 23H20 ---> Mg+2 + 7kaolinite + 8H4Si04o

(13)

34

In summary, it appears that the incongruent dissolution

of montmorillonite or smectite takes place rapidly. This

process is capable of maintaining an equilibrium solution even

at high rates of acid formation. If sufficient time is

available for dissolved silica to polymerize and precipitate

as amorphous silica (which appears to be the case of field

conditions) the montmorillonite-kaolinite equilibrium can

buffer the pH between 3.5 and 4.5. A much lower equilibrium pH

is established if dissolved silica increases beyond the

solubility of amorphous Si02 which happens upon rapid

oxidation associated with high rates of silica release. In the

course of months, equilibrium between kaolinite (as well as

montmorillonite) and its dissolved constituents is established

and can help to maintain pH values between 3.5 and 4 under

field conditions. In very acid conditions, kaolinite may break

down to buffer a change in pH:

Kaolinite + 4W + 28°4-2 ---> 2A10H804 [14]

(basic Al sulfate) (amorphous silica)

It should be pointed out that other processes may be

involved in maintaining the typical field pH values of 3 to 4.

The assemblage jarosite-limonitic goethite will tend to keep

the pH at about 3.7. Mica and feldspar are also particularly

important because they probably provide much of the K+ for

jarosite.

35

3.5.2.4 Products from oxidation and neutralization

The ferrous, hydrogen, and sulfate ions released during

pyrite oxidation normally undergo various further reactions in

the soil. Protons are largely inactivated by different

buffering reactions. Essentially all ferrous iron is

ultimately oxidized to ferric iron, which precipitates as

jarosite (KFe3(S04)2(OH)6)' poorly crystallized goethite, or

amorphous ferric oxide. The larger part of the sulfate

released during pyrite oxidation, however, remains in solution

and is removed from the soil by leaching, and by diffusion

into the surface water. The remaining sulfate is partly

precipitated, either as jarosite or as the basic aluminum

sulfate, AIOHS04 (van Breemen, 1973b) and partly adsorbed,

mainly by ferric oxides. Under relatively dry conditions,

gypsum (CaS04.2H20) can form. Under strongly evaporative

conditions, surface crusts of still more soluble sulfates such

as sodium alum (NaAI (S04) 2. 12H20) , tamarugi te (NaAI (S04) 2' 6H20) ,

pickeringite (MgAI2(S04)4.22H20) and rozenite (FeS04.4H20) have

been observed. Under anaerobic condi tions , sul, fa te can be

reduced again to sulfide, which may, temporarily, be fixed as

FeS. In very young acid sulfate soils still under tidal

influence, this sulfide may even be incorporated in pyrite

again (van Breemen, 1973b). The diagenesis of these compounds

have effects on morphology and profile development of acid

sulfate soils.

36

a. Jarosite. The general formula of jarosite is

AB3(S04)2(OH)6; where A = K+, Na", H30+, NH/, Ag+, or 1/2Pb+2 and

B = Fe+3 (jarosite) or AI+3 (alunite) (van Breemen, 1973a).

If "A" in a jarosite minerals is K+, Na+, H30+, or NH/,

it should be called as jarosite, natrojarosite, hydronium

jarosite, and ammoniumjarosite, respectively. Most jarosite

minerals found in acid sulfate soils are (potassium) jarosite.

Jarosite may form according to the reaction

or

3Fe(OH)/ + 2S04-2 + K+ ---> KFe3(S04)2(OH)6

(Matijevic et al., 1975)

[15]

3Fe2(S04)3 + 1/202 + llH20 + 2K+ ---> 2KFe3(S04)2(OH)6 + 5H2S04 [16]

(Bloomfield and Coulter, 1973; Ivarson et al., 1982).

Typically, j arosi te occurs as conspicuous, pale yellow,

earthy material as fillings of biopores or as efflorescence on

ped faces and pore walls. Individual particles are often

smaller than 1 um and their diameter rarely exceeds 5 f.lTIl

(Berner, 1971; Ross et al., 1982). van Breemen (1973a) found

between 50 to 100% of jarosite contained in most yellow

mottles, and sometimes in brown-yellow mottles or even red

mottles if these mottles are located close to a jarositic

horizon.

Jarosite can be synthesized at room temperature within

1 - 6 months by aeration of a solution of ferrous sulfate

(0.06 - 0.6 mol L- 1) and potassium sulfate (0.01 - 0.1 mol L- 1

)

acidified with H2S04 to a pH of 0.8 to 1. 7 (Brown, 1970).

37

Ivarson et al. (1979) could detect jarosite after 10 days of

aerobic incubation of pyritic mangrove soil. Ivarson (1973)

observed the formation of ammonium jarosite within 4 to 10

days in cultures of T. ferrooxidans in a medium containing

ferrous sulfate. Since the organisms were isolated from

natural jarosite samples, it seems likely that they do in fact

take place in the formation of jarosite in situ.

From synthesis experiments (Brown, 1970) and by the

appearance with grain size of jarosite precipitated in acid

sulfate soils, Nordstrom (1982) suggested that jarosite can

form directly from solution without a precursor solid phase,

generally at strong supersaturation faces in association with

Thiobacilli. In soils, where it is formed more readily than in

pure solutions, supersaturation may be maintained for months

even when the mineral is present (van Breemen, 1976).

In agreement with theoretical stability relations,

jarosite is formed only in acidic (pH 2 to 4), oxidized (Eh >

400 mV) environments. In acid sulfate soils however, it is

metastable and will eventually hydrolyze to goethite. Often,

brown mottles from acid sulfate soils will give a sharp

jarosite X-ray diffraction pattern, indicating that a thin

coating of Fe(III) oxide may prevent recognition of jarosite

in the field. Hydrolysis is enhanced by leaching and by a

supply of bases. Al though the yellow color may turn brown

within a few months by dialysis (van Breemen, 1976), yellow

[17)

38

jarosite mottles sometimes persist for decades at pH values

above 4 in limed soils (Verhoeven, 1973).

Jarosite is stable only under relatively oxidized, acid

conditions. Both a decrease in Eh and increase in pH may lead

to the disappearance of jarosite

(i) by solution reduction:

KFe3(804)2(OH)6 + 6W ---> K+ + 3Fe+2 + 2804-2 + 6H20

(ii) by hydrolysis:

KFe3(804)2(OH)6 + 3H20 ---> K+ + 3Fe(OH)3 + 2804-2 + 3W [18)

or

[19]

b. Iron oxides. In most acid sulfate soils only part of

the iron released as a result of pyrite oxidation (and clay

breakdown, e.g., Fe-chlorite) is tied up in jarosite, the bulk

being present in free ferric oxides and in the clay mineral

beidellite. In acid sulfate soils, Fe(III) oxides may be

formed either directly by oxidation of dissolved, solid,

adsorbed ferrous iron, or by hydrolysis of jarosite. The first

mechanism is dominant in zones where jarosite is absent, for

instance, in the upper part of the pyritic substratum and in

the A horizon. Ferric oxide mottles in these zones are

frequently dark reddish-brown or dark-brown, rather than

yellowish-brown as in the B horizon (van Breemen, 1973b).

From the chemical a.nd morphological changes observed in

the sequence of acid sulfate soils in Thailand, van Breemen

(1973a) indicated that jarosite is formed first and that most

39

of the ferric oxide in the B horizon of the older soils is

formed later by hydrolysis of jarosite. The increase in the

prevalence of brown rims around jarosite masses in thin

section from shallower depths corroborates this (van Dam and

Pons, 1973; Miedema et al., 1974).

At pH values below 6, ferric oxides are incompatible

with pyrite, and indeed, most iron oxide is formed at an

appreciable distance from pyrite. By contrast, at high pH, as

in calcareous pyritic sediments, goethi te is often

pseudomorphic after pyrite (Miedema et al., 1974).

In the better drained, deeply developed acid sulfate

soils, part of the ferric oxides in the B horizon may occur as

hematite, giving conspicuous red mottles. It is unlikely that

the hematite is formed with ferrihydrite [FesHOa .4H20] as a

precursor (Schwertmann and Cornell, 1991), because conditions

favoring ferrihydrite (rapid hydrolysis of dissolved iron at

relative high concentrations) are absent in the red mottled B

horizons. Perhaps the low pH and periodically dry conditions

facilitate transformation of fine-grained goethite, via a

solution stage, to the thermodynamically more stable hematite

(van Breemen, 1976).

c. Water soluble sulfate. Gypsum has been observed in

coastal marine soils over a wide pH range of 3.5 - 7. The

upper limit of the calcium activity product in such soil is

clearly regulated by precipitation as gypsum. Due to its

40

fairly high solubility, gypsum is confined to the dryer soils

or to those with some supply of calcium carbonate.

The soil solution of acid sulfate soils is generally

supersaturated with alunite, the aluminous counterpart of

jarosite (van Breemen, 1973b). Alunite has evidently been

observed however, in some acid sulfate soils, and is more

typical of rock weathering by relatively concentrated sulfuric

acid in sheltered or hydrothermal environments.

3.5.2.5 Profile development toward maturity

Harmsen and van Breemen (1975) proposed a hypothetical

chronosequence (stage A --> C) of seasonally flooded acid

sulfate soils in the Bangkok Plain. Stage A represents an

undrained mangrove soil, and Band C illustrate increasingly

older and deeper developed acid sulfate soils. When a pyritic

soil is drained periodically, e. g., during a dry season,

pyrite within 50 cm of the surface can be removed completely

in a few decades. Jarosite is formed by oxidation of ferrous

sulfate that diffuses upward from the zone where pyrite

oxidizes, and accumulates at shallow depths as yellow mottles

along pores and cracks (stage B). Jarosite is slowly

hydrolyzed to fine-grained goethite in the upper part of the

yellow mottled horizon, causing a residue of brown mottles

(stage C). In the surface soil, reduction during flooding

mobilizes iron, part of which migrates downward. This

eventually leads to a lowering of ferric oxide near the soil

surface (stage C). Thus, as the soils become older (in terms

41

of pedological maturity, not absolute age) and better drained,

the different horizons are found at a progressively greater

depth. For example, while the acid sulfate soils from inland

parts of the Bangkok Plain and the Me Kong Delta, especially

those in the Plain of Reeds, developed in sediments of

probably the same age, most of the soils from Vietnam are more

poorly drained and hence less developed and younger than those

from Thailand.

Due to low decomposi tion rates under acidic conditions,

acid sulfate soils often have a distinctly higher organic

carbon content in the surface horizon than comparable non-acid

marine soils (Kawaguchi and Kyuma , 1969). Still lower rates of

organic matter decomposition prevail in acid sulfate soils in

continuously wet equatorial climates. Such soils often have a

peaty surface layer as in Western Malaysia (Chow and Ng, 1969)

and southern Kalimantan (Driessen and Soepraptohardjo, 1974).

Peaty surface horizons are also found in poorly drained

acid sulfate soils in monsoon climates as in Kerala state,

India (Rao et al., 1975), the Plain of Reeds, Vietnam (Dent,

1986), and Southern Peninsular of Thailand (Krisornpornsan,

1991) . Acid sulfate soils developed in highly organic pyritic

sediments sometimes lack the conspicuous yellow mottles,

probably because they are generally too reduced to permit the

persistence of jarosite (Kosaka, 1971).

42

3.6 Problems of acid sulfate soils to rice production

The problems concerning acid sulfate soils used for rice

production include 1) adverse effect of H+, 2) aluminum

toxicity, 3) iron toxicity, 4) phosphorus deficiency, and 5)

low base status.

3.6.1 Adverse effect of H+

The direct adverse effect of W on plants have been

observed at an acidity stronger than pH 3.5 4. The

probability of soil acidity directly resulting in plant growth

problems on some acid sulfate soils has already been reported

(Moormann, 1963; Brinkman and Pons, 1973). Rice grown in

solution culture is found also to be affected directly by H+

at pH below 3.5 to 4 (Ponnamperuma et al., 1973; Thawornwong

and van Diest, 1974).

Occasionally, in young acid sulfate soils (Sulfaquepts) and

rapidly oxidized potential acid sulfate soils (Sulfaquents),

pH levels as low as 3 or even lower can be found. In addition,

pH values of approximately 1 to 2 have been observed in

oxidized horizons of acid sulfate soils (Tanaka and Yoshida,

1970; van Breemen and Pons, 1978). The W injury can be

eliminated by raising pH above 4.

3.6.2 Aluminum toxicity

Ponnamperuma (1972) reported that aluminum in acid sulfate

soils becomes more soluble at a low pH. At a pH lower than

4.4, a high concentration of aluminum in the soil solution is

found to be toxic to rice plants. Cate and Sukhai (1964)

43

reported that the toxic effects of Al on the growth of rice

seedlings appeared when Al concentrations were greater than

1.2 mg L-1• Young rice seedlings growing in pH 3.5 to 5.4

solutions were adversely affected by 0.5 - 2 mg L-1 dissolved

AI, and 3 to 4 week-old plants were suffered from more than 25

mg Al L-1 (Tanaka and Navasero, 1966a; Thawornwong and van

Diest, 1974). The death of rice plants grown on some acid

sulfate soils of Vietnam which contained 68 mg kg-1 of

dissolved Al was attributed to Al toxicity (Nhung and

Ponnamperuma, 1966).

Regardless of the type of acid sulfate soils, such toxic

levels occur at soil pH below 4.5 - 5.0 for seedlings, and

below 3.5 - 4.2 for older plants. These cri tical values

however, depend on other factors. Phosphate, for instance,

counteracts Al toxicity, partly due to coprecipitation of Al

and P outside the plants and in the roots (Tanaka and

Navasero, 1966b; Rorison, 1973; Attanandana, 1982). Tanaka and

Navasero (1966a) reported that in a culture solution, the

critical level of Al was 25 mg L-1 for normal rice plants, 15

mg L-1 for P-deficient plants, and no toxic symptoms were

observed even at concentrations of Al as high as 40 mg L-1 in

high P treated solutions. The low availability and high

fixation of P in acid sulfate soils (Hesse, 1963; Watts, 1969)

may therefore aggravate Al toxicity (van Breemen and Pons,

1978) .

44

High Al levels affect cell division, disrupt certain enzyme

systems, and hamper uptake of P, K, Ca and several essential

cations (Rorison, 1973). Aluminum can also disrupt the

activities of proteinaceous enzymes located in the cell wall.

The harmful effects of Al are more pronounced in the root.

Aluminum concentrations as low as 1 to 2 mg L- 1 markedly retard

the growth of rice roots (Cate and Sukhai, 1964). Symptoms of

Al toxicity to rice plants may be observed initially by

interveinal orange-yellow discolorations of the tips followed

by brown mottling (Yoshida, 1981; Tadano, 1985). Because Al

contents in the plant do not necessarily reflect aluminum

toxicity, the disorder is easily overlooked (van Breemen and

Pons, 1978).

Aluminum toxicity is predominant when soils are unsaturated

and just after flooding. After prolonged submerging when pH

rises, Al will precipitate, and the toxicity is eliminated. In

acid sulfate soils which contain low reactive reducible Fe and

low easily decomposable organic matter, pH rises slowly, and

Al toxicity may persist for several weeks. Broadcast rice,

which grows for 2 to 4 months as dryland crop before flooding

starts, as in the Bangkok Plain, may suffer from Al toxicity

(Jugsujinda et al., 1978).

Aluminum injury can be eliminated by raising the soil pH

above 5.0 by liming or by keeping the soil submerge for

several weeks before planting (Ponnamperuma et al., 1973) or

by leaching (van Breemen and Pons, 1978). Many researchers

45

have overcome Al toxicity by liming or applying heavy doses of

phosphate (Nhung and Ponnamperuma, 1966; Jones and Fox, 1978:

Attanandana, 1982). Organic residues which accelerate soil

reduction after flooding could eliminate Al injury.

3.6.3 Iron toxicity

In acid sulfate soils, iron toxicity is an important growth

limiting factor (Tanaka and Yoshida, 1970; Ponnamperuma et

al.,1973). Iron toxicity has been observed in soils with pH

below 5.8 when aerobic, and pH below 6.5 when anaerobic (van

Breemen and Moormann, 1978). Reported toxic levels of Fe in

culture solutions vary from 10 to 20 mg L-1 to more than 500

mg L-1 (Ishizuka, 1961; Tanaka et al., 1966). The wide ranges

may be due to differences in criteria used for toxicity (van

Breemen and Moormann, 1978).

At least three criteria have been used to define toxic

levels : plant growth, degree of bronzing, and Fe content of

the plants. Ishizuka (1961) found that rice growth increased

as Fe+2 concentrations in a culture solution increased from 0

to 0.1 mg L-1, but the growth was adversely affected at

concentrations above 10 mg L-1• Bronzing symptoms generally

appear at higher Fe concentrations : 30 to 80 mg kg- 1 in pot

experiments with soils from Sri Lanka (Mulleriyawa, 1966), 100

to 500 mg L-1 in culture solutions (Tanaka et al., 1966), and

300 to 400 mg L-1 in soils well supplied with nutrients (IRRI,

1972) .

46

Likewise, the Fe content in plants of a given variety grown

at a given time and place usually correlates well with the

degree of bronzing (Inada, 1965; Ota, 1968); but Fe contents

in plants of different varieties, all moderately affected by

bronzing, may vary from 110 to 1100 mg Fe kg-1 (Jayawardena et

al., 1977; Ismunadji and von Uexkull, 1977). Furthermore,

because apparently healthy plants may contain more than 1000

to 1500 mg Fe kg-1 (Ota, 1968; Tanaka and Yoshida, 1970), it

is probably impossible to define a generally applicable

critical Fe content in rice tissue. To confirm suspected Fe

toxicity, a comparison of the Fe contents in leaf blades of

affected and healthy plants from the same field may be most

useful (van Breemen and Moormann, 1978).

Iron toxicity in the soil is attributed to a high

concentration of ferrous iron in the soil solution. The

concentration of Fe+2 normally increases in a soil after

flooding due to reduction of ferric oxide by organic matter.

A rapid rise in Fe+2 is favored by a low initial soil pH,

active iron oxide, the absence of compounds with a higher

oxidation state than ferric oxide (Ponnamperuma, 1972),

factors stimulating anaerobic microbial activity such as an

abundant supply of easily decomposable organic matter, and a

high nutrient status (van Breemen and Moormann, 1978). In most

soils, Fe+2 increases to 100 - 600 mg kg-1 in the first 2 to 10

weeks after flooding and later decreases to levels between 50

and 100 mg kg-1 in a few weeks. In acid sulfate soils the Fe+2

47

peaks after several months, reaching 100 - 4000 mg kg- 1 (van

Breemen and Moormann, 1978).

Reduction of iron oxide to Fe+2 consumes Wand increases pH

(Ponnamperuma, 1972)

[20]

In most acid soils, pH normally reaches between 6.4 and 7.0

within 2 to 5 weeks after flooding. But in acid sulfate soils

which are low in easily decomposable organic matter, low in

active iron oxide, and have an extremely low pH, reduction of

iron oxide is minimal. Therefore, the soil pH increases slowly

(van Breemen and Moormann, 1978).

In Sulfaquents and Sulfaquepts, in which pyrite is found at

a shallow depth, a considerable amount of Fe+2 may be derived

from the oxidation of pyrite. The Fe+2 ions (or in the form of

ferrous sulfate) later diffuse upward to the oxidized layers

and surface soils. Some of the Fe+2 will be slowly oxidized to

Fe+3• Although some Fe precipitates in oxidized layers and as

a film on the surface soils, large amounts of iron as Fe+2 and

Fe+3 still exist in aqueous form. In such situations Fe

toxicity may result. De Guzman (1965) showed that Fe(III)-EDTA

apparently induced bronzing only at 5 to 10 mg L-1 of Fe+3•

Iron toxicity is normally found in Sulfaquepts, and sometimes

in Sulfic Tropaquepts.

The critical Fe level in growth media required to produce

bronzing differs with the age, variety, and possibly with the

nutritional status of the plants. Tadano (1970, 1975) showed

48

that rice roots have a built-in protective mechanism which

enables them to exclude an excessive amount of Fe+2• The Fe

excluding power is weakened in plants deficient in P, Ca, Mg,

Mn, and especially in K. On the other hand, if a high level of

Fe is present in the soil solution, the uptake of P, K, and Mn

will also be adversely affected. A high salinity due to NaCl

or MgC1 2 is known to decrease the oxidizing power of rice

roots and thus enhance Fe toxicity. However, in the case of

acid sulfate soils, the extremely low pH would weaken the

protective mechanism of rice roots against the uptake of an

excessive amount of Fe.

In some acid sulfate soils, rice well supplied with

nutrients apparently suffers from Fe toxicity only if

dissolved Fe+2 is higher than 300 to 400 mg L-1• It is not

clear whether varietal differences in tolerance for Fe+2 are

due mainly to exclusion of Fe in the oxidizing rhizosphere, to

reduced translocation of Fe, to tolerance for high Fe levels

in the plant tissue, or to a combination of these factors

(Jayawardena et al., 1977; Tadano, 1985).

Ameliorations of Fe toxicity include liming, drainage and

leaching, presubmergence, prevention of deep drainage, and the

use of tolerant rice varieties. Liming suppresses a high

concentration of Fe by raising pH, thereby precipitating Fe.

Presubmergence tends to hasten the period of decline after

reaching peak concentration. The addition of fresh organic

matter has been found to shorten the period of high Fe+2

49

content (IRRI, 1976). The prevention of deep drainage will

maintain the watertable above the pyritic layer, thereby

minimizing pyrite oxidation. The use of tolerant plants seems

to be economical for Fe-toxic soil.

3.6.4 Phosphorus deficiency

In acid sulfate soils, P is strongly fixed in unavailable

forms such as iron and aluminum phosphate or phosphate

adsorbed to clay surfaces. With time, the phosphate may be

converted to the more insoluble ferric phosphate and even

occluded with iron oxide (Hesse, 1963). At low soil pH, iron

and aluminum playa major role in phosphorus fixation (Yuan et

al., 1960).

Patrick and Khalid (1974) observed that in an aerobic acid

soil, P coprecipitates with Fe and Al and is occluded with

iron oxide; under reducing conditions, some of the iron and

aluminum phosphate becomes available, but the degree of

mobilization by flooding will gradually be reduced by aging

and crystallization of the oxides. Acid sulfate soils

containing low available P, showed a slight increase in P

availability with flooding (Pattrick et al., 1985).

Attanandana and Vacharotayan (1982) reported that Sulfic

Tropaquepts required a high level of applied P, and the

Rangsit very acid soil, with or without lime, was the only

soil not responding to any fertilization in the absence of P.

When the initial soil pH is 4.2 to 4.5, the response to lime

is often low and rarely significant; phosphate application

50

alone at 50 to 100 kg P20 S ha" gives a dramatic positive

effect even on unlimed acid soils (pH 3.6 - 4.2) (van Breemen

and Pons, 1978). The very acid sulfate soils of the Bangkok

Plain show good responses to P and lime (Attanandana, 1982).

Rock phosphate with a high level of citrate solubility proved

to be a good source of P for rice yields in acid sulfate soils

of Thailand (Engelstad et al., 1974). In addition, residual

effects of rock phosphate lasted for five consecutive crops in

Thailand (Jugsujinda and Suwannawoang, 1973).

3.6.5 Low base status

Attanandana and Vacharotayan (1982) reported that in an

extremely acid sulfate soil in Thailand, extractable K and Ca

are low, whereas extractable Al is very high. In a potential

acid sulfate soil of Senegal, exchangeable Ca, K, and Mg were

found to be 2.6, O.7, and 8.2 cmol., kg- 1 of soil , respectively

(Toure, 1982). The quantities of exchangeable Ca and Mg found

in the surface layer of acid sulfate soils in Thailand were

3.5 to 5. a and 3. a to 3.3 cmol , kg- 1 , respectively

(Attanandana, 1982).

51

References

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60

CHAPTER 4

Aluminum toxicity and tolerance of plants to acid soils

4.1 Aluminum toxicity symptoms and mechanisms of plant

tolerance

4.1.1 Physiological and biochemical effects of aluminum

toxicity

Aluminum toxicity is probably the most important growth

limiting factor for plants in strongly acidic soils (Foy,

1988). The problem is particularly severe below pH 5.0, but

has been reported to occur as high as pH 5.5 in acid sulfate

soils (Cate and Sukhai, 1964). The critical soil pH at which

Al becomes toxic depends upon many factors, including the

predominant clay minerals, organic matter levels,

concentrations of other cations, anions and total salts, and

particularly, the plant species or cultivar (Kamprath and Foy,

1985) .

Symptoms of Al toxicity are remarkably similar for most

plants. They are most pronounced on the root system, where

roots tend to be shortened and swollen. The entire root system

has a stubby appearance because lateral-root growth is

inhibited to a greater extent than primary-root growth (Foy,

1974). With increasing severity of Al toxicity, roots become

sparse, shorter, and more swollen, and fine branching

vanishes.

61

Moderate levels of Al toxicity show no characteristic

foliage symptoms and tend to go undetected. Under severe Al

stress, plant tops are stunted and are often indistinguishable

from P-deficiency symptoms (Foy, 1984) . Although Al

phytotoxici ty is not readily recognizable in the field by

foliage symptoms, it can be readily diagnosed by the

appearance of damaged roots.

Despite a large body of literature documenting Al toxicity

and tolerance, the physiological basis remains elusive

(Fageria et al. 1988; Marschner, 1991). Toxicity appears to be

the result of several interactions, and there is no consensus

on the mechanisms of Al toxicity in higher plants. To some

extent, Al clearly has a deleterious effect on numerous

physiological aspects of the affected species. The inability

to use phosphate in the presence of Al appears to contribute

to toxicity (Foy and Campbell, 1984), probably as a result of

the formation of aluminum phosphate complexes wi thin the

tissue. Exposure to Al also results in the reduction of free

Mg and Ca in plant tissue. It has been shown in different

upland rice cultivars that under Al toxic conditions, Mg

levels are reduced below those critical for tissue survival

(Fageria and Carvalho, 1982); Ca uptake was reduced by as much

as 90% (Fageria, 1985). It has been hypothesized that Al

rhizotoxicity is related to disruption of membrane function.

Several biochemical effects are probably due to changes in the

structure and function of the root cell plasmalemma (Hecht-

62

Buchholz and Foy, 1981). Depending on the pH, Al can bind to

membrane proteins and lipids (Foy and Campbell, 1984).

Aluminum can also participate in the formation of cross-links

between proteins and pectins within the cell wall, making the

wall more rigid. These interactions would have a deleterious

effect on membrane integrity. Wheat roots that exhibit severe

symptoms of Al toxicity however, have an undiminished capacity

to extrude protons, suggesting that these membranes are intact

and ATP synthesis is sufficient to supply the proton

trans locating ATPases (Kinraide, 1988). It has been reported

that abnormal root growth is the result of disturbed mitotic

processes (Marschner, 1986). Aluminum is particularly

concentrated in the nucleus (Foy et al., 1978). The cell cycle

is inhibited, probably at the level of DNA replication where

Al inhibits the synthesis of DNA (Foy, 1974, 1983). Kinraide

and Parker (1987) suggest two categories of Al phytotoxicity:

(1) binding or precipitation on cell surfaces, thereby

interfering with membrane function, and (2) combining with

biologically important ligands such as DNA. Most of the

effects of phytotoxic Al are manifested in root growth and

function (Foy, 1983; Taylor, 1988). Root elongation rate is

the most commonly used indicator of plant response to Al (Hue

et al., 1986). Susceptibility to Al toxicity appears to be the

greatest in young seedlings and decreases as plants continue

to grow. Roots of rice (Thawornwong and van Diest, 1974) and

alfalfa (Kapland and Estes, 1985) seedlings have been shown to

63

be more sensitive to Al than roots of the mature plants. At 2

mg L- 1 in nutrient solution, Al was lethal to rice seedlings

but had no effect on dry-matter yields when added to 18 day

old plants (Thawornwong and van Diest, 1974).

4.1.2 Mechanisms of plant adaptation and aluminum tolerance

to acid soils

As high Al concentration in acid soils is a key factor

limiting crop production, research emphasis has been on

mechanisms of Al tolerance. Plant species and genotypes within

species show great differences in Al tolerance (Foy, 1988;

Wright, 1989; Foy, 1991). Genetic control of Al tolerance has

been reported for a number of crops, including maize (Zea mays

L.) , wheat (Triticum aestivum L.), sorghum (Sorghum bicolor L.

Moench), and rice (Oryza sativa L.) (Fageria et al., 1988),

This tolerance can be exhibited in terms of better root and

shoot growth and more efficient utilization of nutrients.

There is currently no unified view on the physiological and

biochemical basis of Al tolerance in crops. Solution studies

with well defined conditions grouped tolerance mechanism into

two categories: external and internal (Marschner, 1991).

External or exclusion tolerance mechanisms occur in the root

apoplasm (portions external to the cell membranes) which

prevent Al from entering the symplasm (portions within the

cell membranes but excluding the vacuoles) and from reaching

sensitive metabolic sites. Internal Al tolerance mechanisms

involve Al passing into the symplasm and detoxification,

64

immobilization or changes in metabolism occurring at that

point (Marschner, 1991).

Exclusion mechanisms may include immobilization of Al at

the cell wall, selective permeability of the plasma membrane,

plant-induced pH changes in the rhizosphere, and exudation of

chelating ligands (Marschner, 1991). A majority of Al in plant

roots is bound to the cell wall (Schaedle et al., 1986).

Plant-induced pH increases in the rhizosphere have not

consistently been associated with Al tolerance (Taylor, 1988).

Organic chelating agents exuded from plant roots or in the

mucilage at the root cap could detoxify AI. A large and

continuous supply of chelating agent would however, be needed

to depress aluminum activities (AI+3) and this would impose a

considerable energetic cost to the plant (Marschner, 1991).

Chelation of Al by organic acids or proteins in the

cytosol, Al compartmentation in the vacuole and evolution of

AI-tolerant enzymes have been proposed as internal tolerance

mechanisms (Marschner, 1991). Thus, it is not surprising that

AI-tolerant plants often produce large amounts of organic

acids (Mengel and Kirkby, 1982). AI-tolerant barley maintains

high concentrations of organic acids in roots (Foy et al.,

1987). In maize roots exposed to AI, there is a large increase

in organic acids, especially malate, citrate, and trans

aconitate (Suhayda and Haug, 1986). In suspension cultures,

the toxic effect of Al decreased greatly after addition of

acid fraction of the conditioned media from tolerant cells

65

(Ojima et al., 1984). This fraction contained four different

organic acids, primarily citrate. Moreover, the toxic effects

of Al could also be reduced by addition of citric or malic

acid to the cells. Therefore, at least in cell culture,

tolerance may be related to the production and release of

organic acids by the cells (Bennett and Breen, 1991).

4.2 Tolerance of green manure crops to acid soil conditions

Green manure crops can be leguminous as well as

nonleguminous and can be grown in situ or brought from outside

as cutting of trees and shrubs. In Thailand, leguminous green

manures are more common.

4.2.1 Type of green manures for in situ incorporation

Leguminous green manure crops contain a relative high N

content (2.5 - 4.5 % of dry matter), and they can grow well on

adverse soils of low organic matter, high or low pH, and can

subsequently help to improve the fertility status of the

problem soils (Singh et al., 1991).

Recently, several experiments have been conducted on green

manuring crops in Thailand (Thawornmas et al., 1977; Donsae et

al., 1981; Sukthumrong et al., 1986.). A promising green

manure should have the following characteristics: fast growth,

a large biomass, well developed root system, tolerant of

drought, alkalinity and acidity, easy incorporation, and rapid

decomposition to yield a large amount of plant nutrients

(Chinapun, 1982). Sunn hemp (Crotalaria juncea) , green gram

66

{Vigna radiata}, cowpea {Vigna unguiculata}, pigeon pea

{Cajanus cajan}, mimosa {Mimosa invisa}, stylo {Stylosanthes

humiles}, leucaena {Leucaena leucocephala}, and Sesbania

{Sesbania spp} are the most widely grown green manures in

Thailand.

Sunn hemp is a root-nodulating green manure that commonly

grows in low rainfall and limited soil moisture areas but less

tolerant of salinity and acidity {Sratongkam, 1976}. Naklang

et al. {1980} found that sunn hemp used as a green manure

increased rice yields. It was also considered as a possible

fodder crop. It could produce significant forage when planted

in bunded rice fields with one buffalo plowing after rice

harvest {Shelton, 1976}. Many insect pests, however,

especially pod borers, cause marked reductions in both forage

and seed yields {Phisikul et al., 1980}. Cowpea, at present,

not a major crop in Thailand, performs well as a short season

crop, with better drought resistance than mungbean {Phaseolus

aureus} or soybean for the uncertain period before the monsoon

of Northeastern area. Its seed has satisfactory flour-milling

quality, and values both as food and as green manures by

possessing higher water-use efficiency in terms of dry matter

and N production {Wallis and Byth, 1988}.

Sesbania species are good green manure crops because of

their vigorous growth, adaptation to various soil conditions,

and ability to enhance soil fertility by transferring fixed N

to the main crop {Evans and Rotar, 1987}. Sesbania favors the

67

moist tropics; many species show exceptional tolerance for

waterlogged soils, which appears in part through their ability

to nodulate. Stem nodulation species, Sesbania rostrata, is

potentially an important green manure crop preceding rice. S.

bispinosa, S. grandiflora and S. sesban have been grown

widely.

Sesbania bispinosa (Jacq.) W.F. Wigh is a shrubby annual or

short-lived perennial that can grow to 4 - 5 m tall. It is

also known as S. aculeata (Willd.) Poir. and as S. cannabina

(Retz.) Poir., and by the common name dhaincha (Duke, 1981).

Dhaincha has thick glabrous stems, large (35 cm) pinnate

leaves, yellow flowers, and long (25 cm) curved pod. It is

self-fertile and seeds freely. It is a multipurpose shrub; the

wood and leaves are used for poles, fiber, fodder, and green

manure. It is adapted to wet areas and heavy soils. It is

viewed as a marsh plant able to produce floating roots with a

spongy aerenchyma, and is also said to withstand drought, with

rainfall as low as 500 mm in the growing season.

Sesbania bispinosa is grown widely as the short-duration

legume before or after rice in the Bangkok Plain and some

areas of the Northeast region (Hongpan, 1962 i Plangkool,

1980). Seeds are sown at the onset of the monsoon; 2-to 3

month-old plants are incorporated before rice is transplanted.

Its rapid growth resulted in N accumulation of 80 kg ha- 1 in

30 d and 230 kg N in 60 d at Khon Kaen (Chandrapanya et al.,

1982). Evans and Rotar (1987) reported yield averages of 26

68

tons fresh foliage ha! from growth periods averaging 75 d. Its

yields greatly exceed those of other green manure crops, such

as crotalaria, mungbean and pigeon pea. Its leaves are

pinnate, averaging about 38 cm long, with 20 - 50 leaflets,

1.2 - 2.5 cm long, per leaf. The leaflets degrade rapidly in

moist soil and release 50% of their N contents within 4 weeks

of incorporation (IRRI, 1985).

Among the stem-nodulating legumes, Sesbania rostrata and S.

cannabina have received particular attention. These are

characterized by profuse stem nodulation, and faster growth

than most root-nodulating legumes. Also, S. rostrata exhibits

two unique properties: it has high N2-fixing potential

(measured as fixing 200 kg N2 ha? in 50 days) and it has the

ability to nodulate and fix N even with high rate of combined

N in the soil (about 200 kg N ha-1) . That S. rostrata can

assimilate both soil and atmospheric N constitutes a

significant advantage (Dreyfus et al., 1985). The stem

nodulating legumes usually show a high sensitivity to climatic

variations, particularly to temperature and photoperiod

(Rinaudo et al., 1988). Besides being a green manure, sesbania

can be grown as a forage crop and as human food. In Thailand,

flowers of S. grandiflora and S. roxburghii are an accepted

vegetable (Arunin et al., 1988).

Leucaena has been the most successful species because of

its deep rooting, however it is slow to establish and has poor

growth on acid soils. Potential of Leucaena with wetland rice

69

in Thailand is limited because it does not tolerate flooding,

and must be cut off aggressively to avoid shading (Arunin et

al., 1988). Species of Sesbania (mostly S. sesban) appear

promising, with yields and reasonable leaf retention over the

dry season, and use for in situ green manure as well as GLM

(Topark-Ngarm and Gutteridge, 1985).

Green leaf manures (GLM) are preferred when raising green

manures in situ is not possible, especially in areas with

limitations such as lack of irrigation water and due to loss

of main crop growing season. Woody species of the genera

Leucaena and Sesbania, which are widely used in food crop

systems, are the important GLMs. Leucaena is of interest

primarily as animal fodder (Brewbaker, 1985, 1987b). It is

planted widely for fuel and post wood, increasingly is being

studied as a source of pulpwood and timber, is used as nurse

trees with plantation crops, and is planted for shade or soil

amelioration. Leucaena herbage yields generally exceed those

of other shrubby tropical legumes and are comparable to those

of the best herbaceous legumes. They range from 40 to 80 tons

fresh weight ha? when moisture is not limiting (Brewbaker,

1987a). Yields are reduced to 20 50 tons ha' in the

seasonally dry tropics. Yields are optimized with a harvest

cycle of 2 - 4 months. Other variables affecting yield include

variety, cutting height, and plant densities.

70

4.2.2 Sesbania tolerance to adverse conditions

4.2.2.1 Flood tolerance

Sesbania species are known for their ability to

withstand flooding. In China, sesbania survived for 15 - 20

days in water at 5 - 30 cm deep (Evans and Rotar, 1987).

Tolerance of sesbania to flooding develops after the

seedling stage. Although a few centimeters of standing water

aids germination of S. bispinosa, the seedlings appear to need

several weeks of nonflooded conditions before they can readily

withstand flooding. The basis for flood tolerance in sesbania

is the development of aerenchyma, a spongy tissue having

enlarged cell with large intercellular spaces. This

development allows the plant to avoid anoxia in the root zone.

In a study of the performance and yields of four

Sesbania species under flooded (30 em deep) and upland

conditions, the ability to tolerate flooding was increased in

the order S. rostrata > S. aculeata > S. cannabina > S.

speciosa (Arunin et al., 1987).

4.2.2.2 Salt tolerance

Many Sesbania species are tolerant of saline and

alkaline conditions. African species of sesbania are

segregated in habitat according to the degree of salinity in

the edaphic environment. This tolerance may be related to

water requirement; species with greater adaptation to drought

might be expected to encounter increasing salinity as soils

71

dry out or as seasonal surface water evaporates (Evans and

Rotar, 1987).

Singhabutra et al. (1987) studied the effect of

salinity levels - 0.56, 1.89, 4.40, 5.64, and 6.18 dS m-1 - on

Sesbania spp. As salinity increased, plant height, fresh

weight, and number of nodules decreased in the order of S.

speciosa > S. cannabina > S. rostrata > and S. aculeata.

4.2.2.3 Acid tolerance

Plants which are able to colonize problem soils are

important in stabilizing and reclaiming such lands. The

ability of Sesbania species to grow in a wide range of soil

conditions has resulted in an expanded range of adaptability

and utility compared to many otheL legumes.

Sesbania species have been recognized for their

tolerance to soil salinity and flooding; some also grow well

in acid soils. Sesbania cannabina or S. bispinosa, known for

its tolerance of soil alkalinity in India, appears tolerant of

soil acidity. The crop was grown as a green manure on acidic

tea-growing soils in Assam (Patel, 1966). Sesbania sesban is

reportedly grown successfully on acid sulfate soils in Viet

Nam. Tran phuoc Duong of Cantho Agricultural College, Viet

Nam, in an illustrated lecture given at the University of

Hawaii in 1983, presented a photograph of Sesbania species

growing in a soil crusted with aluminum salts, and stated that

Sesbania was generally sown at the end of the monsoon season

72

(Evans and Rotar, 1987). No research has been carried out in

Thailand on Sesbania tolerance to soil acidity.

Tolerance diversity of Sesbania was demonstrated by

Yost et al. (1985), who grew S. grandiflora across a pH

gradient established by liming a manganiferous Oxisol in

Hawaii. One annual variety of sesbania used (USDA PI 180050)

appeared quite sensitive to low pH and the associated high

levels of available soil Mn. This variety has been found to be

different from other species accessions received as S.

cannabina or S. bispinosa (Evans, 1983). Although amounts of

N accumulated in S. grandiflora were low relative to the more

rapidly growing annual crops, it showed a tolerance to low pH

comparable to that of Crotalaria juncea.

A group of Sesbania species were tested for response to

lime applied to two acid soils (Evans, 1986). The 28

accessions grown in pots showed wide diversity in response to

an aluminous Ultisol pH 3.8 - 5.0 and a manganiferous Oxisol

pH 5.2 - 6.0. Yield increases due to the lime additions

averaged approximately 80% over no-lime treatments in each

soil. Yield variation among accessions was manyfold in the Mn

dominated soil, varying by a factor of 40 at both pH levels.

Variation was much less in the AI-dominated soil, by factor of

2.3 at both pH values. The results suggested that Sesbania

cultivars could be selected to provide good yields on acid

soils.

73

References

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Arunin, S., C. Dissataporn, Y. Anuluxtipan, andD. Nana. 1988.Potential of sesbania as a green manure in saline ricesoils in Thailand. In Green Manure in Rice Farming, pp.83-95. Int. Rice Res. Inst., Los Banos, Philippines.

Bennett, R.J., and C.M. Breen. 1991. The aluminum signal: Newdimensions to mechanisms of aluminum tolerance. PlantSoil 134 : 153-166.

Brewbaker, J.L. 1985. Leguminous trees and shrubs forSoutheast Asia and the South Pacific. In G. J. Blair etal. (eds.) Forages in Southeast Asia and South PacificAgriculture, pp. 43-50. Australian Center forInternational Agricultural Research. Canberra, Australia.

Brewbaker, J.L. 1987a. Leucaena : a genus of multipurposetrees for tropical agroforestry. In Ten Years ofDevelopments in Agroforestry. International Council forResearch in Agro-Forestry. Nairobi, Kenya.

Brewbaker, J.L. 1987b. Nitrogen fixing trees for fodder andbrowse in Africa. In B.T. Kang (ed.) Alley Farming forHumid and Subhumid Regions of Tropical Africa.International Institute of Tropical Agriculture. Ibadan,Nigeria.

Cate, R.B.Jr., and A.P. Sukhai. 1964. A study of aluminum inrice soils. Soil Sci. 98 : 85-93.

Chandrapanya, D., T. Charoenwatana, A. Pookpakdi, and G.Banta. 1982. Cropping systems research in Thailand. InReport of Workshop on Cropping Systems Research in Asia,pp. 295-316. IRRI, Los Banos, Philippines.

Chinapan, W. 1982. Technical report on use of green manure.Dept. of Land Development. Bangkok. 19 p.

Donsae, M., C. Thongyai, K. Thapbunhan, and D. Thawornrnas.1981. Comparative study on the use of green manure toincrease cassava yield. Research Report. Dept. ofAgriculture, Ministry of Agriculture and Cooperatives.Bangkok. pp. 22-25.

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Dreyfus, B., G. Rinaudo, and Y.R. Dommergues. 1985.Observations on the use of Sesbania rostrata as greenmanure in paddy fields. Mircen J. 1 : 111-121.

Duke, J.A. 1981. Handbook of legumes of world economicimportance. Plenum Press, New York. 345 p.

Evans, D.O. 1983. Sesbania flowering observations. NitrogenFixing Tree Research Report 1 : 42.

Evans, D.O. 1986. Sesbania research in Hawaii : Summary of aproject. Nitrogen Fixing Tree Research Report 4 : 57- 58.

Evans, D.O., and P.P. Rotar. 1987. Sesbania in Agriculture.Westview Tropical Agriculture Series, No 8. WestviewPress. Boulder, Colorado. 189 p.

Fageria, N.K. 1985. Influence of aluminum in nutrient solutionon chemical composition in two rice cultivars atdifferent growth stages. Plant Soil 85 : 423-429.

Fageria, N.K., and J.R.P. Carvalho. 1982. Influence ofaluminum in nutrient solutions on chemical composition inupland rice cultivars. Plant Soil 69 : 31-44.

Fageria, N.K., V.C. Baliger, and R.J. Wright. 1988. Aluminumtoxicity in crop plants. J. Plant Nutr. 11 : 303-319.

Foy, C.D. 1974. Effects of aluminum on plant growth. In E.W.Carson (ed.) The plant root and its environment, pp. 601642. Univ. Press of VA. Charlottesville, VA.

Foy, C.D. 1983. Plant adaptation to mineral stress in problemsoils. Iowa J. Res. 57 : 339-354.

Foy, C.D. 1984. Physiological effects of hydrogen, aluminumand manganese toxicities in acid soils. In F. Adams (ed.)Soil Acidity and Liming. 2nd ed. Agronomy 12 : 57-97.

Foy, C.D. 1988. Plant adaptation to acid, aluminum toxicsoils. Commun. Soil Sci. Plant Anal. 19 : 959-987.

Foy, C.D. 1991. Soil chemical factors limiting plant rootgrowth. Adv. Soil Sci. 16 : 85-124.

Foy, C.D., and T.A. Campbell. 1984. Differential tolerances ofamaranthus strains to high levels of aluminum andmanganese in acid soils. J. Plant Nutr. 7 : 1365-1388.

Foy, C.D., R.L. Chaney, and M.C. White. 1978. The physiologyof metal toxicity in plants. Ann. Rev. Plant Physiol. 29: 511-566.

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Foy, C.D., E.H. Lee, and S.B. Wilding. 1987. Differentialaluminum tolerance of wheat and barley varieties in acidsoils. J. Plant Nutr. 10 : 1089-1101.

Hecht-Bucholz, C., and C.D. Foy. 1981. Effect of aluminumtoxicity on root morphology of barley. In R. Brouwer etale (eds .) Structure and Function of Plant Roots, pp.343-345. Matin Nijhoffl Publishers. Netherlands.

Hongpan, S. 1962. Handbook for green manuring and soilimprovement application in the tropics. 1st ed. NitivesPublishing, Bangkok. (in Thai).

Hue, N.V., G.R. Craddock, and F. Adams. 1986. Effect oforganic acids on aluminum toxicity in subsoils. Soil Sci.Soc. Am. J. 50 : 28-34.

IRRI. 1985. Annual Report 1984. International Rice ResearchInstitute. Los Banos, Philippines. 504 p.

Kamprath, E. J., and C. C. Foy. 1985. Lime-fertilizer-plantinteractions in acid soils. In O.P. Engelstad (ed.)Fertilizer Technology and Use, 3 rd ed., pp. 91-151. SoilSci. Soc. Am. Madison, WI.

Kapland, D.I., and G.O. Estes. 1985. Organic matterrelationship to soil nutrient status and aluminumtoxicity in alfalfa. Agron. J. 77 : 735-738.

Kinraide, T.B., and D.R. Parker. 1987. Nonphytotoxicity of thealuminum sulfate ion, AIS04+. Plant Physiol. 71 : 207212.

Kinraide, T.B. 1988. Proton extrusion by wheat rootsexhibiting severe aluminum toxicity symptoms. PlantPhysiol. 88 : 418-423.

Marschner, H. 1986. Mineral nutrition of higher plants.Academic Press. London. 686 p.

Marschner, H. 1991. Mechanisms of adaptation of plants to acidsoils. Plant Soil 134 : 1-20.

Mengel,K., and E.A. Kirkby. 1982. Principles of plantnutrition. 3rd ed. Int. Potash Inst., Bern, Switzerland.

Naklang, K., S. Roj anakusol, V. sommut , B. Sawantarach, and T.Songawong. 1980. Effect of long-term application ofcompost, green manure and rice straw on soil organicmatter and rice yield. Rice Div., Dept. of Agriculture,Ministry of Agriculture and Cooperatives. Bangkok. pp.133-138.

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Ojima, K., H. Abe, and K. Ohira. 1984. Effect of non-metaboliccondi tions on the uptake of aluminum by plant roots.Plant Cell Physiol. 25 : 855-858.

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Phisikul, S., S. Teerapongtanakorn, P. Lorwilai, A. Moonsiri,and A. Topark-Ngarm. 1980. Insect pests on sunn hemp(Crotalaria juncea L.) growing in two different seasons.In Annual Report of the Pasture Improvement Project, pp.26-29. Fac. of Agriculture, Khon Kaen Univ., Thailand.

Plangkool, B. 1980. Land resources. Dept. of Land Development,Bangkok. 223 p.

Rinaudo, G., D. Alazard., and A. Moudiongui. 1988. Stemnodulating legumes as green manure for rice in WestAfrica. In Green Manure in Rice Farming, pp. 97-109. Int.Rice. Res. Inst., Los Banos, Philippines.

Schaedle, M., F.C. Thornton, and D.J. Raynal. 1986. Nonmetabolic binding of aluminum to roots of loblolly pineand honeylocust. J. Plant Nutr. 9 : 1227-1238.

Shelton, H.W. 1976. An evaluation of dry season legume foragegrowth following paddy rice. In Annual Report of thePasture Improvement Project, pp. 45-47. Fac. ofAgriculture, Khon Kaen Univ., Thailand.

Singh, Y., C.S. Khind, and B. Singh. 1991. Efficientmanagement of leguminous green manures in wetland rice.Adv. Agron. 45 : 135-189.

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Sratongkam, S. 1976. Technical report on green manure and soilimproving crops. In Proc. of the National Seminar onCropping Systems. Fac. of Agriculture, Chiang MaiUniversity.

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78

CHAPTER 5

Chemistry of acid sulfate soils and aluminum detoxification by

organic amendments

5.1 Chemistry and mineralogy of aluminum, iron and sulfur in

acid sulfate soils

5.1.1 Acid development by oxidation of pyrite

The fine-grained pyrite typical of tidal sediments oxidizes

readily upon exposure to the air, giving Fe(II) sulfate and

sulfuric acid:

FeS2 + 7/2°2 + H20 --->Fe+2 + 2S04-2 + 2W [1]

Sulfite forms as an intermediate, and there are indications

that appreciable amounts of sulfur are sometimes removed as

gaseous sulfur dioxide from young acid sulfate soils (van

Breemen, 1976). Complete oxidation and hydrolysis of iron to

Fe (III) oxide yields 2 moles of sulfuric acid per mole of

pyrite:

FeS2 + 15/402 + 7/2H20 ---> Fe (OH) J + 2S04-2 + 4W [2]

The term "sulfuricization or hyperacidity" has been used to

denote these acidification processes (Fanning, 1978) .

Oxidation of Fe(II) to Fe(III) by O2 is slow in vitro but the

reaction rate is greatly enhanced by the chemoautotrophic

organism Thiobacillus ferro-oxidans. Pyrite is oxidized more

rapidly by dissolved Fe(III) than by 02' according to

FeS2 + 14Fe+J + 8H20 ---> 15Fe+ 2 + 16W + 2S04-2 [3]

79

Because a low pH and the activi ty of T. ferrooxidans are

required to maintain appreciable concentrations of dissolved

Fe(III), these conditions also promote pyrite oxidation (van

Breernen, 1988). The importance of catalytic effect of the

Fe(III)-T. ferrooxidans couple is well established for acid

mine drainage, where the pH commonly drops below 3 (Taylor et

al., 1984). T. ferrooxidans is commonly present in acid

sulfate soils (Bloomfield and Coulter, 1973), but since the pH

of the soil rarely falls below the 3 3.5 range, its

importance for the oxidation of pyrite may be limited.

Thus, pyrite oxidation involves a series of complex

processes, many of which are potentially rate determining. The

rate of pyrite oxidation in the field is simply determined by

the rate of O2 supply (van Breemen, 1976). This has an

important practical consequence : soil acidification due to

pyrite oxidation is slower when pyrite occurs at an

appreciable depth below the soil surface (i.e., below 50 cm)

than at very shallow depth or when pyritic soils is dug up and

is directly exposed to the air.

5.1.2 Mineralogy and oxidation products of pyrite

Most of the iron(II) , hydrogen, and sulfate ions released

during pyrite oxidation undergo various further reactions in

the soil. Essentially all Fe (II) is further oxidized to

Fe(III) in oxides, in the basic sulfate jarosite and in clay

minerals. Most of the sulfate remains in solution and is

removed by leaching, together with cations (mainly Mg, Ca and

80

Na) derived from ion exchange and mineral weathering. The

remainder of the sulfate is precipitated as jarosite and as

basic aluminum sulfate, and part is adsorbed, especially at

low pH. Gypsum and other more readily soluble sulfates may

form, often temporarily, when evaporation exceeds rainfall.

Jarosite. The predominant Fe sulfate mineral in acid

sulfate soils is the sparingly soluble jarosite

[KFe3(S04)2(OH)6]. It commonly occurs as earthy fillings of

voids or as mottles in the soil matrix, and gives a sharp x

ray diffraction pattern. Jarosite is a member of the alunite

group, a series of basic sulfates with the general formula

AB3(S04)2(OH)6 in which A stands for K+, Na", H30+, or NH/, and

B for Fe(III) (jarosite) or Al (alunite). A virtually complete

solid solution exists between jarosite (the K form),

natrojarosite, and hydronium jarosite, but K-form is the most

common in coastal acid sulfate soils. The theoretical pe-pH

stability diagram indicates that jarosite is formed only in

acid (pH 2 - 4), oxidized (Eh > 400 mV) environments (Ivarson

et al., 1982). The pale yellow (2.5 - 5Y 8/3 - 8/6) mottles

are so characteristic that they are used, together with pH, as

a diagnostic criterion for classifying acid sulfate soils. On

the other hand, jarosite is lacking in some acid sulfate

soils, particularly those high in organic matter (Kosaka,

1971) .

Iron oxides. Eventually most of the iron from oxidized

pyrite ends up in Fe(III) oxides. Fine-grained goethite may

81

form either directly, and quickly, upon oxidation of dissolved

Fe (II) sulfate released during pyrite oxidation, or more

slowly, by hydrolysis of jarosite. Both reactions are acidic

and part of the sulfuricization process

Fe+2 + 804-2 + 1/402 + 3/2H20 ---> FeOOH + 2W + 8°4-2 [4]

jarosite ---> 3FeOOH + 28°4-2 + K+ + 3W [5]

At pH values below 6, Fe(III) oxides are incompatible with

pyrite, and indeed, most of the iron oxide is formed at an

appreciable distance from pyrite. By contrast, at high pH, as

in calcareous pyritic sediments, goethite is often

pseudomorphic after pyrite.

In the better drained, deeply developed acid sulfate soils,

part of the Fe (III) oxides in the B horizon may occur as

hematite, giving conspicuous red mottles. In Thailand, these

red mottles are unknown from non-acid marine soils, and they

are used as a field indicator of moderately to strongly acidic

conditions (van der Kevie and Yenrnanas, 1972).

Basic aluminum sulfates. The soil solution of acid sulfate

soils is generally supersaturated with alunite, the aluminous

counterpart of jarosite (van Breemen, 1973). The activity of

dissolved Al in acid sulfate soils in the field, as well as in

more oxidized pyritic soil in the laboratory, seems to be

regulated by jurbanite [AIOHS04 ] (van Breemen, 1973; 1982).

Water soluble sulfates. Gypsum has been observed in coastal

marine soils over a wide pH range (3.5 - 7.0). Due to its

fairly high solubility, gypsum is confined to dryer soils or

82

to those containing calcium carbonate. Under arid conditions

most soluble sulfate such as Na-alum, tamarugite

[NaAl(S04)2(H20)6]' pickeringite [MgA12(S04)4(H20 ) 22 ] and rozenite

[FeS04(H20)2] , can be formed, particularly on the surface of

young acid sulfate soils or of excavated pyritic soil, where

pyrite oxidation is relatively rapid (van Breemen, 1976).

Precipitates of malanterite [FeS04(H20)7] and copiapite

[Fe(II)Fe(III)4(S04)6(OH)2(H20)20] and coquimbite [Fe2(S04)3(H20)g]

have been found in association with acid mine drainage (van

Breemen, 1988).

5.1.3 Solution chemistry of iron in acid soils

The concentration of Fe+3 in aqueous solution in equilibrium

with ferric oxides is controlled by pH, whereas that of Fe+2

is controlled by a combination of pH and pe (16.9xEh). The

solubility characteristics of the two species will be

considered separately.

5.1.3.1 Ferric species

The measured activity of Fe+3 in well-aerated soil

suspension (Norvell and Lindsay, 1982) is 500 times higher

than that generated by goethite (a-FeOOH) , the most stable of

the ferric oxyhydroxides in water, and about 13 times higher

than that generated by lepidocrocite (y-FeOOH). Norvell and

Lindsay termed the solid phase responsible "soil-Fe", which

might correspond to ferrihydrite. In acid soils, ferrihydrite

readily transforms to goethite or hematite, but its total

Fe(III) concentration (consisting of Fe+3 + Fe(OH)+2 + Fe(OH)2+

83

+ Fe (OH) 3) is extremely low, falling below 10-6 M at around pH

3.4. Thus, acidity alone can mobilize little Fe{III).

The presence of complexing agents can increase

significantly the amounts of Fe (III) in equilibrium solutions;

e.g., oxalate, which is a strong complexing agent for Fe{III),

at 10-4 M can maintain the concentration of total soluble

Fe{III) above 10-5 M at pH 4.4 in the presence of soil-Fe.

Although lower concentrations (< 10-5 M) of oxalate are less

effective, it is possible that local concentrations of

microbially generated oxalate could be effective in the

mobilization of iron in acidic surface horizons (Jones and

Wilson, 1985).

5.1.3.2 Ferrous species

The concentration of Fe (II) in equilibrium with soil-Fe

is described in the equation

[6]

for which the relationships

log (Fe+2) = 15.47 - (pe + pH) - 2pH

has been derived (Lindsay, 1979). The term (pe + pH) is

constant for a constant partial pressure of °2 , or H2 , and

ranges from about 17 on well-aerated soils to 3 in strongly

reducing soils. It follows from this relationship that, in

acid soils of pH 3 - 4, the activity of Fe{II) exceeds that of

Fe{III) species in non-complexing solutions for values of pe

+ pH below 15 (Lindsay and Schwab, 1982).

84

In soils subjected to periodic waterlogging, redox

processes assume prime importance in determining the chemistry

of iron. In paddy fields, large fractions of the iron present

as free Fe(III) oxides in aerated conditions become reduced to

Fe(II) after comparatively short periods (a few months) of

waterlogging (van Breemen, 1988). In most instances only a

small fraction of the Fe(II) remains in solution, most of it

ending up either in exchanged forms or as in sulfide minerals,

such as mackinawite or pyrite (FeS2 ) .

5.2 Aluminum chemistry in acid soils

Several recent reviews of Al chemistry discuss the

important forms and transformations of Al in details (Thomas,

1988 i Paterson et al., 1991). Aluminum exists in numerous

forms in both the solid and the liquid phases of soil. The

forms of Al in the liquid phase (i. e. soil solution) are

governed by hydrolysis, complexation and polymerization.

Hence, the level of Al in soil solution will depend on the

soil pH, amount and type of primary and secondary AI

containing minerals, exchange equilibria with inorganic

surfaces and complexation with organic constituents (Bell and

Edwards, 1986).

5.2.1 Hydrolytic reactions of Al

Aluminum has a high ionic charge, and a small crystalline

radius (0.054 nm). Thus, it is very reactive in soil solution.

When an AI-containing mineral dissolves, the released AI+3

85

coordinates with six OH2 groups and forms an octahedral

hydration sphere (AI{H20)/3). As pH increases, each OH2 group

sequentially dissociates a H+ to produce the mononuclear

hydrolysis products of AI+3, AI{OH)+2, AI{OH)2+' Al{OH)3° and

Al (OH) 4- (coordinated OH2 groups have been deleted for clarity)

over the pH range of soils (Martin, 1988). This simple

monomeric hydrolysis provides a satisfactory description of

the pH and the AI+3 activity in dilute aqueous solutions at low

basicities.

At high basicities or elevated OH:AI ratio in solution,

polynuclear hydroxy-AI species, which are metastable

intermediates in the precipitation of solid phase Al (OH) 3' may

form (Driscoll and 8checher, 1988; Nordstrom and May, 1989).

Various inorganic ligands including fluoride (F) and sulfate

(804 ) and a wide variety of organic ligands form soluble

complexes with AI.

5.2.2 Solubility and precipitation of Al

Various solid phases have been implicated in the control of

Al solubility in acid soils. Richburg and Adams (1970)

suggested that gibbsite or gibbsite-like mineral controls Al

solubility. Marion et al. (1974) concluded that gibbsite,

halloysite, kaolinite or montmorillonite may govern Al

solubility, depending on the soil. van Breemen (1973)

suggested that basic aluminum sulfates may control Al

solubility in acid sulfate soils. This hypothesis was

supported by Nordstrom (1982), who claimed that KAl3(804 ) 2(OH) 6

86

(alunite), and Al4(OH) 10S04 (basaluminite) likely controls Al

solubility under acidic conditions if S04-2 activities are high

enough.

Under some circumstances, the solubility equilibria cannot

be used readily to predict Al concentration in acid soils for

three reasons. (1) Soils generally do not contain

thermodynamically pure solid phases and therefore cannot reach

true equilibria (Helmke, 1988). (2) The kinetics of Al

dissolution, polymerization, and precipitation reactions are

so slow compared to changes in dissolved forms and activities

from leaching and changes in soil water content that

equilibrium is not likely to be approached (Baham, 1984;

Goenaga and Williams, 1988). (3) Aluminum solubility, when

expressed on a concentration basis, is generally increased by

the presence of strong ligands (Burrows, 1977). Organic acids

inhibit hydrolysis and precipitation of AI, perhaps by

disrupting hydroxyl bridging (Kwong and Huang, 1979), and

accelerates the dissolution of AI-bearing minerals (McColl and

Pohlman, 1986). Huang (1988) reviewed the effects of soil

solution ligands on the solubility and precipitation products

of AI.

5.2.3 Aluminum speciation as an index of phytotoxicity

The relative phytotoxicity of mononuclear and polynuclear

Al species have been assessed in short-term solution culture

studies (Hue et al., 1986; Parker et al., 1988; Parker et al.,

1989). These studies established the nontoxicity of Al-S04

87

(Kinraide and Parker, 1987a), AI-F (Cameron et al., 1986) and

organic complexes of Al (Hue et al., 1986). One polynuclear

hydroxy Al species (Al l3 ) proved to be quite toxic in

artificial solutions (Parker et al., 1989), but its role in

soil Al toxicity is uncertain given the high affinity of

negatively charged soil surfaces for Al polymers (Zelazny and

Jardine, 1989). While Al+3 is generally considered to be the

main toxic mononuclear species, the toxicity of Al(OH)+2 and

Al(OH)2+ has been claimed (Alva et al., 1986). Some

investigators use the summation of the activities of these

three mononuclear Al species as a measure of toxicity (Bell

and Edwards, 1986). In recent studies wheat (monocotyledon)

was shown to be sensitive to Al+3 but not to Al (OH) +2 and

Al(OH)2+ (Kinraide and Parker, 1989a). Dicotyledonous species,

however, appeared to be sensitive to mononuclear hydroxy-AI

and unaffected by AI+3 (Kinraide and Parker, 1989b). The

authors suggest that determining the relative toxicities of

AI+3 and mononuclear hydroxy-AI may be an intractable problem

because the hydroxy-AI monomers can be expressed as a function

of the activities of Ar3 and W. Therefore, toxicity

attributed to mononuclear hydroxy-AI may be AI+3 toxicity

influenced by pH (Kinraide and Parker, 1989b).

Plant growth limitations have generally been expressed as

a function of activity of Al+3 [(Al+3) ] . A plot of relative

taproot length of soybean (Glycine max (L.) Merr.) versus

calculated (Ar3) is reported by Noble et al. (1988). This

88

graph illustrates the sensitivity of root elongation to (AI+3 )

in a complete nutrient solution and is typical of the response

pattern to Al stress exhibited by a number of plant species.

Treatments that reduce (AI+3) , including increasing the pH of

the system and the addition of Al complexing ligands, have

been shown to ameliorate Al toxicity (Parker et al., 1988).

Lime addition is often used to overcome soil acidity but

materials containing Al complexing ligands (S04' F) have also

been used (Sumner and Carter, 1988). Calcium and other cations

have been shown to reduce Al toxicity in solution culture

experiments (Kinraide and Parker, 1987b). These salt additions

increase ionic strength and hence reduce (AI+3) . The cations

also have a direct physiological effect on the plants, perhaps

by competing with Ar3 for external binding sites on root

cells. In general, some function of solution (Ar3) and (Ca+2 )

gives a better prediction of root growth than (Ar3) alone

(Noble and Sumner, 1988).

5.3 Al detoxification by organic amendments

Polyvalent aluminum ions are bound reversibly on cation

exchange sites, and almost irreversibly by ligand bonds, to

organic surfaces (Hargrove and Thomas, 1981a). Addition of

organic amendments (crop residues, animal waste, and green

manure) appears to provide alternative adsorption sites for

Al+3.

89

5.3.1 Influences of plant residues, sewage sludge and

animal wastes

In fact, several studies have demonstrated a considerable

reduction in soluble and exchangeable Al concentration by

organic matter applications to very acid soils (Brogan, 1967;

Hoyt and Turner, 1975). The incorporation of crop residues,

such as ground coffee (Coffea sp) leaves and pangola grass

(Digi taria decumbens Sent) into a strongly acid soil of

Puerto Rico significantly reduced both exchangeable and soil

solution AI, and enhanced growth of sorghum (Wahab and

Lugolopez, 1980). It has been observed that plant residues

like alfalfa (Medicago sativa) meal (Hoyt and Turner, 1975),

wheat (Triticum aestivum) straw (Ahmad and Tan, 1986), green

manure leaves of cowpea (Vigna unguiculata) and leucaena

(Leucaena leucocephala), and pineapple (Ananus comosus) leaves

(Hue and Arnien, 1989; Lu, 1991) added to AI-toxic soils

reduced the toxicity of Al and increased the biomass

production of plants grown in those soils. Similarly, the

critical Al concentration above which Al phytotoxicity occurs,

increases with increasing the content of soil organic matter

(Adams and Hathcock, 1984).

Sewage sludge and animal wastes, such as poultry and cattle

manures, have also been reported to reduce Al toxicity and

increase rice (Oryza sativa) yields (Ragland and Boonpuckdee,

1986), and forage yield of Desmodium intortum (Hue, 1992). In

particular, chicken manure is known to contain significant

90

amounts of uric acid in addition to many other organic

molecules (Tan et al., 1971; Hue, 1992). The structure of uric

acid is a heterocyclic molecule with chemical structure

similar to that of 8-hydroxy quinoline in terms of forming a

5-membered ring with AI. Since the latter compound is a strong

Al complexer as indicated by its use in the colorimetric

determinations of Al (Bloom et al., 1978), uric acid and its

supply source (chicken manure) is expected to strongly complex

AI.

5.3.2 Ai detoxification by organic complexation

Al ions are detoxified by interaction with organic matter

and its decomposition by-products through complexation,

chelation and peptization (Bartlett and Riego, 1972; Cabrera

and Talibudeen, 1977; Hue et al., 1986). Organically complexed

Al would not be absorbed by roots or may be taken up by plants

without the adverse effects on growth and yields.

Low-molecular-weight organic acids have been shown to

reduce Al phytotoxicity without reducing its total

concentrations (Bartlett and Riego, 1972). By using the

elongation rate of cotton (Gossypium hirsutum L.) taproots as

a bioassay index of Al toxicity, Hue et al. (1986) measured

the relative toxicity of Al in the presence of several

naturally occurring organic acids in acid forest soils. The

detoxifying capabilities of these acids appear to increase

with concentration and the proximity of hydroxyl and

carboxylic groups within the molecule, and follow the order

91

citric > oxalic > tartaric > malonic > salicylic. This

suggests that complexation involving AI, hydroxyl and carboxyl

groups, resulting in stable 5- or 6-membered ring structures,

was primarily responsible for the Al detoxification. Humic

acids reduced the toxicity of Al by reducing the concentration

of Al in soil solution and the quantity of KCI-extractable Al

(Bloom et al., 1979; Hargrove and Thomas, 1981b). Addition of

100 and 350 Ilg g-l humic acid to maize in sand culture

increased yields and reduced leaf Al concentration from 86.6

Ilg g-l to 57.4 Ilg g-l (Tan and Binger, 1986). Soluble organic

ligands can reduce the toxicity of Al without necessarily

reducing the soil solution concentration or plant uptake.

Citric acid, which occurs at 10- 6- 10-5 M concentrations in

soil solution, increased translocation of Al to the tops of

ryegrass (Lolium multiflorum Lam) with no apparent effect on

dry matter yield (Muchovej et al. 1986).

In addition to transforming soluble Al from phytotoxic to

nontoxic forms without changing the soluble Al concentrations,

the added organic materials could also precipitated Al by

increasing the soil solution pH as a result of intense

microbial activities or of ligand exchange between organic

anion and terminal OH- of the variable-charge soils (Parfitt

et al., 1977b; Hue and Amien, 1989) . Consequently,

exchangeable AI+3 as extracted with unbuffered 1 M KCI can

also be reduced by the addition of organic matter (Evans and

Kamprath, 1970; Hansen et al., 1988). Chelation of AI+ 3 by

92

different functional groups (-OH, -COOH and -NH2 ) of the solid

portion of organic matter could also play an important role in

reducing exchangeable Al (Hoyt and Turner, 1975; Hargrove and

Thomas, 1981a).

5.4 Green manures and organic management for acid soils

Past research has concentrated on green manure application

to lowlands to improve rice yield (Naklang et al., 1980a,

1980b). The results often suggest that the incorporation of

sunn hemp markedly increased the growth and yield of rice,

especially when sunn hemp residue was used continuously and/or

combination with chemical fertilizer.

For upland acid soils under field crops, some experimental

resul ts have been encouraging. Effect of green manure on

sorghum yield has been reported by Thawornmas and Dechsongchan

(1979). It was found that the incorporation of Vigna spp and

Crotalaria juncea increased the yield of sorghum by 2350 and

3437 kg ha-1, respectively. Also, long term rotations of green

manure-corn were practiced for six years at the National Corn

and Sorghum Research Center in Nakhon Ratchasima. The

experiment plots were on a Pak Chong soil (Kandiustults, pH

5.2). It was observed that the yields of corn improved under

repeated rotation of Mimosa invisa and Sesbania speciosa, and

reached a plateau of 6 - 7 tons ha-1 in the fourth rotation.

Treatments of Nand NP fertilizer accelerated the rate of

yield improvement to a certain extent but did not show any

93

distinct advantage after the fourth rotation. Soil analyses

clearly showed an accumulation of total N (2.12%) and organic

matter (1.8%) under both mimosa and sesbania rotations,

indicating an abundant supply of organic N from the legumes

(Sukthumrong et al., 1986).

In Thailand, the use of organic industrial wastes as a

source of nutrients for paddy rice has been of considerable

interest in recent years (Vacharotayan and Yoshida, 1985;

Vacharotayan et al., 1988). Cooperative work between Thailand

and Japan has indicated that organic industrial wastes,

especially castor meal and activated sludges, could be

effectively used as source of nitrogen for paddy rice and

upland crops (Panichsakpatana et al., 1988). The efficiency of

glutamic mother liquor (GML) and 'humus', the organic waste

materials from a monosodium glutamate factory, as a source of

N for paddy in acid sulfate soils (Rangsit series; Sulfic

Tropaquepts) was evaluated in the laboratory and pot

experiments (Chanchareonsook et al., 1989). It was found that

grain yield of the rice plants grown in the soil amended with

GML was 83 and 82% of those grown in the soil amended with

{NH4)2S04 and urea, respectively. 'Humus' increased both the

growth and yield of rice but the effectiveness was very low as

compared to GML. The low efficiency of 'hw~us' might be due to

the effects of some toxic substances released from

decomposition which reduced the number of grain sets per

panicle.

94

Some organic wastes have been shown to reduce the adverse

effects of acidity in some soils from the Northeast Thailand.

Ratanarat et al. (1977) showed that the application of lime

together with organic compost resulted in the highest yield of

soybean grown on acid Roi-et soil (Aerie Paleaquult, pR 4.8

and 0.9% a.M.). Lower tissue concentrations of Al and Mn in

the plants treated with lime-compost were probably responsible

for such a superior growth.

5.5 Influence of soil organic materials on aluminum

transformation

5.5.1 Formation of soluble organic-aluminum complexes

5.5.1.1 Formation and stability of bonds

Aluminum may form soluble or insoluble complexes with

organic matter or it may be non-specifically adsorbed onto

exchange sites. Evidence for insoluble organic Al complexes

comes from studies using infra-red spectroscopy. Vinkler et

al. (1976) considered that the antisymmetric carboxylate

stretching frequency at 1625 - 1630 cm-1 for Al humates and

fulvates indicated the formation of covalent rather than ionic

bonds. By bonding with various functional groups of a humic

acid, Ritchie et al. (1982) demonstrated that carboxyl and

phenolic OR-groups were responsible for formation of complexes

with AI+3 , Fe+3 and Cu+2 • The high pH at which phenolic

hydroxyls deprotonate prevent them from binding AI+3 in acid

solution. If Al binds to a carboxyl adjacent to phenolic OR,

95

however, the OH is more easily deprotonated, and may form part

of bidentate chelate ring (Lind and Hem, 1975). Aluminum also

forms chelates with adjacent carboxyl groups on ligands.

Complexes of Al and organic ligands have been reported as AI

oxalate (oxalate forming a 5 member ring), AI-tartrate 2 (2

tartrate ligands forming 7 member ring), and AI-citrate

(citrate forming 3 rings with 7, 6, and 5 members) (Motekaitis

and Martell, 1984).

Factors which influence the stability of AI-organic

complexes include the number of ligand atoms bonded to the

metal atom, the number and arrangement of rings formed, and

the presence of competing cations and ligands, especially H+

and OH- (Stevenson, 1982). Formation of stable chelates

requires that reactive functional groups are in close enough

to form rings with the central metal atom. Therefore, the

stability of Al complexes with humified polymers is inversely

related to the distance between carboxyl groups (Arai and

Kumada, 1981).

5.5.1.2 Solubility of organic-AI complexes

Low molecular weight organic ligands are capable of

forming soluble complexes with Al (Motekaitis and Martell,

1984; Hue et al., 1986). Complexation with Al can cause larger

fulvic acid molecules to precipitate out of solution

(Schnitzer, 1978).

Aluminum forms insoluble complexes with insoluble

humified organic matter. The formation of complexes with

96

insoluble organic polymers can also be described as

adsorption. Increasing soil organic matter content may result

in decreasing soil solution and KCI-extractable Al (Thomas,

1975) .

5.5.1.3 Significant role of pH

Organic-aluminum complexation is pH dependent. The

addition of organic matter to a soil or solution can decrease

or increase the level of soluble Al depending on the

concomitant changes in pH (Bloom et al., 1979). Increases in

soluble Al could be explained by the high pH of the soil

causing some dissociation of H+ from the organic matter which

lowered the soil pH and resulted in the dissolution or release

of AI. On the other hand, the addition of organic matter could

also decrease soluble Al because the extent of Al binding by

the organic matter more than counterbalances any increased Al

dissolution caused by pH decreases (Hargrove and Thomas,

1981b) . Hargrove and Thomas (1982) observed a pH increase with

Al adsorption when the soluble Al was present mainly as

hydrolyzed species initially. This reaction was probably due

to the low pH of the organic matter suspension partially

dehydroxylating some of the added AI. In contrast, Arai and

Kumada (1981) and Hargrove and Thomas (1982) observed a pH

decrease when Al was adsorbed by organic matter from solution

at pH < 4. In the latter cases, the Al would have been present

primarily as Ar3 and its adsorption could have involved

97

exchange with H+ ions as well as some surface hydrolysis. Both

mechanisms would decrease pH.

5.5 .2 Adsorption of organic anions on variable-charged

surfaces of aluminum, and iron minerals

Organic anions which form complexes with dissolved aluminum

and iron can also be specifically adsorbed on variable-charge

surfaces of Al and Fe clays and hydrous oxides. Specific

adsorption denotes adsorption through other than electrostatic

binding (Stumm et al., 1980). In ligand exchange, one or more

reactive groups form an inner-sphere complex with Al or Fe on

hydroxylated mineral surfaces, displacing surface hydroxyls

(Sposito, 1984). The surface hydroxyls are amphoteric, and may

be protonated (-OH2 ) or deprotonated (-0), depending on pH.

Ulrich and Sumner (1991) illustrate possible results of the

specific adsorption of a di- and tricarboxylate anion on a

variable-charge Al mineral with varying degrees of

protonation.

5.5.2.1 Evidence for ligand exchange

Several authors present evidence that organic anions

are specifically adsorbed on mineral surfaces through ligand

exchange with carboxylate and surface hydroxyls. Stumm et al.

(1980) inferred the formation of inner-sphere complexes of

various ligands with goethite and gibbsite from the similarity

of stability constants of those complexes with analogous

complexes with soluble iron and aluminum. The decrease in

organic anion adsorption as pH increased above the pKa of the

98

carboxyl, reported by several authors (Bowden et al., 1980;

Kummert and Stumm, 1980), has been suggested as evidence for

a ligand exchange reaction analogous to the formation of

solution complexes.

Parfitt et al. (1977a) measured adsorption of oxalate

and benzoate on gibbsite and goethite. The adsorption of

oxalate was consistently greater than benzoate under identical

conditions. By comparing the infrared spectra of the surface

complexes with model compounds and complexes, the authors

concluded the following. At low concentrations of 100 umol s'.

oxalate was adsorbed on goethite as a binuclear complex across

adjacent Fe atoms. At higher concentrations a monodentate

complex formed. Oxalate formed a bidentate complex with single

atom of Al on gibbsite edges. Oxalate and benzoate did not

adsorb onto faces of the gibbsite used in the study. At pH 3.5

fulvate was adsorbed onto the gibbsite faces, perhaps by

hydrogen bonding. The authors suggested that poorly

crystalline hydrous Al oxides would contain more sites for

ligand exchange than the gibbsite used in their study (Parfitt

et al., 1977b).

Huang et al. (1977) confirmed the importance of

amorphous hydrous oxides in organic anion adsorption.

Dissolution of amorphous hydrous oxides from four Taiwan soils

reduced adsorption of five aromatic carboxylic acids. One of

the soils (an Alfisol) was coated with hydroxy-AI and -Fe

99

precipitates. The hydroxides coatings increased adsorption of

all of the aromatic acids studied.

Specific adsorption of organic anions alters soil

properties in three ways which may benefit plant growth. (1)

Specific adsorption can potentially raise soil pH by

displacing surface hydroxyls into the soil solution, and

precipitate AI. (2) Specific adsorption of organic anions may

increase cation nutrient retention by increasing negative

charge on hydrated oxide or clay surfaces. (3) At low pH,

adsorption of organic anions may enhance the availability of

nutrient oxyanions, such as phosphate, by competing for

adsorption sites and by reducing positive charge.

5.5.2.2 Effects of adsorption on soil pH

Parfitt et al. (1977b) measured OH- release during

adsorption of oxalate by gibbsite, and fulvate by gibbsite and

goethite (Parfitt et al., 1977c) by back-titrating suspensions

to their original pH after organic anion additions. The first

25 umol g-l oxalate released approximately 1.3 moles of OH- per

mole of oxalate adsorbed. Adsorption of 50 g kg-1 fulvate

released approximately 10 cmol (OH) kg- 1 from goethite and 20

g kg-1 released approximately 2.5 cmol (OH) kg- 1 from gibbsi te.

A few report of increases in soil pH with additions of

organic material can be found in the literature. After 14 days

of incubation, coffee (Coffea arabica L.) leaves raised the pH

of a Puerto Rican Typic Humult (Wahab and Lugo-Lopez, 1980).

Lopez-Hernandez et al. (1986) reported a pH increase in soil

100

suspensions after malate was added. After ground alfalfa

(Medicago sativa L.) hay was incorporated into a Canadian

soil, pH increased for 6 weeks and remained nearly constant

for an additional 14 weeks before decreasing (Hoyt and Turner,

1975). An excess of pH 6.1 fulvate added to suspensions of

gibbsite or goethite at pH 6.3 raised pH over 2 units (Parfitt

et al., 1977c). Sodium humate also raised pH of suspensions of

the same minerals. Incorporation of 8 Mg ha-1 fresh weight of

Kudzu (Pueraria Phaseoloides) tops in a Peruvian UI tisol

increased soil pH with a corresponding reduction of KCI

extractable Al and Al saturation relative to an unamended

treatment (Wade and Sanchez, 1983). The difference between the

green manure and control was greatest 15 months after

incorporation.

Asghar and Kanehiro (1980) incubated large quantities

(5 and 10% of soil mass) of sugarcane (Saccharum officinarum)

and pineapple (Ananas comosus) residue in a Rhodic Eutrustox.

Each treatment resulted in an initial increase in soil pH. The

pH of the pineapple amended soil remained higher than the

unamended throughout the experiment (120 days). The increase

and subsequent decrease of pH was linearly related (r2 = 0.71,

P < 0.01) to changes in Eh measured with a platinum electrode.

The authors attributed the changes in soil pH to the redox

reaction,

Mn02 + 2W + 2e-1 ---> Mn+2 + 20W , [7 ]

101

with the availability of e- positively related to microbial

activity.

5.5.2.3 Effects of adsorption on surface charge

Adsorption of organic anions may improve the retention

of nutrient cations. By reducing the number of protonated

sites on a hydrous oxide surface, adsorption of organic anions

tends to reduce surface positive charge over a range of pH,

and reduce the zero point of net charge of the oxide (Kummert

and Stumm, 1980). Plant materials (Canavalia ensiformis and

Panicum maximum) incorporated into a Brazilian Typic Haplustox

increased negative charge of the soil (Motavalli et al.,

1988) .

102

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105

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110

CHAPTER 6

Selected physical and chemical properties of acid sulfate

soils used in the present study

6.1 General description of the soils

Two acid sulfate soils from the Bangkok Plain, Thailand

were used in this dissertation. They are Rangsit very acid

phase series (Ra) , and Bang Pakong series (Bg) r and are

hydromorphic alluvium. Details of site selection, morphology

and profile description are listed in Appendix A-1 and A-2.

The Ra soil (Sulfic Tropaquepts), an extremely acid soil,

represents the suitability class P-IVa (in Thailand) . Soils in

this class have severe limitations due to acidity constraints

that restrict their use for rice production and/or require

very special management.

The Bg soil (Typic Sulfaquents) represents potential acid

sulfate soils. Its suitability classification for rice

production is dependent on the level of salinity and the

degree of oxidation of the surface soil. The soil will turn

into class P-IVa after intense oxidation.

6.2 Site selection and soil sample preparation

Figure 6.1 shows the locations where the two acid sulfate

soils were collected. The Ra soil was sampled from two local

farms and the Ongkarak Acid Sulfate Soil Experiment Station in

the Nakhon Nayok Province. The Bg soil was located about 45 krn

D·'·'.. "...

~

Fan Complex

Old Delta

'"

t

k~O, ,

~ Deltaic High}New Deltao Delta Flat

.:t=f.- Irrigation Canal

111

Figure 6.1. Generalized soil map of the Southern Bangkok Plain

(after Province Series No.8, Dept. of Land Development,

Bangkok. 1972) showing the sampling site as : II. Rangsit very

acid phase, at Nakhon Nayok Province; and IV. Bang Pakong, at

Chachoengsao Province.

east of the Ra soil. The Bg soil was collected from two local

farms and the Bang Pakong Soil Conservation Center in the

112

Chachoengsao Province. The soil samples were collected in the

early rainy season (August, 1992). Approximately 800 kg of a

composite sample from each soil series was collected from the

plow layer (20 cm) and the jarositic horizon (30 - 60 cm) in

a half hectare area. The samples were transported to the

Department of Soil Science, King Mongkut' s Institute of

Technology, Ladkrabang, Bangkok, where the soils were spread

on large floors and allowed to air dry for 2 - 3 weeks.

Finally, the soils were ground with a mechanical grinder and

sieved to pass a 2-mrn screen before use. About 20 kg of each

soil was shipped to the University of Hawaii for an incubation

study.

6.3 Chemical characteristics of the aerated soils

Composite samples were used for each analysis. The

parameters analyzed, methods and instruments used are as

follows.

The pH and electrical conductivity (EC) in water (1:1) were

determined by a pH meter equipped with a combination glass

calomel electrode, and conductivi ty meter as described by

Attanandana and Chancharoensook (1980).

Organic matter was analyzed by the Walkley-Black method

(Black, 1965). Total nitrogen was determined by the Kjeldahl

method (Bremner, 1965). Extractable P was determined by the

Bray II method (Bray and Kurtz, 1945).

113

determined using

as described by

Cation exchange capacity (CEC) was

ammonium saturation-distillation method

Chapman (1965).

Exchangeable K, Ca, Mg and Na - the soils were extracted

with 1 M NH40Ac (pH 7.0) as described by Black (1965).

Potassium, Ca, Mg, and Na in the extract were determined by

atomic absorption spectrophotometry. Total exchangeable base

was the sum of exchangeable K, Na, Ca, and Mg.

Total exchange acidity, and exchangeable Al were extracted

with 1 M KCI as described by Mclean (1965). Following

extraction, a known aliquot was titrated. Exchangeable H was

calculated as the difference between total exchange acidity

and exchangeable AI.

For water soluble S04-2, the soils were extracted by the

method described by Freeney (1986). Then, S04-2 ions were

determined by the turbidimetric method.

For extractable Fe and Mn, the soils were extracted with

0.005 M DTPA, pH 7.30 as described by Lindsay and Norvell

(1978) .

Selected chemical properties of the two acid sulfate soils

are listed in Table 6.1. The air-dried pH of both Bg and Ra

soils was very low of 3.3 - 4.3 (average 3.8 for the Bg soil),

and pH of 3.3 - 5.2 (average 4 for the Ra soil). The pH of the

air-dried surface layers was usually above 4 because of high

organic matter content.

114

Organic matter content in the Bg and Ra soils used in this

study was higher than expected in as much as ranged from 2.9 -

5.5%, with the highest average value of 4.1% in the Bg surface

soil. These values are probably due to slow decomposition of

organic residues under strongly acid conditions.

In general, the extractable P in the Bg and Ra soils was

insufficient for plant growth. The Bg soil had only 8.4 rng P

kg-I. Exchangeable K, Ca, Mg and base saturation in both soils

Table6.1. Some important chemical properties of the aerated acid sulfate soils.

Parameters

pH (l: I; soil : water)

Organic matter (%)

Total N (%)

Extractable P (mg kg")

CEC (cmol, kg")

Total exchangeable base

(cmol, kg")

Exchangeable K (cmol, kg")

Exchangeable Ca (cmol,kg-I)

Exchangeable Mg (cmol, kg")

Exchangeable Na (cmol,kg")

Exchange acidity (cmol, kg")

Exchangeable AI (cmol,kg")

Exchangeable H (cmol,kg")

Extractable Fe (mg kg")

ExtractableMn (mg kg")

Water solubleSO/ (mgS kg")

Soil Series

Bang Pakong Rangsit

(Bg) (Ra)

3.8 4.0

4.15 3.83

0.19 0.21

8.40 9.0

21.40 23.82

11.28 3.34

0.15 0.22

2.20 1.08

6.75 1.89

2.17 0.14

14.36 8.14

7.56 4.31

6.80 3.83

1440.92 71U5

36.80 22.66

502.44 134.49

115

were low, whereas exchangeable Al was high. From the data

presented, the Bg soil had 7.5 cmol., kg-1 exchangeable AI, and

the Ra soil, 4.3 cmoL, Al kg- 1• Moreover, exchangeable H was

high with contributing 40% of the total acidity.

The high proportion of exchangeable Mg to exchangeable Ca

in the Bg soil is indicative of old marine sediments as the

parent material. Extractable Fe, Mn and water soluble 8°4-2

also differed markedly between the two soils, and they were

much lower in the Ra soil. These inherent properties of two

acid sulfate soils together with their relatively low pHs

indicate a low fertility status and potential for the release

of soluble Al and Fe in toxic amounts upon land use.

6.4 Particle size distribution

Particle size determination was carried out by the method

of Gee and Bauder (1982). The percentages of clay, silt, and

sand size fractions and the corresponding texture class are

listed in Table 6.2. The two soils had clay textures, and the

clay content ranged from 61.7% in the Ra soil to 55.7% in the

Bg soil. A considerable vertical and lateral variation in soil

texture however, may be found in field conditions, especially

in smaller coastal plains, as near Bang Pakong. In the Bangkok

Plain, textures are more uniform in the older acid sulfate

soils (e.g., Ra, typically 60 - 65% clay in most soils) than

in the zone closer to the coast, where soil with a silty clay

texture (e.g., Bg) can be found.

116

Table 6.2. Particle size distribution and textures of the two acid sulfate soils.

Soil

Series

Bg

Ra

% Texture

Clay Silt Sand

« 2 urn) (50 - 2 um) (> 50 urn)

55.76 39.76 1.04 Clay

61.71 35.63 2.63 Clay

6.5 Lime titration curves

According to Frink (1973), when an acid soil is

neutralized, at least three reactions occur. First,

exchangeable H is neutralized. Next, exchangeable and some

parts of non-exchangeable Al are neutralized over a pronounced

buffer range of 4.5 - 5.0. Finally, H bound at the edge sites

of clay minerals and the weak acid groups of organic matter

are neutralized, and the buffer range is above pH 5.5 - 6.0.

Figure. 6.2 shows the lime titration curves of the two acid

sulfate soils (using the methods described by Attanandana and

Chancharoensook, 1980). By normal liming standards, these

soils require enormous quantities of lime to raise soil pH by

one unit. For example, 10 cmol (OW) kg-1 (or 10 ton CaC03 ha")

is needed to raise pH of the Ra soil from 4.0 to 5.0, and

nearly 15 cmol (OW) kg- 1 is required to raise pH of the Bg

soil from 3.8 to 4.8. If pH 5.5, a traditionally recommended

pH value, is attempted, then at least 13 and 21 ton CaC03 ha'

must be applied to the Ra and Bg soils, respectively.

8.5

7.5

:r: 6.5Co-'0

5.5en

4.5

3.5I0

I10

I20

I30

I40

I50

117

CaCOH)2 added, ernol COH) kg-'

Figure 6.2. Titration curves of the two acid sulfate soils. Bg

= Bang Pakong, Typic SUlfaquents; Ra = Rangsit, Sulfic

Tropaquepts,

118

References

Attanandana, T., and J. Chancharoensook. 1980. Soil and plantanalysis. Dept. of Soil Science, Kasetsart Univ.,Bangkok. 103 p. (in Thai) .

Black, C.A. 1965. Methods of soil analysis, Part II. Am. Soc.Agron., Inc. Publ., Madison, WI.

Bray, R.H., and L.T. Kurtz. 1945. Determination of totalorganic and available forms of phosphorus in soils. SoilSci. 59 39-45.

Bremner, J.M. 1965. Total nitrogen. In C.A. Black (ed.)Methods of Soil Analysis, Part II, pp. 1149-1178. Am.Soc. Agron., Inc. Publ., Madison, WI.

Chapman, H.D. 1965. Cation exchange capacity. In C.A. Black(ed.) Methods of Soil Analysis, Part II, pp , 891-901. Am.Soc. Agron., Inc. Pub., Madison, WI.

Freeney, J.R. 1986. Analytical methods for determining sulfurin soils and plants. In Sam Partch and Ghulam Hussain(eds.) Proc. Int. Symp. on Sulfur in Agric., pp. 67-84.Bangladesh Agricultural Council and Sulfur Institute.

Frink, C. R. 1973. Aluminum chemistry in acid sulfate soils. InH. Dost (ed.) Acid Sulfate Soils Proc. Int. Symp., pp.131-160. ILRI Publ. 18, Vol. I. Wageningen, TheNetherlands.

Gee, G.W., and J.W. Bauder. 1982. Particle-size analysis. InA.L. Page et al. (eds.) Methods of Soil Analysis, Part I,2nd ed., pp. 383-412. Agronomy Monog. 9. ASA-SSSA.Madison, WI.

Lindsay, W.L, and W.A. Norwell. 1978. Development of a DTPAsoil test zinc, iron, manganese, and copper. Soil Sci.Soc. Am. J. 42 : 421-428.

Mclean, E.O. 1965. Aluminum. In C.A. Black (ed.) Method ofSoil Analysis, Part II, pp. 978-998. Am. Soc. Agron.,Inc. Publ., Madison, WI.

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119

CHAPTER 7

Differential acidity tolerance of tropical legumes grown for

green manure in acid sulfate soils

7.1 Abstract

The growth and soil acidity tolerance of four tropical

green manure legumes (Cajanus cajan, Sesbania aculeata, s.

rostrata, and S. speciosa) were studied in a greenhouse

experiment. Two acid sulfate soils (Typic Sulfaquents, Bg

Series; and Sulfic Tropaquepts, Ra Series) were adjusted to

four pH levels: 3.8 and 4.0 (original soil pH), 4.5, 5.5, and

6.5 (amended with lime). Plant dry weight was determined 49

days after sowing along with N, P, K, Ca, Mg, Fe, Mn, and Al

concentrations in aerial plant parts.

The legumes responded differently to soil acidity and

liming, but not to soil type. Cajanus cajan had the highest

biomass production, followed by S. aculeata, S. rostrata and

S. speciosa, in this order. Nitrogen content closely

paralleled biomass production, suggesting that the symbiotic

rhizobia and nodulation were perhaps more susceptible to soil

acidity than the host plants. Liming to pH 5.5 was recommended

for the legumes' growth based on sharper yield increases as

soil pH was raised from 4.5 to 5.5 than from pH 5.5 to 6.5. In

the unlimed soils, C. cajan and S. aculeata accumulated Ca

concentration (0.32%) twice as much as the other two low

yielding legumes (0.15%). Furthermore, plant Ca increased

120

exponentially (or quadratically in the case of S. speciosa) as

lime additions increased. It was estimated that c. cajan

required approximately 1.2% Ca, S. aculeata 1.0%, S. rostrata

0.8%, and S. speciosa 0.5% for adequate growth. In contrast to

the Ca accumulation pattern, Fe and to a lesser extent Mn were

significantly lower in C. cajan and S. aculeata than in S.

rostrata and S. speciosa. The ratio of Ca to Al in plant tops

was used to characterize plant tolerance to soil acidity, and

to quantify the critical Al concentration in the plants. It

appears that growth ~ 90% of the maximum is only attainable

when Ca/AI is ~ 150 for C. cajan and S. speciosa, ~ 200 for S.

rostrata, and ~ 300 for S. aculeata. Cajanus cajan tolerated

up to 80 mg Al kg-1, whereas significant growth reduction

occurred in the Sesbania species at levels > 30 mg Al kg-1 in

their tops.

7.2 Introduction

Use of fast growing legumes, such as Cajanus cajan,

Leucaena and Sesbania species, as green manure has long been

an important cultural practice of subsistence farmers in the

tropics (Evans and Rotar, 1987; Yost and Evans, 1987). Little

scientific information concerning edaphic adaptability of

these multipurpose plants is available. Data from India, where

most green manuring research has been done, indicate that

these green-manure plants can grow well in calcareous or sodic

soils, but their performance in acid soils has not been

121

vigorously tested (Evans et al., 1983; Mappaona and Yoshida,

1993). As for crop tolerance to soil acidity, c. cajan and S.

rostrata were the only tropical legumes for which data were

available (Joshua et al., 1989; Nakano et al., 1992). Cajanus

cajan reportedly grew better than S. rostrata in acid soils

(Nakano et al., 1992). Perhaps because of this tolerance, C.

cajan has been recommended as a promising crop for the

Northeast Thailand (Khon Kaen Province), where most soils are

sandy and acid (Wallis et al., 1988). Recently, Thai farmers

are growing legumes for green manure in annual crop rotation

over an increasingly large area of clay-textured acid sulfate

soils of the Bangkok Plain (Poolpipatana' s personal

observation). These soils are strongly acidic and potentially

phytotoxic when aerated because of the oxidation of pyrite and

jarosite minerals (Parkpian et al., 1991). Given these

circumstances, characterizations of green-manure plants that

are tolerant of soil acidity are urgently needed. Thus, the

objective of this study was to evaluate the acidity tolerance

of four leguminous species commonly used as green manure when

grown on acid sulfate soils.

7.3 Materials and methods

7.3.1 Plant selection from a field survey

Initially a field survey was conducted to investigate the

species by soil environment interaction for local tropical

legumes in the Bangkok Plain. Studied plants were potential

122

green manure, leguminous species of Cajanus, Crotalaria,

Desmodium, Leucaena, Vigna, and Sesbania. Each location was

visited several times in order to question farmers on growth

behavior, biomass production and ability to survive under

adverse conditions. After evaluation, the four most promising

species for green manure were selected for the greenhouse

study. They were Cajanus cajan, Sesbania aculeata (also known

as S. bispinosa) , S. rostrata, and S. speciosa.

7.3.2 Soil sampling and analysis

Acid sulfate soils were taken from two locations where

paddy rice and tropical fruits were grown. They were the Bang

Pakong (Bg) and Rangsit very-acid phase (Ra) series. The Bg

was characterized as a preoxidized potential acid sulfate soil

(Typic Sulfaquents) and the Ra was an actual acid sulfate soil

(Sulfic Tropaquepts) .

Soil samples for the experiment were collected from Ap

horizons and jarositic layers. The samples were air-dried,

ground, sieved to pass a 2-mm screen and stored for chemical

analysis. Selected chemical properties of the soils are given

in Table 7.1.

Soil pH and electrical conductivity (EC) of 1:1 soil to

water suspensions were measured after 1 h of intermittent

shaking. CEC was determined by 1 M NH40Ac, pH 7.0 (Chapman,

1965). Extractable P was determined by the Bray-II method

(Bray and Kurtz, 1945). Ca and Mg from the NH40Ac extract were

123

Table 7.1. Selected physical and chemical properties of the acid sulfate soils used in the greenhouse

experiment.

Soil properties

pH (1:1; soil: water)

EC (1:1; dS m")

Organic carbon (%)

Extractable P (mg kg") tl

CEC (cmol, kg") *'Exchangeable K (crnol, kg")

Exchangeable Ca (cmol, kg-I)


Exchangeable Na (cmol, kg")

Exchangeable acidity (cmol, kg")

Extractable Al (cmol, kg")

Al saturation, % of CEC

DTPA-Extractable Fe (mg kg")

DTPA-Extractable Mn (mg kg")

Water soluble SO/ (rngS kg")

Particle size distribution

Silt (%)

Clay (%)

tl Bray P-II method

*' 1M NHPAc, pH 7.0 method

Soil Series

Bg Ra

3.8 4.0

0.7 0.2

2.4 2.2

8.4 9.0

21.4 23.8

0.1 0.2

2.2 1.0

6.7 1.8

2.1 0.1

14.3 8.1

7.5 4.3

29.5 38.4

1440.9 711.1

36.8 22.6

502.4 134.4

40.8 35.6

56.8 61.7

determined by atomic absorption spectrophotometry, and K and

Na by flame photometry. Al was extracted by 1 M KCl and

determined colorimetrically (Barnhisel and Bertsch, 1982).

Water-soluble sulfate-S was extracted by shaking 10 g soil

with 50 mL water for 30 minutes, followed by 5 minutes

124

centrifugation at 2000 g (relative centrifugal force). The

supernatant was filtered and measured turbidimetrically, using

a spectrophotometer at 420 nrn (Freeney, 1986). Soil texture

was determined by the pipette method of Gee and Bauder (1982).

7.3.3 Pot experiment

The treatments consisted of factorial combinations of four

legume species and four soil pH levels (unamended, 4.5, 5.5,

6.5). Thus, there were 16 treatments replicated three times in

a randomized complete block design. The soil pH levels of 4.5,

5.5 and 6.5 were established by adding 10, 22, and 38 cmol

(OH-) kg- 1 as Ca(OH)2 to the Bg soil, and 5.5, 14 and 22 cmol

(OH-) kg- 1 to the Ra soil. To ensure that plant growth was not

limited by nutrient deficiencies, the following nutrient

solution was applied (mg kg-1 soil): Na2Mo04 ' 2H20, 0.67 i H3B03 ,

o.83 i CuS04 ' 5H20, 5 i ZnS04 ' 7H20, 10 i MnS04 ' H20, 15; and KH2P04 ,

176. Nitrogen was applied only with a rate of 24 mg N kg- 1 as

NH4N03 at the beginning of the experiment to initiate growth.

The P rate, based on a previous P response experiment, was

selected to produce 90% of maximum growth and to minimize the

ameliorating effects that higher rates of P could have on Al

toxicity (Jugsujinda et al., 1978). The nutrients were mixed

with the soils, then incubated for 14 days at field water

holding capacity before planting.

Five seeds of each legume, inoculated with appropriate

rhizobial strains, were planted in 15-cm diameter pots filled

125

with 2 kg of soil. After germination, seedlings were thinned

to 2 plants per pot. The pots were watered daily with

deionized water, initially to 80% field capacity, and later to

field capacity as demand increased. The experiment was carried

out in a greenhouse at King Mongkut's Institute of Technology,

Bangkok, during September - November 1991. Day temperatures

were about 30 - 35°C and night temperatures 18 - 22 °C.

At 49 days after sowing, plants were removed from the pots

and aerial biomass (dry weight of tops) was obtained from oven

dried samples (70 - 80°C for 48 h). A subsample from each

replicate was digested in a 3: 1 nitric perchloric acid

mixture and analyzed for K, Ca, Mg, AI, Fe, and Mn by atomic

absorption spectrophotometry and P by the molybdate/ascorbic

acid colorimetric method. Plant N was measured by the micro

Kjeldahl method.

Analysis of variance was used to test the effect of soil

pH, soil type, and plant species on dry-matter weight and

nutrient uptake of the legumes. The relationship between

relative dry-matter yield and Ca/AI ratio of plant tissues was

established by nonlinear regression analysis, using PLOTIT~

software (Haslett, MI., USA) to derive critical Al concentra

tions.

7.4 Results and discussion

The two acid sulfate soils selected for this study had at

least two nutritional problems for the growth of the green-

126

manure legumes: Al toxicity and Ca deficiency (Table 7.1).

The data on extractable Mn and Fe also indicated the toxicity

potential of these elements. The effect of soil type on growth

parameters was however, statistically insignificant; thus,

growth data from the two soils were combined for subsequent

analysis and discussion.

7.4.1 Differential growth as measured by dry matter yield

and N accumulation

The dry-matter weights of the legumes as affected by soil

pH are listed in Table 7.2. Dry weight production was affected

significantly (P < 0.01) by species, soil pH and species x pH

interactions, but not by soil series. Soil series also had no

significant effects on plant composition (statistical analysis

not presented). For this reason, the plant dry weights from

the two soils were combined for regression analysis (Table

7.3) (unlimed pH was designated as 3.9 which is the average of

3.8 and 4.0, actual pH of the 2 unamended soils). Among the

four legumes, S. speciosa and S. rostrata had the lowest

yields, whereas C. Cajan and S. aculeata grew much better

throughout the pH range tested (Table 7.2). Since high biomass

is an important factor in selecting legumes for green

manuring, C. cajan and S. aculeata appear to be much better

than S. speci osa and S. rostra ta as green manures. wi thin

species, growth improved considerably as pH increased (Table

7.2) .

127

Table 7.2. Dry-matter yield of the four green manure legumes grown on two acid sulfate soils from

Thailand, and the associated analysis of variance.

Dry weight. g pori

Soil pH C. cajan S. aculeata S. rostrata S. speciosa

Bang Pakong soil (Typic Sulfaquents)

3.8 1.64 1.56 1.27 0.93

4.5 3.48 2.61 2.40 1.95

5.5 5.04 3.96 3.29 2.36

6.5 5.55 4.61 4.50 2.64

Rangsit very acid-phase soil (Sulfie Tropaquepts)

4.0 1.97 1.86 1.70 1.17

4.5 5.16 2.77 2.31 2.32

5.5 6.76 4.55 4.17 3.54

6.5 6.58 4.76 4.18 3.00

Analysis of Variance (ANOVA)

Variance df MS F-value Significance

Block 2 0.36 1.05 NS t

Treatment 31 7.57 22 ** t

Soil series (S) 9.2 27 NS

Soil pH 3 44.7 131 **Legume (L) 3 21.8 64 **S x pH 3 0.9 2.65 NS

SxL 3 1.8 5.3 NS

pH x L 9 1.6 4.7 **S x pH x L 9 0.4 1.2 NS

Error 62 0.34

t NS : non-significant at the 0.05 level; •• : significant at the 0.01 level.

128

Table 7.3. Regression equation of dry-matter weight (Y, in g pori) against soil pH (X) and plant N

concentration (N, in %). There were24 observations for each legume.

Regression equation

Y =-30.1 + IZ.ZX - 1.0 X2

Y =-Z.15 + Z.07N

Y =-IZ.8 + 5.ZX - 0.39X2

Y =-0.59 + I.ZZN

Y =-9.7 + 3.9X -0.Z7X2

Y =-1.69 + I.72N

Y =-14.8 + 6.IX -0.5ZX2

Y =O.ZI + 0.9ZN

.. Significant at the 0.01 level.

Cajanus cajan

Sesbania aculeata

Sesbania rostrata

0.83··

Sesbania speciosa

Coefficient of determination (r)

0.72··

0.59··

0.83··

0.47··

O.5Z··

0.78··

0.46··

Quadratic relationships between dry-matter yield and soil pH

(Table 7.3) suggest that 95% of the maximum yield would be

attained when soil pH is 5.5 for C. cajan, 5.9 for S.

aculeata, 6.3 for S. rostrata, and 5.3 for S. speciosa.

Along with biomass, high total N accumulation (mostly from

N fixation) is another desirable characteristic of legumes to

be used as green manure. This N criterion clearly indicates

that C. cajan and S. aculeata are better green manures than S.

speciosa and S. rostrata (Fig. 7.1 A). In fact, S. speciosa

grew slowly, produced the least biomass and had the lowest N

concentration among the four legumes studied (Fig. 7.1 B).

129

Thus, this Sesbania species, in spite of its photoperiod

insensitivity (i.e., a potentially short-season, fast-growing

legume) does not appear to be suited as a green manure crop

when grown in acid sulfate soils. Furthermore, strong linear

relationships between biomass and plant N concentration (Table

7.3) indicate that N nutrition partly controlled dry matter

production, suggesting that either the symbiotic rhizobia and

the host legume had similar responses to soil acidity and

liming or the rhizobia were more susceptible to soil acidity.

The latter possibility is more likely based on the facts that

(i) both S. speciosa and S. rostrata, the two low-yielding

legumes, always contained ~ 3.0% N, a level considered

inadequate for good growth, and (ii) the appropriate rhizobia

for S. speciosa often fail under adverse environmental

conditions as reported by Evans and Rotar (1987) and Dr. P.

Prabuddham (personal communication) while the stern-nodulated

rhizobia for S. rostrata might not have enough time to be

fully active. This conclusion is supported by the work of Alva

et al. (1986a) and Suthipradit (1989), who reported that

rhizobial nodulation was more sensitive to Al toxicity than

host plants, including soybean (Glycine max), cowpea (Vigna

unguiculata), and green gram (Vigna radiata).

7.4.2 Soil acidity and chemical composition of the le~~es

Phosphorus. Increasing soil pH from 3.9 to 6.5 steadily

increased P concentration in the plants probably because of

250 CA) 130

~ 20000--.....ZOJ) 150E'"~

s:= 100(])~

s:=0u

Z 50

oI

3.5 3.9I

4.5I

5.5I

6.5

4.0 (8) cc

3.0

2.0

toI

3.5 3.9I

4.5I

5.5

SA

I6.5

Soil pH

Figure 7.1. Total N uptake (A) and plant N concentration (B) of the four green manure legumes grown

in acid sulfate soils at different levels. CC : Cajanus cajan, SA : Sesbania aculeata, SR : S. rostrata; SS

: S. speciosa. Vertical bars are standard errors. Unlimed pH is designed as 3.9 which is the average of 3.8

and 4.0, the actual pHs of the two unamended soils.

131

more vigorous growth at higher pH and because soil P becomes

more available for plant uptake at pH range of 5.5 - 6.5. In

C. cajan, plant P increased from 0.19% in the unamended soils

to 0.30% at pH 5.5 and to 0.36% at pH 6.5 (Table 7.4). Given

the plateauing of the dry-matter yield at pH 5.5 (Table 7.3),

it appears that a plant P concentration of 0.30% would be

adequate for Cajanus cajan growth. In fact, this P level

agrees well wi th the adequate range of 0 .30 0 .35% P

tabulated by Reuter and Robinson (1986) for this legume

species sampled 60 days after sowing.

In the Sesbania, P increased steadily from 0.16% at pH 3.9

to about 0.30% at pH 5.5 then practically leveled off at

higher pH (Table 7.4). Thus, 0.30% P also seems to be the

adequate level for Sesbania species growth. Evans and Rotar

(1987) also reported 0.30% P as a "normal" concentration in

many Sesbania species and Singh et al. (1992) listed 0.32% as

the average P concentration (with a range of 0.21 - 0.40%) in

S. aculeata. In general, the four legume species behaved

similarly in terms of P nutrition in response to soil acidity.

Potassium. Al though herbage K increased steadily from 1.87%

to 2.38% in c. cajan (the increases were not statistically

significant, however), and fluctuated between 1.8 and 2.4% in

S. aculeata and remained virtually constant at 1.9% in S.

rostrata and 1.6% in S. speciosa, as lime rates increased

(Table 7.4). Thus, in our experiment, soil acidity apparently

had little effect on K nutrition of the legumes, perhaps

132

Table 7.4. Nutrient composition t of four tropical legumes used as green manures as affected by different

pH in two acid sulfate soils.

Soil pH Plant top composition

P K Ca Mg Al Mn Fe

<------------ % ---------------------------------> <----- mg kg" ------------------>

Cajanus cajan

3.9 0.19 1.87 0.32 0.47 175 146 236

4.5 0.21 2.03 0.77 0.92 125 126 lSI

5.5 0.30 2.25 1.35 1.22 82 93 143

6.5 0.36 2.38 2.13 1.47 75 88 81

LSD 0.05 0.08 0.60 0.37 0.37 32 35 32

Sesbania aculeata

3.9 0.16 1.76 0.31 0.55 147 173 409

4.5 0.24 2.42 1.03 0.64 121 148 322

5.5 0.30 1.89 U8 0.98 76 133 228

6.5 0.31 2.36 1.25 1.58 57 71 130

LSD 0.05 0.05 0.43 0.46 0.32 29 28 73

Sesbania rostrata

3.9 0.17 1.90 0.15 0.44 161 207 513

4.5 0.20 1.99 0.51 0.72 124 173 464

5.5 0.29 1.94 0.81 1.04 76 151 371

6.5 0.30 1.97 0.88 1.03 44 144 158

LSD 0.05 0.06 0.50 0.33 0.26 30 34 84

Sesbania speciosa

3.9 0.16 1.64 0.14 0.38 141 272 616

4.5 0.21 1.51 0.44 0.56 88 243 490

5.5 0.27 1.66 0.88 0.66 41 223 244

6.5 0.29 1.46 0.47 0.61 37 162 165

LSD 0.05 0.07 0.32 0.21 0.21 24 49 72

t Average of data from the two soils.

133

because K was supplied adequately by all the treatments.

[Adequate concentration of K was reportedly ~ 1.7% in c. cajan

(Reuter and Robinson, 1986) and 1.6 - 1.8% in Sesbania species

(Singh et al., 1992).

Calcium. Adding Ca(OH)2 to raise the soil pH also raised

plant Ca significantly (Table 7.4). In C. cajan, plant Ca

increased linearly from 0.32% in the unamended treatment to

2.13% in the highest limed treatment. By assuming that Ca was

the most limiting factor to the legume growth in the acid

soils, a rough estimate of critical Ca levels was obtained by

plotting the dry-matter yield vs. plant Ca (Fig. 7.2). This

figure shows that 1.75% and 1.20% Ca would be required by C.

cajan to maintain growth at 90% and 80% of the maximum,

respectively. The lower value agrees well with the adequate

range of 0.8 - 1.2% Ca reported by Reuter and Robinson (1986),

and is probably closer to the "true" Ca requirement because

under the Al toxicity stress (as in our case) Ca requirement

has been shown to be higher than in the absence of such a

stress (Alva et al., 1986b). On the other hand, Ca levels of

0.32% and 0.77% in plants of the unamended soils (pH 3.9) and

the lowest lime treatment (pH 4.5) were clearly deficient,

which were partially responsible for the observed poor growth.

using the same approach, adequate Ca levels were identified

as 0.82% for S. aculeata, 0.57% for S. rostrata, and 0.42% for

S. speciosa (Fig. 7.2). Evans and Rotar (1987) also listed 0.8

- 1.1% Ca as the normal range for good Sesbania growth.

134

10.0 6.0 0

• • c •8.0 • a •a c. ,

c. "- 4.0-, OJ • •OJ 6.0 .:•.: • 2 • •2 iii • ••iii 4.0 E •E 2.0 •~ •>. • .. • •"- 0 Sesbanla aculeala0 • C8lanus calan •2.0 • •..

y = 6.75'(1.0 - e-1.31X).r'=.73 V =4.36'(1.0 - e-1.98X). r'=.42

0.0 i I , , , I I 0.0

0.0 1.0 2.0 3.0 0.0 0.4 0.8 1.2 1.6 2.0

Plan! ca. % Plant Ca, %

6.0 4.0

• •• •

(5 3.0 41

e •• ••OJ •.: 841

2 2.0 • 0 8

iii •E •>. •"- to0 0 • Sesbanla speclosa

•Y = 3.04*(1.0 - e-3.85X).1"=.65

0.00.0 0.5 1.0 1.52.01.6120.8

•Sesbanla roslrala

'( = 4.20'(1.0 - e-2.80X), 1"=.60

0.4

•••8

•••

0.0 -!---,---,-.,.-....--,....--,.-,.----,---.---,0.0

• • •• • •4.0 •

•• • •• •e.

.:2iiiE..... 2.0Co

ac.

"OJ

Plan! Ca,% Plan! Ca,%

Figure 7.2. Relationship between dry-matter yield of the four

green manure legumes and the Ca concentration in plant tops.

It is worth noting that the two high-yielding legumes (C.

cajan and S. aculeata) were able to obtain and accumulate more

Ca at each pH level than the other two low-yielding species

(Table 7.4). For example, in the unamended soils, C. cajan and

s. aculeata contained about 0.32% Ca as compared to 0.15% Ca

135

in S. rostrata and S. speciosa. Perhaps, the ability to

efficiently absorb Ca from Ca-poor sources has made C. cajan

and S. aculeata well adapted to acid soils of Thailand as our

survey indicated.

Magnesium. In the unlimed treatment, C. cajan contained

0.47% Mg, S. aculeata 0.55%, S. rostrata 0.44%, and S.

speciosa 0.38%. Despite these rather high initial

concentrations, plant Mg did increase two to three fold as

soil pH increased (Table 7.4). Given the fact that the soils

were inherently high in Mg (Table 7.1) and that the adequate

Mg level was ~ 0.3% in both C. cajan (Reuter and Robinson,

1986) and S. sesban (Singh et al., 1992), this Mg increase was

due mainly to a more vigorous growth of the legumes at higher

pH and at better Ca supplies. Since Mg was not a growth

limiting factor, its uptake pattern among the four legume

species was not clearly different, except that Mg in the two

high-yielding species seems to increase steadily with liming

while Mg in the two low-yielding species seems to level off at

1.0% (S. rostrata) and 0.6% (S. speciosa).

Iron and Manganese. The four legumes responded differently

to the potentially excessive Fe in the acid sulfate soils

(Fig. 7.3). C. cajan accumulated only 236 mg Fe kg- 1 in the

unlimed soils and maintained its internal Fe between 151 and

81 mg kg-1 as lime quantities were added to raise the soil pH

to 4.5 and 6.5 (Table 7.4). According to Reuter and Robinson

(1986), Fe concentrations are considered adequate for C. cajan

... C. calan 136

600 .. S.aculeata<,

t»<, A-A S.rostrata

~ ** S.speciosa

"0)r- 400c:..CD

I.L.-cas 200a:

0-+-----..,------,-------,

300

200

..e~

C 100asc:

3.9 4.5 5.5 6.5

O-t-----..,------,- --,3.9 4.5

Soil pH

5.5 6.5

Figure 7.3. Plant Fe and Mn concentrations in the four green-

manure legumes as a function of soil pH. vertical bars are

standard errors.

137

growth at 150 - 190 mg Fe kg-1 , and would not be deficient

unless < 60 mg Fe kg-1• Thus, it seems logical to speculate

that C. cejen has an ability to regulate Fe uptake and to

prevent Fe from becoming detrimental to its metabolism when

grown on acid sulfate soils. By contrast, S. rostrata and

particularly S. speciosa, accumulated between 513 and 616 mg

Fe kg-1 when grown on the unlimed soils (Table 7.4). Whether

these high Fe levels were a result or a cause of poor growth

(low dry matter yield) could not be resolved in this

experiment because the data were taken only at harvest (after

7 weeks of growth). It is clear however, that the two high

yielding legumes (C. cajan and S. aculeata) always had lower

Fe concentration than the other two low-yielding legumes,

especially at the two lowest soil pH levels of 3.9 and 4.5

(Fig. 7.3).

Plant Mn shows a response pattern similar to plant Fe:

lowest in the high-yielding C. cajan and S. aculeata and

highest in the low yielding S. rostrata and S. speciosa (Table

7.4 and Fig. 7.3). This observation agrees well with that made

by Nakano et al. (1992) in explaining a high biomass

production of C. cajan as compared to s. rostrata and

Crotalaria juncea, when grown in acid red soil of southern

Japan. The differences in Mn concentrations in the four

legumes were however, smaller than those of Fe; and absolute

Mn concentrations were all below 275 mg kg- 1• Since Mn levels

in C. cajan would not be detrimental until they exceeded 300

138

Mg kg-1 (Reuter and Robinson, 1986), it is unlikely that Mn was

a cause of poor growth for any legume in this experiment.

Thus, the greater accumulation of Mn by the low-yielding

legumes than their high-yielding counterparts might have been

due to their genetic differences because even at pH 6.5 where

dry matter yields were relatively high, S. speciosa and S.

rostrata still contained 162 and 144 mg Mn kg-1 , respectively,

nearly twice the Mn concentration in c. cajan (88 mg kg- 1 ) and

S. aculeata (71 mg kg- 1 ) •

Aluminum. Given the high exchangeable Al in these acid

sulfate soils (Table 7.1), Al phytotoxicity was likely when no

lime was added. In fact, in the unlimed soils, all four

legumes contained 140 to 175 mg Al kg- 1 , which subsequently

declined exponentially as lime rates increased, a typical

response of a living organism to a toxicant (Table 7.4). At pH

6.5, plant Al was 37 mg kg-1 in S. speciosa and 75 mg kg- 1 in

c. cajan. Regarding the Al toxicity threshold, we found no

published data for the legumes used in this study. In a

published abstract, however, Licudine and Hue (1992) suggested

that Al levels ~ 40 mg kg-1 and ~ 85 mg kg-1 would reduce dry

matter yield of 6-week-old S. cochinchinensis by 10% and 50%,

respectively. Similarly, Hilyar (1979) related Al toxicity to

concentrations exceeding 40 mg Al kg- 1 in soybeans at 43 days

after planting. It is worth noting that these reported

critical Al levels were in the lower limit of Al concen

trations found in our experimental plants even at pH 6.5.

139

In our experiment, however, defining the critical Al

concentration is not simple because Ca was also affecting

growth. It has been well accepted that Ca strongly interacts

with Al in terms of ameliorating Al phytotoxicity (Alva et

al., 1986b). To deal with this Ca/AI interaction, we plotted

the relative yield of each legume as a function of its Ca/AI

ratio (Fig. 7.4). Such plots show that approximately 80% of

the variation in the dry matter yield could be attributed to

Ca/AI ratios in the plants. Also, relative yields would attain

~ 90% of the maximum if Ca/AI equals 150 for C. cajan and S.

speciosa, 200 for S. rostrata, and 300 for S. aculeata (Fig.

7.4). Based on Ca levels of 1.25% for C. cajan, 0.82% for S.

aculeata, 0.57% for S. rostrata, and 0.42% for S. speciosa,

that were considered minimum requirements for adequate growth

of these respective species (Fig. 7.2), we estimated that

critical Al concentration, above which significant yield

reductions would be expected, was 80 mg kg-1 for C. cajan, 40

mg kg-1 for S. rostrata, and 33 mg kg- 1 for both S. aculeata

and S. speciosa. Thus C. cajan apparently was the most Al

tolerant while the three Sesbania species had similar AI

toxicity but different Ca-deficiency tolerances.

7.5 Summary and conclusions

Increasing use of fast growing legumes as green manure for

improving soil productivity and crop production necessitates

an evaluation of their adaptability to acid soils, especially

140

~ 100~

100Ell eEll~

CIl 80 80CIl EllaI Ell Ell eE 60 fill 600 Sesbania rostrata Sesbania aculeata:0

40 40CD Ell> Y =-71.0 *e-0.01X + 97.5 Y =-83.7 *e-0.0055X + 104:0:: 20 20 EllaIQ) R2 =0.85 R2 =0.86II: 0 0

I I I I I I I I I I I I

0 100 200 300 400 500 0 200 400 600 800 1000

Ga/AI ratio Ga/AI ratio

~ 100 Ell 100 EllEll

~Ell

CIl 80 80en E!lD eaI (!)fl)

E (J) 6060 Ell (!)0 Sesbania speciosa:c 40 Cajanus cajanCD 40.;:: Y =-59.7 * e-0.017X + 81.3 20 Y =-67.9 *e-0.015X + 74.8Ciia; R2 =0.79 R2 =0.80II: 0 a

I i I I I I I I I I I I

0 100 200 300 400 500 0 200 400 600 800 1000

GaiAI ratio Ga/AI ratio

Figure 7.4. Relationship between relative biomass of the four

green manure legumes and the Ca/Al ratio in plant tops.

to acid sulfate soils which occupy a major portion of

Thailand's coastal areas. Based on green manuring criteria of

high biomass production and high N content, c. cajan and S.

aculeata were better suited to the acid soils than S. rostrata

and S. speciosa. Since dry-matter increases were most

pronounced in the pH range of 4.5 - 5.5, liming to pH - 5.5

141

was recommended for the growth of these legumes after taking

a cost/benefit analysis into consideration. (In many

developing countries where green manures are most needed, lime

may be expensive or not readily available because of poor

infrastructure. Thus a small yield increase after pH 5.5 might

not cover the lime cost.).

The legumes responded differently to stresses imposed by

soil acidity : C. cajan and S. aculeata were able to absorb

much more Ca but much less Fe and Mn than the other two

legumes. Calcium concentrations required for adequate growth

were estimated to be 1.2% for C. cajan, 0.8% for S. aculeata,

0.6% for S. rostrata and 0.4% for S. speciosa. Although the

four legumes had similar P and K uptake patterns, C. cajan

could tolerate nearly three times the levels of Al as the

Sesbania: critical Al concentrations (for 10% dry-matter

reduction) were 80 mg kg-1 for the former and 33 - 40 mg kg-1

for the latter.

142

References

Alva, A.K., D.G. Edwards, C.J. Asher, and F.P. C. 1986a.Relationships between root length of soybean seedlingsgrown in aluminum-toxic soils. Soil Sci. Soc. Am. J. 50: 656-661.

Alva, A. K., C. J. Asher, and D. G. Edwards. 1986b. The roleof calcium in alleviating aluminum toxicity. Aust. J.Agric. Res. 37: 375-382.

Barnhisel, R., and P.M. Bertsch. 1982. Aluminum. In A.L. Pageet al. (eds.) Methods of Soil Analysis, Part II, 2nd ed.,pp. 275-300. Agronomy. Monog. No.9. ASA-SSSA. Madison,WI.

Bray, R. H., and L. T. Kurtz .1945. Determination of totalorganic and available forms of phosphorus in soils. SoilSci. 59 : 39-45.

Chapman, H.D. 1965. Cation exchange capacity. In C.A. Black(ed.) Methods of Soil Analysis, Part II, pp. 891-901. Am.Soc. Agron., Inc. Pub., Madison, WI.

Evans, D. a., and P. P. Rotar. 1987. Sesbania in Agriculture.Westview Tropical Agriculture Series, No.8. WestviewPress, Boulder, CO. 189 p.

Evans, D. a., R. S. Yost, and G. W. Lundeen. 1983. A selectedand annotated bibliography of tropical green manures andlegume covers. Res. Ext. Series No. 028, College ofTropical Agricul ture and Human Resources, Univ. of Hawaii.211 p.

Freeney, J.R. 1986. Analytical methods for determining sulfurin soils and plants. In Sam Portch and Ghulam Hussain(eds.) Proc. Int. Symp. on Sulfur in Agric., pp. 67-84.Bangladesh Agricultural Council and Sulfur Institute.

Gee, G.W., and J.W. Bauder. 1982. Particle size analysis. InA.L. Page et al. (eds.) Methods of Soil Analysis, Part I,2nd ed., pp. 383-412. Agron. Monog. No.9. ASA-SSSA.Madison, WI.

Hilyar, K.R. 1979. Effects of aluminum and manganesetoxicities on legume growth. In C.S. Andrew and E.J.Kamprath (eds.) Mineral Nutrition of Legumes in Tropicaland Subtropical Soils. CSIRO, Australia.

Joshua, D.C., S.B. Anjali, J.D. Gadgil, and C.R. Bhatia. 1989.Sesbania rostrata Brem. A stem nodulating legume-itspotential as green manure crop. In Proc. Int. Symp.

143

Biological Nitrogen Fixation Associated with RiceProduction. CRRI. Cuttack, India.


Licudine, D. L., and N. V. Hue. 1992. Residual liming effectsof organic manures. Agron. Abstract, pp. 283.

Mappaona, M.K., and S. Yoshida. 1993. Biomass production andnitrogen fixation of tropical green manure legumes grownunder pot condition. J. Trop. Agr. 37 : 124-127.

Nakano, H., A. Sugimoto, H. Nakagawa, M. Matsuoka, T. Terauch,Y. Owaki, K. Shibano, and T. Momonoki. 1992. Evaluationof legume species for use as green manure crops in thesub-tropics in Japan. In Tropical Agr. Res. Center. Res.Highlights '92, pp. 23-26.

Parkpian, P., P. Pongsakul, and P. Sangtong. 1991.Characteristics of acid soils in Thailand: A review. InR.J. Wright (ed.) Proc. 2nd Int. Symp. on Plant-SoilInteractions at Low pH, pp. 397-405. Beckley, WestVirginia.

Reuter, D. J., and J. B. Robinson. 1986. Plant analysis: aninterpretation manual, pp. 62-63. Inkata Press,Melbourne, Australia.

Singh, Y., B. Singh, and C.S. Khind. 1992. Nutrienttransformation in soils amended with green manures. Adv.Soil Sci. 20 : 237-310.

Suthipradit, S. 1989. Effects of aluminum on growth andnodulation of some tropical crop legumes. Ph. D. Thesis,Univ. of Queensland, Brisbane, Australia.

Wallis, E.S., R.F. Woolcock, and D.E. Byth. 1988. Potentialfor pigeon pea in Thailand, Indonesia and Burma. CGPRTNo. 15. Bogor, Indonesia.

Yost, R.S., and D.O. Evans. 1987. Green manures and legumecovers in the tropics. Research Series 055. HawaiiInstitute of Tropical Agriculture and Human Resources,Univ. of Hawaii. 40 p.

xxxxxxxxx

144

CHAPTER 8

Ameliorating aluminum toxicity in upland rice grown on acid

sulfate soils, using green manures

8.1 Abstract

Green manuring reportedly can reduce Al toxicity in acid

soils. A greenhouse experiment was conducted to quantitatively

compare the effects of two green manures and lime on Al

detoxification in upland rice (Oryza sativa L. var. RD 15)

grown on two acid sulfate soils. The soils used were Bg (Typic

Sulfaquents) and Ra (Sulfic Tropaquepts) from the Bangkok

Plain with initial pH 3.8 and 4.0, respectively. Treatments

were 0, 4, 8 Mg CaC03 ha:", and 0, 20, 40 and 80 Mg ha! ground

tops of sesbania (Sesbania aculeata) or pigeon pea (Cajanus

cajan). Plant height, shoot and root dry weight, and relative

root length of rice plants at 60 days were measured. Soil

solutions were analyzed at harvest.

The results indicated that green manure applications

effectively detoxified AI. Pigeon pea was better than sesbania

in reducing AI+3 activities in the soil solution. Major

chemical processes were increased soil solution pH, EC, Ca and

Mg, followed by precipitation of soluble Al and/or the

formation of AI-organic complexes.

Rice plants were moderately tolerant of Al toxicity.

Nevertheless, Al+3, AI(S04)+ and Al sum activities in the soil

solution were strongly correlated with rice growth. Relative

145

root length predicted rice growth better than plant height,

dry weights of shoots and roots. Soil solution attributes

[ (AI+3, Al (S04) + and AI sum) ] were good indicators of Al toxicity.

Green manure amendments and CaC03 treatments were compared

by estimating the amounts of the materials required to

decrease Al +3 activity until relative root length of rice

plants was ~ 90%. The rates of CaC03 , pigeon pea and sesbania

applications required to reach this critical level were 5.3,

44.4, 57.5 Mg ha", respectively.

8.2 Introduction

Acid sulfate soils are developed from pyrite oxidation when

the soils are exposed to air after drainage. These soils are

characterized by very low pH and high Al concentrations, and

by yellowish jarositic mottles in the Band/or Ap horizons

(van Breemen, 1982). In the Bangkok Plain of Thailand, there

are approximately 900,000 ha of both actual and potential acid

sulfate soils, of which 80% are under rice cultivation

(Charoenchamratcheep et al., 1987).

Al toxicity is a major constraint to rice growth on acid

sulfate soils (Moore et al., 1990). Rice grows poorly in these

soils if the pyritic layer occurs within the top meter

(Satawathananont et a I . , 1991) . Strategies used for

ameliorating acid sulfate soils include liming, flooding, and

organic amendments. Liming to raise soil pH and to precipitate

Al has been a common practice to alleviate surface soil

146

acidity, but the movement of surface-applied lime (CaC03 ) down

the soil profile is extremely limited. Therefore, surface

liming has little effect on Al toxicity in the subsoils.

Furthermore, lime may not be available nor affordable by

subsistence farmers of the tropics. Flooding is not possible

for upland rice.

An alternative to lime for reducing Al toxicity is the use

of organic amendments. Additions of organic materials to acid

soils could prevent Al toxicity (Hoyt and Turner, 1975;

Hargrove, 1986). Evans and Kamprath (1970) found that as the

organic matter content of the soil increased, less Al +3 was

present in the soil solution at a given pH. It is believed

that Al ions are detoxified by interactions with organic

matter to form Al-organo complexes and chelates (Cabrera and

Talibudeen, 1977). Hue et al. (1986) and Suthipradit et al.

(1990) have shown that soluble organically complexed Al is

nonphytotoxic.

Additions of ground leaves of some legumes to acid soils

from Hawaii reduced Al toxicity and increased plant growth

(Hue and Amien, 1989), and such applications should be tested

on acid sulfate soils. Thus, the objectives of this study were

(i) to quantify changes in soil-solution compositions of acid

sulfate soils as affected by lime or green manure

applications, and (ii) to measure rice responses to these soil

amendments.

147


8.3.1 Soils and organic amendments

Soil samples were collected from cultivated paddy areas

representing two soil series: Bang Pakong (Bg) , and Rangsit

very acid phase (Ra). The Bg soil was a preoxidized potential

acid sulfate soil (Typic Sulfaquents); the Ra soil was an

actual acid sulfate soil (Sulfic Tropaquepts). The soils were

air-dried, and ground to pass a 2-mm sieve. Selected physical

and chemical properties of the soils are listed in Table 8.1.

Sesbania aculeata and pigeon pea (Cajanus cajan) were

selected as green manures for soil amendments because a

previous study has shown that these legumes adapted well to

acid sulfate soils (Chapter 7). Leaves and sterns were chopped

to approximately 1 ern, oven dried at 70°C, ground to < 1 mm,

mixed and stored at room temperature in air-tight containers

until use. Nutritional values (%) of the green manures are :

(Sesbania 3.20 N, 0.25 P, 2.12 K, 0.94 Ca and 0.93 Mg; pigeon

pea 3.22 N; 0.26 P, 2.13 K, 1.14 Ca and 1.01 Mg).

8.3.2 Plant growth study

The pot experiment had a factorial design with 3 repli

cations. Treatments included lime rates equivalent to 0, 4,

and 8 Mg CaC03 ha? and 4 rates of the green manures 0, 10, 20,

and 40 g kg- 1, which were equivalent to 0, 20, 40, and 80 Mg

ha" , respectively. Basal nutrients (in mg kg- 1 soil

Na2Mo04.2H20, 0.67; H3B03 , 0.83; CuS04.5H20, 5; ZnS04.7H20, 10;

148

Table 8.1. Some selected physical and chemical properties of the two unamended acid sulfate soils used

in the greenhouse experiment.

Soil properties

pH (1:1; soil: water)

EC (1:1; dS m")

Organic carbon (%)

Extractable P (mg kg") 1/

CEC (cmol, kg") 2J

Exchangeable K (cmol, kg:')

Exchangeable Ca (cmol, kg-I)


Exchangeable Na (cmol, kg")

Exchangeable acidity (cmol, kg")

Extractable Al (cmol,kg")

Al saturation, % of CEC

DTPA-Extractable Fe (mgkg:')

DTPA-Extractable Mn (mg kg:')

Water soluble S04·2 (mgS kg:')

Particle size distribution

Silt (%)

Clay (%)

1/ Bray P-II method

2/ 1 M NH40Ac, pH 7.0 method

Soil Series

Bang Pakong Rangsit

3.8 4.0

0.7 0.2

2.4 2.2

8.4 9.0

21.4 23.8

0.1 0.2

2.2 1.0

6.7 1.8

2.1 0.1

14.3 8.1

7.5 4.3

29.5 38.4

1440.9 711.1

36.8 22.6

502.4 134.4

40.8 35.6

56.8 61.7

MnS04.H20, 15i KH2P04 , 176) were applied to all pots. Nitrogen

was also applied at 24 mg N kg- 1 as NH4N03 • Each pot contained

2 kg of air-dried soils. The pots were placed randomly in the

greenhouse and incubated for 21 days at field water holding

capacity.

149

Upland rice var. RD 15 was planted at the rate of 6 seeds

per pot, and thinned to 3 seedlings 7 days later. Thereafter,

the pots were moistened to field water holding capacity daily.

The plants were allowed to grow for 60 days after germination.

Plant height and tiller shoot dry weight were determined.

Roots were carefully separated from the adhering soil. The

shoots and roots were oven dried for 48 h at 70°C, weighed,

ground and digested for chemical analysis. Total N was

analyzed by the micro Kjeldahl method. Total AI, Ca, K, Fe and

Mn in the digest were determined by atomic absorption

spectrophotometry (AAS). The data obtained were subjected to

analysis of variance, and the differences among means were

evaluated by the LSD (0.05) test.

8.3.3 Soil-solution collection and analysis

The air-dried soil samples from various treatments after

harvest were rewetted with distilled water to field capacity,

and equilibrated for 24 hr. The soil solution was extracted by

a centrifuge method (Menzies and Bell, 1988). Extracted soil

solution was filtered through a 0.45 ~m membrane, and pH and

electrical conductivity (EC) were measured immediately. The

rest of the soil solution was analyzed for Ca, Mg, K, Na,

S04-2, P and N03 - . Calcium, Mg, K and Na were measured with AA8.

Phosphorus and N03 - were determined colorimetrically using a

Technicon Auto-Analyzer II. 8°4-2 was measured

turbidimetrically at 420 nm (Freeney, 1986). Total dissolved

150

organic C in the soil solution was determined by the method of

Bartlett and Ross (1988). Total Al in the soil solution was

determined by the catechol violet method (Kerven et al.,

1989). Analytical data from the soil solution were used to

calculate ionic strength (I) and single-ion activities. The

calculation of ionic strength was based on EC of the soil

solutions (Griffin and Jurinak, 1973). The single-ion

activities were computed from the measured pH, EC and

concentrations of each element by the SOILSOLN computer

program (Wolt, 1987). Speciation of monomeric Al was

calculated by taking into account complex formation of Al with

S04-2 (all S measured in this experiment was regarded as

sulfate), OH-, and soluble carbon.


8.4.1 Effects of lime and green manure amendments on

chemical properties of the soil solution

pH and electrical conductivity. Although the initial pH

values of the soils were very low (3.8 - 4.0), significant

increases in pH and EC were resulted from the amendments

(Table 8.2). Soil solution pH increased to 5.2 in the Bg soil

and to 5.4 in the Ra soil when CaC03 was applied at 8 Mg ha",

The pigeon pea and sesbania additions at the highest rates of

80 Mg ha- 1 also increased soil pH to 5.0 and 5.6. Pigeon pea

seemed more effective (5 - 9 %) than Sesbania as green manures

in raising soil pH. A similar effect of organic materials on

increasing soil pH was noted by Ritchie and Dolling (1985) for

151

Table 8.2. Chemical properties of the soil solution as affected by various levels of CaCOJ (Ca), sesbania (Ses) and

pigeon pea (Pea)application to two acidsulfate soils.

Soils/ pH EC Ionic § K Ca Mg Na

Treatments (dS strength <---------------------------------- >m") (mM) (pM)

Bg

Ca 0 t 4.3 1.4 18.2 569 424 866 1320

Ca 4 4.9 1.7 22.1 1030 676 1276 1850

Ca 8 5.2 1.9 24.7 1210 907 1590 1913

Ses 20 * 4.5 1.5 19.5 742 632 888 1645

Ses 40 4.7 1.6 20.8 875 586 1255 1772

Ses 80 5.0 1.8 23.4 1042 1042 783 1468

Pea 20 * 4.9 1.8 23.4 999 735 1360 1770

Pea 40 4.9 1.7 22.1 1029 1029 1494 1851

Pea 80 5.4 2.1 27.3 1248 948 1757 1900

LSDo.os 0.1 0.1 1.8 77 79 103 93

Ra

CaO 4.5 1.6 20.8 841 808 1232 1607

Ca 4 5.0 1.9 24.7 1144 1135 1554 1838

Ca 8 5.4 2.2 28.6 1440 1386 1843 2208

Ses 20 4.7 1.7 22.1 908 887 1445 1717

Ses 40 4.8 1.8 23.4 1107 1109 1265 1827

Ses 80 5.3 2.2 28.6 1482 1259 1469 1834

Pea 20 5.0 1.9 24.7 1116 1106 1650 1925

Pea 40 5.2 2.0 26.0 1358 1278 1893 2178

Pea 80 5.6 2.4 31.2 1455 1509 2344 2441

LSDo.os 0.1 0.1 2.1 113 85 96 97

t Application ratesof CaC03 : 0, 4, 8 Mg ha:'

*Application ratesof sesbania and pigeon pea : 0, 20,40, 80 Mg ha'

§ I (mM) = 13*EC(dS m:')

152

lucern (Medicago sativa), and by Hue and Amien (1989) for

cowpea (Vigna unguiculata) , and leucaena (Leucaena

leucocephala) . Various reasons for such pH increases have been

proposed, e.g., production of OH- by (1) dissolution of solid

Fe oxides in reduced conditions or (2) ligand exchange

occurring through the replacement of terminal OH- of Al and

Fe-hydroxy oxides by organic anions (Hue and Amien, 1989).

soil-solution EC of the unamended soils was 1.4 - 1.6 dS

m", which increased to 1.7 - 2.2 dS mol in the lime treatments

and 1.5 - 2.4 dS m? in the green manure treatments (Table

8 .2). The increases in EC in the sesbania and pigeon pea

treatments were related to the substantial amounts of

nutrients in these materials. Higher EC in the pigeon pea

treatments than the sesbania treatments presumably resulted

from the higher Ca and Mg contents of the pigeon pea.

Basic cations. Soil-solution Ca and Mg concentrations

increased significantly with lime and green manure additions

(Table 8.2). Additions of 4 and 8 Mg ha- l of CaC03 increased

soil-solution Ca by 252 - 578 jJ.M, Mg by 322 - 724 jJ.M.

Additions of 20 to 80 Mg ha' green manure increased Ca by 22 -

644 jJ.M and Mg by 7 - 1204 jJ.M. The increases in K (54 - 601

jJ.M) and in Na (7 - 863 jJ.M) in the CaC03 treatments might have

resulted from Ca replacing K and Na on the exchange complex.

The changes in K concentration in the green manure treatments

were probably due to both the direct contribution of K by the

green manures and the exchange of K with added Ca, Mg on the

153

soil surfaces. Regardless of the reactions involved, liming or

green manuring improved the availability of Ca, Mg and K,

making the soils more favorable for plant growth.

Sulfate and phosphate concentrations. The concentration of

8°4-2 in the soil solution increased significantly with

increasing rates of CaC03 or green manures (Table 8.3). In the

CaC03 treatments (4 - 8 Mg ha- 1) , 8°4- 2 increased by 618 to 1527

JlM in both acid sulfate soils which had jarositic layers close

to the soil surface. The jarositic layer contained sulfur

bearing minerals, which dissolved as pH increased (De Coninck,

1978). This explanation also seems applicable to the green

manure treatments. A sulfate increase in the Ra soil from

approximately 2500 JlM to nearly 4500 JlM with the application

of 80 Mg ha? of pigeon pea was caused mainly by an increase

in soil-solution pH from 4.6 to 5.6 (Table 8.3). Although

these high 8°4-2 levels are not uncommon in acid sulfate soils,

they are about 10 times more than the level needed by many

crops for normal growth (Hue et al., 1984). The role of

excessive 8°4- 2 in P availability and Al toxicity warrants

closer investigation.

8oil-solution P increased from 1.5 JlM to 2.1 and 2.5 JlM in

the Bg soil, and from 2.6 JlM to 3.7 and 4.8 JlM in the Ra soil

as 4 and 8 Mg CaC03 ha' were applied, respectively. It is

reasonable to believe that liming reduced positive charge on

the soil particle surfaces as well as concentration of Al and

Fe in the soil solution (discussed later). Thus, lesser

154

Table 8.3. Chemical properties of the soil solution as affected by various levels of CaCO:J (Ca), sesbania

(Ses) and pigeonpea (Pea) application to two acid sulfate soils.

Soils! pH Total AI Fe Mn P S04·2

Treatments <---------------------------------------------------------------------------------- >pM

BgCa 0 t 4.3 345.6 55.9 23.2 1.5 1789Ca 4 4.9 99.0 28.2 13.4 2.1 2408Ca 8 5.2 85.8 18.8 10.3 2.5 2882

Ses 20 t 4.5 188.9 43.9 19.5 1.7 1917

Ses 40 4.7 129.2 32.1 15.4 1.9 2209Ses 80 5.0 108.4 24.9 11.2 2.2 2555

Pea 20 t 4.9 133.2 28.0 14.9 2.2 2413

Pea 40 4.9 104.3 25.1 12.2 2.2 2516Pea 80 5.4 75.4 17.2 8.6 2.8 3187

LSDo.()5 0.1 30.5 4.6 2.5 0.2 246

Ra

Ca 0 4.5 267.5 41.0 17.2 2.6 2526Ca 4 5.0 132.5 21.3 11.4 3.7 3355

Ca 8 5.4 76.2 15.7 9.2 4.8 4054

Ses 20 4.7 193.5 32.2 16.1 3.1 2897

Ses 40 4.8 164.5 24.4 11.9 3.4 3044

Ses 80 5.3 107.7 17.6 10.4 4.5 3891

Pea 20 5.0 144.3 22.4 10.5 3.6 3306

Pea 40 5.2 110.0 18.3 10.0 4.1 3669

Pea 80 5.6 78.7 14.1 7.9 5.4 4497

LSDo.()5 0.1 19.8 4.8 2.0 0.4 348

t Application rates of CaC03 : 0, 4, 8 Mg ha'

t Application rates of sesbania and pigeon pea: 0, 20, 40, 80 Mg ha"

155

amounts of the added P were adsorbed or precipitated and more

P remained in the soil solution.

Adding organic manures also increased P substantially

(Table 8.3). Pigeon pea was the most effective, and sesbania

the least among the three amendments in increasing soil

solution P. For example, the 80 Mg ha? treatment of the pigeon

pea doubled P concentration from 2.6 ~M to 5.4 ~M in the Ra

soil. Besides raising soil pH as with liming, organic manures

could also increase solution P via their direct P

contributions and via producing organic molecules that can

compete effectively with P for sorption sites on the soil

particle surfaces (Hue, 1991; 1992).

Since rice is known to adapt well to relatively P-depleted

soils (R.L. Fox, personal communication), and given the

external P requirements of related plant species (e.g., corn

and sorghum) as 1.6 - 1.9 ~M for 95 % maximum yields (Fox,

1979), P did not appear to limit rice growth in this

experiment, especially in those treatments receiving lime or

green manures.

Total Al concentrations in soil solution. There was a

decrease in total Al concentration from 345. 6 ~M to 65. 8 ~M in

the Bg soil, and from 267.5 ~M to 76.2 ~M in the Ra soil with

an increase in soil solution pH (4.3 - 5.4) by the CaC03

treatments. An exponential equation of Y = a*explbX) was fitted

to the regression of total Al concentration and pH for both

soils. In general, Al concentration in the soil solution was

156

negatively correlated with pH (r2 = 0.89**). When pH was raised

to 5.2 - 5.4 after applications of 8 Mg CaC03 ha? to both acid

sulfate soils, the total Al concentration was reduced to 65.8

- 76.2 ~M, a level considered as nontoxic to a moderately acid

tolerant plant such as upland rice (Table 8.3). Liming,

therefore, is an important agronomic practice for crop

production on acid sulfate soils, although it may be costly.

Liming has been shown experimentally to be successful with

rice, corn, and tropical fruit trees in the Bangkok Plain

(Muensangk, 1991).

The pigeon pea treatments decreased total Al concentrations

similarly to the CaC03 treatments (Table 8.3). On the other

hand, the sesbania treatments resulted in higher total Al

concentrations than the pigeon pea treatments at the same

application rate. Nevertheless, application of sesbania at 80

Mg ha-1 significantly reduced total Al concentration in the

soil solution.

Al speciation and activities in soil solution. Table 8.4

lists the activities of various Al species, Mn, and 804 as

calculated by the 80IL80LN computer program (Wolt, 1987).

Aluminum species existed principally in the soil solution as

Al+3 and as complexes with 8°4- 2 and OW. Addition of various

treatments decreased Al activities in a similar manner to that

observed with total Al concentrations, although the activities

were approximately 14.4% of the total Al concentration on

average. The decrease in AI+3 activity in both acid sulfate

157

Table 8.4. Calculated activities of monomericAI species and sulfate in the soil solutionas affected by various levels

of CaC03 (Ca), sesbania (Ses) and pigeon pea (pea) application to two acid sulfate soils.

Soils! pH AI+3 AI(OHl+2 AI(OH)2+ AI(OH>,D AI(S04l.:....A!.u~ Mn S04·2

Treatments (p.M) (p.M) (p.M)

Bg

Ca 0 t 4.3 55.9 13.7 18.5 0.9 91.0 180.1 15.8 1621

Ca 4 4.9 7.1 6.3 30.7 5.8 14.8 64.8 8.9 1995

Ca8 5.2 1.4 2.8 31.4 13.3 3.4 52.5 6.6 2238

Ses 20 * 4.5 26.1 9.5 19.2 1.5 44.4 100.9 13.1 1778

Ses 40 4.7 12.9 8.0 27.6 3.6 24.9 77.3 10.2 1905

Ses 80 5.0 5.9 6.6 40.6 9.6 12.9 75.8 7.2 2137

Pea 20 * 4.9 9.3 8.4 42.2 8.1 19.3 87.6 9.7 2041

Pea 40 4.9 6.1 6.4 37.9 8.6 13.2 72.3 7.9 2137

Pea 80 5.4 0.6 2.1 35.9 23.8 1.7 64.4 5.4 2344

Ra

CaO 4.5 32.9 12.9 27.9 2.3 72.9 149.1 11.2 2041

Ca4 5.0 5.9 7.4 51.3 13.8 16.6 95.1 7.2 2290

Ca 8 5.4 0.7 2.2 36.0 22.9 2.4 64.3 5.7 2511

Ses 20 4.7 17.8 11.1 38.4 5.1 44.3 117.0 10.2 2137

Ses 40 4.8 12.0 9.9 45.2 7.9 31.1 106.3 7.5 2187

Ses 80 5.3 1.6 3.8 51.2 26.4 5.0 88.2 6.6 2290

Pea 20 5.0 7.5 8.3 51.6 12.3 20.8 100.7 6.6 2344

Pea 40 5.2 2.6 4.9 50.5 20.1 7.9 86.2 6.1 2511

Pea 80 5.6 0.2 1.2 33.4 38.3 0.9 69.8 4.6 2754


*Application rates of sesbania and pigeon pea: 0, 20, 40, 80 Mg ha:

§ AI"m = Sum of all monomeric AI species

158

soils was significantly related to soil solution pH (Fig.

8.1). Aluminum activities vs. pH in all treatments, can be

described by the relationship: (AI in p.M) = 1.1 (10 8 ) *exp(-3.34*Soil

solution pH) (r2 = 0.98**) . The reduction in AI+3 activities by plant

material application has been reported by Bell and Edwards

(1987). Strong relationships between Al activity and soil

solution pH for the green manure and CaC03 treatments in the

present experiment indicate that the major effect of the green

manures on Al may have been through pH increase and Al

precipitation. The pH increase could also make native organic

matter to complex Al more strongly.

The relative proportions of Al species with Al sumactivities

were also related to soil solution pH (Table 8.4). In the

unamended soils, AI+3 was 22 - 31% of total Al activity, only

surpassed by Al (S04) + (48.9 - 50.5%) . When soil solution pH was

raised to about 4.9 by either 4 Mg CaC03 ha? or 40 Mg sesbania

ha" or 20 Mg pigeon pea ha", Al +3 decreased to less than 10.6

- 11.3% of the AI~m activities.

Basic cation activities in soil solution. In the unamended

treatments, the activities of K were 512 - 741 p.M and Na 1202

- 1445 p.M, which were rather high. Applications of CaC03 or

green manures released even more of the cations into the soil

solution (Table 8.5). The applications of CaC03 at 4 -8 Mg ha"

markedly raised Ca activities from 158 p.M to 326 p.M, and also

increased the other cation activities. When the soils were

159

Y" =1.1*1 0" 8exp(-3.34Soil pH)r"2 = 0.98**

...."" 8g soil

+Rasoil

+

""

+

"" +A... ..t+

+... + - -'-

60

555045-

~ 402>- 35+-'os;U 30rn 25C'?+<t 20

15

10

5

o4.25 4.5 4.75 5 5.25

pH in soil solution5.5 5.75

Figure 8.1. Relationship between AI+3 activity and soil

solution pH as affected by all treatments for two acid sulfate

soils.

limed with 8 Mg caco, ha", increases in the activities of Ca+2

and 8°4-2 may result in a precipitation of gypsum (caso., 2H

20).

The formation of gypsum in acid sulfate soil via this

mechanism had been reported (De Coninck, 1978).

In the case of pigeon pea treatments, Mg activity increased

in the range of 309 ~M - 686 ~M with increasing application

rates from 20 to 80 Mg ha", In the sesbania treatments of 20 _

80 Mg ha! , large increases in activities of K (54 - 559 ~M)

160

Table 8.5. Calculated activities of basic cations in the soil solution as affected by various levels of CaC0:J

(Ca), sesbania (Ses) and pigeon pea (Pea) application to two acid sulfate soils.

Soils/ pH Ca+2 Mg+2 K+ Na+

Treatments (pM)

Bg

Ca 0 t 4.3 288 602 512 1202

Ca4 4.9 446 851 912 1659

Ca 8 5.2 575 1047 1071 1698

Ses 20 ~ 4.5 426 602 660 1479

Ses 40 4.7 389 831 776 1584

Ses 80 5.0 501 977 933 1659

Pea 20 ~ 4.9 478 912 891 1584

Pea 40 4.9 524 977 912 1659

Pea 80 5.4 602 1148 1096 1698

Ra

CaO 4.5 524 812 741 1445

Ca4 5.0 707 1000 1000 1621

Ca 8 5.4 851 1174 1258 1949

Ses 20 4.7 562 954 812 1513

Ses 40 4.8 707 831 977 1621

Ses 80 5.3 794 954 1318 1621

Pea 20 5.0 691 1071 977 1698

Pea 40 5.2 776 1202 1174 1905

Pea 80 5.6 933 1445 1258 2137


~ Application rates of sesbania and pigeon pea: 0, 20, 40, 80 Mg ha'

161

and Na (101 - 400 ~M) were observed.

8.4.2 Effects of CaC03 and green manure amendments on rice

growth

Plant height, dry weights, Al and Fe concentrations of

shoots and roots, and relative root length at the time of

harvest were used to measure rice response to the treatments

(Table 8.6). Regression analysis of these growth parameters

with activity of ions in the soil solution and plant nutrient

compositions was performed (Table 8.7).

Plant height. The height of 60 -d-old rice plants was

drastically increased when either CaC03 or green manure

treatments was applied (Table 8.6). In the unamended soils,

the plant height were 73.0 - 74.4 em and plant tops were

stunted similar to those described by Foy (1974). In contrast,

plant height was substantially increased to 77.6 - 85.7 cm by

the addition of sesbania or pigeon pea at 20 Mg ha".

Applications of 40 or 80 Mg ha' of the green manures appeared

to be sufficient to suppress the harmful effects of AI, and

produced the tallest plants (88.2 - 91.7 cm).

Dry weights of shoots and roots. Shoot dry weight increased

with increasing rates of added CaC03 and green manures (Table

8. 6). However, lime applications of 8 Mg CaC03 ha' yielded

lower dry matter (3.3 - 4.5 g pot-1) than did the addition of

80 Mg ha- 1 pigeon pea. It is worth noting that the maximum

yield (3.4 g pot- 1) was obtained at pH 5.4 in the Bg soil and

at pH 5.6 in the Ra soil (4.8 g pot- 1) . It is likely that the

162

Table 8.6. Plant height, dry weights, Aland Fe concentrations in shootsandroots, and relative rootlength

of rice plants as affected by various levels of CaC03 (Ca), sesbania (Ses) andpigeon pea (Pea) application

to two acid sulfate soils.

Soils/ Plant Drv weight Al cone. Fe cone. Relative root

Treatments height shoots roots shoots roots shoots roots length §

(em) <--- g pori ---> <--- mg kg:' ---> <--- mg kg:' ---> (%)

Bg

Ca 0 t 73.0 2.09 0.41 749 5271 754 4459 69

Ca4 81.5 2.82 0.50 614 4272 654 3954 84

Ca 8 91.0 3.39 0.76 525 3542 592 3188 90

Ses 20 t 78.6 2.55 0.46 695 4975 722 4608 62

Ses 40 84.4 2.75 0.57 652 4448 666 4146 92

Ses 80 90.8 3.01 0.60 576 4056 634 3700 89

Pea 20 t 81.1 2.86 0.60 613 4131 655 3922 87

Pea 40 91.0 3.28 0.74 590 4068 635 3692 92

Pea 80 88.8 3.49 0.80 481 3218 566 2923 95

LSDo.05 4.5 0.27 0.10 35 351 31 309 6

Ra

Ca 0 74.4 3.04 0.71 473 4237 514 3994 76

Ca4 84.1 3.87 1.00 387 3401 448 3183 85

Ca 8 90.5 4.50 1.41 337 2752 389 2509 90

Ses 20 77.6 3.33 0.83 431 3871 491 3606 81

Ses 40 89.5 3.64 1.07 416 3580 466 3415 86

Ses 80 88.2 4.58 1.21 354 3013 413 2627 91

Pea 20 85.7 4.22 1.12 396 3412 450 3234 84

Pea 40 91.7 4.59 1.30 364 3114 416 2846 90

Pea 80 88.8 4.87 1.49 304 2506 357 2249 93

LSDo.05 4.7 0.32 0.13 26 298 29 256 5

t Application rates of CaC03 : 0, 4, 8 Mg ha- l

t Applicationrates of sesbania and pigeon pea: 0, 20,40,80 Mg ha"

§ Data are average of three replications; the individual longest length was assigned to 100 %

163

Table 8.7. Relationship between growth parameters of riceplantsand various Al toxicity indices derived

from soil solution attributes in two acid sulfate soils (all treatments combined).

Parameter and indices t Regression equation ~

ReI. dry weight of shoots vs. (Al+3) Y = 74.4exp(.lJ·OIX)

ReI. dry weight of shoots vs. (AISO/) Y = 76.3exp(.lJ·0059X)

ReI. dry weight of shoots vs. (SO/) Y = 0.8 + 0.03X

ReI. plant height vs. (Al+3) Y = 89.2exp(.lJ·0043X)

ReI. plant height vs. (S04,2) Y = 51.2 + O.OIX

ReI. plant height vs. (AISO/) Y = 90.1exp(.lJ·OO2SX)

ReI. root length vs. (S04,2) Y = 37.8 + 0.02X

ReI. root length vs. (Al+3) Y =91.5 - 0.49X

ReI. root length vs. (AlS04+) Y =92.4 - 0.27X

ReI. root length vs. (Alsum) Y = 103.5 - 0.19X

t ( ) = activity in pM

~ Y = Growth parameters; X = index

§ •• Significant at p < 0.01

r § Al 90% max.

root length

(p.M)

0.82"

0.89··

0.67··

s.n:0.48··

0.76··

0.47··

0.67·· 3.0

0.64·· 8.6

0.50·· 70.0

most favorable growth conditions had not been attained in

either soil by adding 8 Mg ha? CaC03 or 80 Mg ha! green

manures.

CaC03 and green manure treatments had similar but less

pronounced effects on roots than shoots (Table 8.6).

Generally, additions of sesbania or pigeon pea green manure

appeared to reduce Al toxicity in these acid sulfate soils. As

mentioned previously, green manure treatments decreased

164

soluble Al with respect to the unamended treatments,

suggesting Al precipitation/adsorption. Furthermore, shoot dry

weights in the green manure treatments at 80 Mg ha" were

either similar to or higher than those of CaC0 3 treatments (8

Mg ha") in both soils. It is possible that besides the pH

effect on soluble AI, the forms of soluble Al and their

corresponding phytotoxicity might be different among the

treatments. In fact, many studies have demonstrated that low

molecular weight organic acids obtained from decomposition of

organic materials can form non-toxic Al species by

complexation (Bartlett and Riego, 1972; Hue et al., 1986).

Aluminum concentration in shoots and roots. Concentration

of shoot Al was significantly affected by the treatments

(Table 8.6). In general, CaC03 application at 4 Mg ha!

reduced shoot Al to 387.0 mg kg- 1 in the Ra soil and 614.5 mg

kg-1 in the Bg soil. The higher rate of 8 Mg ha? further

reduced shoot Al to 337.9 and 525.3 mg kg- 1 in the Ra and Bg

soils, respectively. The green manure treatments of 80 Mg ha-1

also decreased shoot Al to 304. 6 mg kg- 1•

Like shoot AI, root Al also decreased with CaC03 and green

manure applications. Note that root Al was always 6 to 11

times higher than that in shoots. This is because Al is known

to accumulate in roots of AI-tolerate plants, such as upland

rice and cassava (Fageria et al., 1991).

Relative root length (RRL). Relative root length of rice

plants after 60-day growth was increased with CaC03 or green

165

manure applications (Fig. 8.2). The relative root length was

calculated as the percentage of the individual longest root in

each soil.

a). b).

..... 100 ..... 100

C 90 C 90

.c 80 .c 80til 70

iil 70ce oS! 60.! 600 50 '0 50

2 0 4040 ..Gl 30

Gl 30~ ~

iii 20 iii 20

Gi 10 Qi 10a: a: 00

0 4 8 0 20 40 80

CaC03 (Mg/ha) Sellbanla (Mg/ha)

c).

..... 100C 90.c 80~ 70.! 60o 502 40~ 30iii 20Gi 10a: 0

~

m <~:< :::",' 1-

\\~\1':..

~~~:~:~; ;:: I-

I- ::: I-

III

"v-, .'1- H ':.. -

f-- :. :;:;:; t -"

\\jjj\- '..; ; -

'. ;:; -- '.- :. :' ,'.~, f--'. ,;

=:::;~:. ,".'

o 20 40 80

Pigeon pea (Mg/ha)

Figure 8.2. Effects of CaC03 (a), sesbania (b) and pigeon pea

(c) application on relative root length of rice plants at 60

days after planting.

The 4 Mg ha' CaC03 treatment resulted in similar values of

RRL (84 and 85%) for both soils, which wer-e significantly

lower than the RRL (90%) at 8 Mg ha:' CaC03 but much higher than

those of the unlimed control (Fig. 8.2). The results clearly

166

clearly demonstrated the beneficial effects of liming to rice

growth on acid sulfate soils.

The sesbania treatment at 20 Mg ha? had no significant

effects on RRL, especially in the Bg soil. Higher rates

increased RRL, however. In contrast, the RRL in both soils

increased with increasing pigeon pea additions up to 40 Mg

ha- 1, beyond that rate, RRL leveled off (Table 8.6).

8.4.3 Relationship between rice growth and soil-solution

composition

Table 8.7 summarizes the correlations of plant growth with

various soil solution parameters. The relationships between

the growth parameters and the various indices of soluble Al

are shown in Fig 8.3; all treatments in both soils were

combined. The relation in Fig. 8.3a, indicates that relative

plant height is positively correlated with soil solution pH

(r2 = 0.63**), suggesting that rice plants were affected by

soil acidity. A 10% reduction in relative plant height

occurred at pH 4.8, implying that rice plants are moderately

tolerant of acidity as previously noted by Blarney et al.

(1987). Relative plant height was also negatively correlated

with Al sum activity (r2 = 0.60**; Fig. 8.3b).

Strong relationships were obtained between relative root

length and Al +3, Al (S04) + activities in a similar experiment

reported by Bruce et al. (1988). The AI+3 activity

corresponding to 80% RRL was 20 JlM. At 40 Jll'1 AI+3 activity, the

RRL was reduced to 62% (Fig. 8.3c). An Al (S04)+ activity

167

a). V" =14.9 + 14Xr A 2 = 0.63**

.- .... . ..... . .. .. ..

b). V"'" =98.6· O.14Xr A 2 = 0.60**

l100

:E 95Cl

] 90

~ 85Ei. BOII)

~ 75iiiGi 70a: 50 70 90 110 130 150 170 190

Allium activities CuM)

'" "X ~

x '" '"x

x..x

'"x

l100

:E 95Cl

] 90

E 85III

Ei. 80II)>~ 75

~ 7~.2 4.4 4.6 4.8 5 5.2 5.4 5.6 5.8pH in soil solution

c). V"'" = 91.5· O.49Xr " 2 = 0.67*·

lr..- ---- - -

d). V"'"= 92.4 • O.27Xr A 2 =0.64**

-........ ........ ....

........

-

l100.l: 95'6lc 90.! 85so BO..~ 75iii 70Gi 65a: 0 10 20 30 40 50 60 70 80 90 100

AIS04+ activities (uM)

l100

.l: 95

~ 90

.! 85o2 BO~ 75iii 70Gia: 65 0 5 101520253035 40 45 50 55 60

AI+3 activities (uM)

Figure 8.3. Relationship between relative growth and soil

solution composition of two acid sulfate soils.

of 30 J.1.M reduced the RRL of rice plants by 20%. Perhaps,

Al (8°4 ) + is less toxic than Al +3 as suggested by Cameron et ale

(1986). Thus, both Al +3 and Al (8°4 ) + activities seem to be good

indicators of Al toxicity for rice plants grown in acid

sulfate soils. The critical value of 3.0 J.1.M of AI+3 activity

(Table 8.7) was in the same order of magnitude as 4 J.1.M for

upland rice root growth reported by Fageria et ale (1987) .

Relative root length and relative dry weight of shoots were

negatively correlated with Alsum activity. When the Al sum

activity was ~ 76.9 J.1.M, the relative root length was ~ 90%.

168

Thus AI.urn can also be used as an index of Al toxicity .

However I the AI.urn index provided a lower r 2 value than that of

Al +3 activity (Table 8.7).

There was a strong correlation between RRL and Al and Fe in

roots (Fig. 8.4). A 10% reduction in RRL occurred when root Al

was 0.32% (Fig. 8.4a) or when root Fe was 0.29% (Fig. 8.4b).

a). Y A =117.6 - O.008Xr A 2 =0.51**

... .. ..~ .. .... ..

...

7: 100;; 95'5J 90e.! 85'0 80E 75

~ 70i 65Qj

a: 6~500 3000 3500 4000 4500 5000 5500

AI concentration in roots (mg/kg)

b). Y A =117.3 • O.009Xr A 2 = 0.49**

+ + +

+ + +

+

+

+

7: 100;; 95

~ 90.! 85'0 80!:! 75

! 70iii 65Qj

a: 6~200 2700 3200 3700 4200 4700

Fe concentration in roots (mg/kg)

Figure 8.4. Relationship between relative root length and Al

or Fe concentration in rice roots.

Nutrient concentrations in rice plants were plotted against

corresponding ionic activities in the soil solutions. Shoot K,

Ca, Mg and S increased linearly with their corresponding

169

nutrients in the soil solution (Fig. 8.5a-d). All nutrients

were very high in the soils after they were amended with CaC03

or green manures. Therefore, these nutrients were regarded as

sufficient for rice growth.

..••-.

••• •

•.

b). V"" = -8.3 + 6.9Xr" 2 = 0.90**

Ci 6200~ 5700Cl.s 5200fIl 4700

'0 4200

,g 3700

~ 3200

ClI 2700

U 220~75 475 675 875 1075

Ca activities in soil solution (uM)

•x x x

xX

II

Xx X

X X

X XX

11.50

16.00

14.50

13.00

10.00500 800 1100 1400

K activities in soil solution (uM)

a). V"" = 8856.21 + 5.19Xr " 2 = 0.66**

17.50Ci~--Cl '0.§. "tl

CUl ca(5 III

::l0 0.c: ..cUl '=-.Elll::

c). V"" = 1413.5 + 3.9Xr" 2 =0.84**

+.+ .+

!--1'-.~'"

+

d). V,... = 998.5 + 1.5Xr"2 =o.n**

... ...... ............

...

... ......... .. ...

......

'"

_ 5400Cl~--~ 4950-III'0 4500oJ:III 4050

.=(/) 3600

1600 1900 2200 2500 2800

504 activities in soil solution (uM)

Ci 7100

='! 6600Cl.§. 6100

Ul 5600'5o 5100.c:Ul 4600.ECl 4100

:E 3600600 850 1100 1350 1600Mg activities in soil solution (uM)

Figure 8.5. Relationship between K, Ca, Mg and s

concentrations in rice shoots and their respective activities

in soil solution.

170


Acid sulfate soils in the Bangkok Plain of Thailand can be

amended with either lime or green manures to grow upland rice.

Sesbania and pigeon pea green manures at 40 Mg ha " were quite

effective in Al detoxification of acid sulfate soils.

Decreases in soluble Al as a result of CaC03 or green manure

amendments were probably responsible for better rice growth.

The reduction in soluble Al was more pronounced in the pigeon

pea treatments than in the sesbania treatments.

Plant height, shoot and root dry weight, and relative root

length were good growth parameters in acid sulfate soils. Soil

solution pH, and activities of Al +3, Al (S04) + and Al sum are among

the soil-solution parameters which can be used to predict Al

toxicity. Rice plants are also moderately tolerant to Al

toxicity and grow well at pH ~ 4.8, with AI+3 and Alsum

activities s 3.0 and s 70.0 ~M, respectively.

A 90% maximum RRL is attainable when soil-solution Al+3

activity s 10 ~M , which is achievable by applying 44.4 Mg ha"

of pigeon pea or 57.5 Mg ha' of sesbania, or 5.3 Mg ha? of

CaC03.

171

References

Bartlett, R.J., and D.C. Riego. 1972. Effect of chelation onthe toxicity of aluminum. Plant Soil 37 419-423.

Bartlett, R.J., and D.S. Ross. 1988. Colorimetricdetermination of oxidizable carbon in acid soil solution.Soil Sci. Soc. Am. J. 52 : 1191-1192.

Bell, L.C., and D.G. Edwards. 1987. The role of aluminum inacid soil infertility. In M. Latham (ed.) Soil Managementunder Humid Conditions in Asia (ASIALAND) , pp. 201-223.IBSRAM, Bangkok.

Blarney, F.P.C., C.J. Asher, and D.G. Edwards. 1987. Hydrogenand aluminum tolerance. Plant Soil 99 : 31-37.

Bruce, R.C., L.A. Warrell, D.G. Edwards, and L.C. Bell. 1988.Effects of aluminum and calcium in the soil solution ofacid soils on root elongation of Glycine max cv. Forrest.Aust. J. Agric. Res. 38 : 319-338.

Cabrera, F., and O. Talibudeen. 1977. Effect of soil pH andorganic matter on labile Al in soils under permanentgrass. J. Soil Sci. 28 : 259-270.

Cameron, R.S., G.S.P. Ritchie, and A.D. Robson. 1986. Relativetoxicities of inorganic aluminum complexes to Barley.Soil Sci. Soc. Am. J. 50(5) : 1231-1235.

Charoenchamratcheep, C., C.J. Smith, S. Satawathananont, andW.H. Patrick Jr. 1987. Reduction and oxidation of acidsulfate soils of Thailand. Soil Sci. Soc. Am. Proc. 51 :630-634.

De Coninck, F. 1978. Physico-chemical aspects of pedogenesis.ITC, University of Ghent, Belgium. pp. 154.

Evans, C.E., and E.J. Kamprath. 1970. Lime response as relatedto percent Al saturation, solution AI, and organic mattercontent. Soil Sci. Soc. Am. Proc. 34 : 893-896.

Fageria, N.K., V.C. Baligar, and C.A. Jones. 1991. Growth andmineral nutrition of field crops. Marcel Dekker, Inc. NewYork. 476 p.

Freeney, J.R. 1986. Analytical methods for determining sulfurin soils and plants. In Sam Portch and Ghulam Hussain(eds.) Proc. Int. Symp. on Sulfur in Agric., pp. 67-84.Bangladesh Agricultural Council and Sulfur Institute.

172

Fox, R.L. 1979. Comparative responses of field grown crops tophosphate concentrations in soil solution. In H. Musselland R. C. Staples (eds.) Stress Physiology in Crop Plants,pp. 81-106.

Foy, C.D. 1974. Effects of aluminum on plant growth. In E.W.Carson (ed.) The Plant Root at Environment, pp. 601-642.Univ. Press of Virginia, Charlottesville.

Griffin, R.A., and J.J. Jurinak. 1973. Estimation of activitycoefficients from the electrical conductivity of naturalaquatic systems and soil extracts. Soil Sci. 116 : 26-30.

Hargrove, W.L. 1986. The solubility of aluminum-organic matterand its implication in plant uptake of aluminum. SoilSci. 142 : 179-181.

Hoyt, P.B., and R.C. Turner. 1975. Effects of organicmaterials added to very acid soils on pH, aluminum,exchangeable NH4+, and crop yields. Soil Sci. 119 : 227237

Hue, N.V., F. Adams, and C.E. Evans. 1984. Plant-availablesulfur as measured by soil-solution sulfate andphosphate-extractable sulfate in an Ultisol. Agron. J. 76: 726-730.



Hue, N.V. 1991. Effects of organic acids/anion on P sorptionand phytoavailability in soils with differentmineralogies. Soil Sci. 152 : 463-471.

Hue, N.V. 1992. Correcting soil acidity of a highly weatheredUltisol with chicken manure and sewage sludge. Commun.Soil Sci. Plant Anal. 23 : 241-264.

Kerven, G.L., D.G. Edwards, C.J. Asher, P.S. Hallman, and S.Kokot. 1989. Aluminum determination in soil solution. II.Short-term colorimetric procedures for the measurement ofinorganic aluminum in the presence of organic acidligands. Aust. J. Soil Res. 27 : 91-102.

Menzies, N.W., and L.C. bell. 1988. Evaluation of theinfluence of sample preparation and extraction technique

173

on soil solution composition. Aust. J. Soil Res. 26 :451-464.

Moore, P.A. Jr., T. Attanandana, and W.H. Patrick Jr. 1990.Factors affecting rice growth on acid sulfate soils. SoilSci. Soc. Am. J. 54 : 1651-1656.

Muensangk, S. 1991. Interaction between the application ofmarl and rock phosphate upon nutrient availability andyield of rice grown on three soil suitability classes ofacid sulfate soils. Ph. D. Thesis. Kasetsart Univ.,Bangkok. (in Thai) .

Ritchie, G.S.P., and P.J. Dolling. 1985. The role of organicmatter in soil acidification. Aust. J. Soil Res. 23569-576.

Satawathananont, S., W.H. Patrick Jr., and P.A. Moore Jr.1991. Effect of controlled redox conditions on metalsolubility in acid sulfate soils. Plant Soil 133 : 281290.

Suthipradit, S., D.G. Edwards, and C.J. Asher. 1990. Effectsof aluminum on tap-root elongation of soybean (Glycinemax), cowpea (Vigna unguiculata) and green gram (Vignaradiata) grown in the presence of organic acids. PlantSoil 124 : 233-237.

van Breemen, N. 1982. Genesis, morphology and classificationof acid sulfate soils in coastal plains. In J.A. Kittricket al. (eds.) Acid Sulfate Weathering, pp. 95-108. SSSASpecial Publ. No. 10, Madison, WI.

Wolt, J. 1987. Soil solution. Documentation, source code, andprogram key. Version 1.4 ed. Univ. of Tenn. Agric. Exp.Stn. Res. Rep. 87-19.

xxxxxxxxx

174

CHAPTER 9

Mineralogical and chemical properties of two acid sulfate

soils as affected by lime and green manure application

9.1 Abstract

An incubation study was conducted to quantify the effects

of lime or green manure on chemical and mineralogical

properties of two acid sulfate soils from Thailand. The soils

were a potential acid sulfate soil (Bg series; Typic

Sulfaquents) and an actual acid sulfate soil (Ra series;

Sulfic Tropaquepts); lime rate was 6 Mg ha? CaC03 ; and

sesbania green manure, 40 Mg ha-1. The temporal changes in

chemical and mineralogical compositions of the soils were

determined at 2, 14, 42 and 90 days after incubation. In the

unamended soils, strong and rapid acidification occurred,

resulting in pH decreases to 2.8 - 3.2. By contrast, liming

and green manuring increased soil pH, EC, total exchangeable

bases, and reduced Al saturation percentage. The reduction in

Al was assumed to be the precipitation of soluble Al from pH

increases resulted from lime or green manure addition.

X-ray diffraction analysis, on the other hand, showed no

detectable change in clay minerals by either amendment, in

spite of strong acidification occurring in the control.

Detailed mineralogical study of handpicked yellow particles

from soils 90 days after incubation showed the presence of

175

jarosite. Soil solution analysis suggested that jarosite might

be formed by precipitation of K+, Fe+3 and S04-2.

Based on the ion activity product (pIAP) and stability

diagram, AI-hydroxy sulfate minerals existed in the acid

sulfate soils. The AI+3 activities were pH dependent and

apparently controlled by the solubility of an alunite-like

mineral having a pKsp of 81.4.

9.2 Introduction

Acid sulfate soils are developed from the oxidation of

pyrite (FeS2 ) on exposure to the atmosphere due to drainage or

crop cultivation. When the water table drops, oxygen enters

the soil system and oxidizes pyrite by the following process

(van Breemen, 1976):

2Fe82 + 2H20 + 702 ---> 2Fe+ 2 + 4W + 48°4-2 [1]

4Fe+2 + 10H20 + O2 ---> 4Fe (OH) 3 + SH+ [2]

The initial products are ferrous, sulfate and hydrogen ions;

further hydrogen ions are released by the hydrolysis of

ferrous iron and the precipitation of ferric oxide. Under acid

conditions, ferric ions can also oxidize pyrite :

Fe82 + 14Fe+3 + 8H20 ---> lSFe+2 + 2S04 - 2 + 16H+ [3]

The oxidation of ferrous to ferric is catalyzed by the

bacterium Thiobacillus ferrooxidans under acid conditions.

Oxidation of pyrite by this route is nearby 10 6 times faster

than reactions [1] and [2] (Ritsema et al. 1992).

176

Acid sulfate soils are characterized by a low pH and the

presence of yellowish jarosite (KFe3(804) 2 (OH) 6) mottles in the

soil profile at various depths. According to van Breemen

(1982), jarosite is usually found under conditions of low pH

(2 - 4) and strongly oxidizing environments (Eh > 400 mV).

Incomplete hydrolysis of ferric iron can also produce

jarosite, and acidity (W)

FeS2 + 15/402 + 5H20 + 1/3K' ---> 1/3KFe3(804)2(OH)6 + 4/38°4 -2 + 3H' [4]

Besides jarosite and/or natrojarosite, AI-hydroxy sulfate

minerals such as alunite (KA13(OR) 6 (804) 2)' jurbanite (AlOH804),

and basaluminite (A14(OH) 108°4) can also be found in acid

sulfate soils. Low pH and high Al'3, Fe+3, 8°4- 2, and K+

activities favor the formation of these minerals.

Under a scanning electron microscope, jarosite appears as

cubes occupying voids and/or root channels. It has been

suggested that j arosi te is formed by pseudomorphic replacement

of pyrite (8hamshuddin et al., 1986). Mermut et al. (1985)

however, questioned this mechanism in soil as pyrite crystals

were far too big compared to jarosite crystals.

some acid sulfate soils in the Bangkok Plain have been

reclaimed for paddy rice, tropical fruit trees and upland

crops (Panichapong, 1990). The routine management and

agronomic practices for traditional rice production on these

soils consist of 1) leaching and flushing the excess acidity

by rain water and flood-swollen stream water, 2) liming and/or

3) incorporating rice straw or organic residues into the top

177

soil. Liming has been practiced on rice fields, but the

practice is not effective in acid sulfate soils because large

amounts of lime are required. For clay-textured acid sulfate

soils, repeated applications of 20 Mg of CaC03 ha ? year"

(Moore et ale 1990) are cornman. Hence, alternative practice

using organic amendments to reduce or prevent Al toxicity in

the rice fields is worth investigating. Some green manure,

crop residue, chicken manure and sewage sludge have been shown

to decrease Al toxicity in acid soils (Hue and Amien, 1989;

Hue, 1992). The possible reactions include chelation of AI+3

by functional groups on the solid phase of organic amendments

or by the soluble organic components. The objectives of this

study were to assess chemical and mineralogical changes in

some acid sulfate soils of Thailand as a consequence of lime

and green manure applications.


9.3.1 Soil selection and properties

Two soils from the Bangkok Plain, one at the Bang Pakong

Soil Conservation Center in the Chachoengsao Province, and the

other from the Ongkharak Acid Sulfate Soil Experiment Station

in the Nakhon Nayok Province, were selected for the study. The

soils belonged to the Bang Pakong (Bg) and Rangsit-very acid

phase (Ra) series.

The Bg soil is classified as a Typic Sulfaquent (Soil

Survey Staff, 1990), which was under mangroves that have been

178

cleared for rice production. The soil is subjected to salt

water intrusion at high tide during dry seasons. It is

protected from saltwater intrusion by local farmers for rice

cultivation during the wet season after the first rain waters

have flushed surface salts that accumulated during the

previous dry season. During our 1992 investigation, this area

was undergoing intense reclamation/management practices. The

area was being drained by the construction of a deep canal and

protected from sea water by a seawall dam. This soil was

selected to represent the potential acid sulfate soils.

The Ra soil is classified as a Sulfic Tropaquept (Soil

Survey Staff, 1990). This soil has been drained and leached by

rain water since 1970. The soil had been planted to continuous

rice for 15 years. Fresh water was used to irrigate rice

during the dry season. Lime, Nand P had been used during rice

production. This soil was selected to represent the actual

acid sulfate soils.

Soil samples, about 60 kgs from each series, were collected

from the Ap horizons and jarositic layers. The samples were

air-dried, ground, sieved through a 2-mm screen, and shipped

to the Department of Agronomy and Soil Science, University of

Hawaii, for an incubation study.

9.3.2 Chemical analysis

Soil samples were analyzed for texture and relevant

chemical constituents. Soil pH was measured on previously air-

179

dried samples suspended in a 1:1 soil: water. Soil electrical

conductivity (EC) of 1:1 soil: water ratio was obtained by a

conductivity bridge. Extractable bases were determined in the

1M NH40Ac extract, buffered at pH 7. Calcium and Mg were

measured by atomic absorption spectrophotometry (AAS) , while

Na and K were determined by flame photometry. Aluminum was

extracted by 1M KCl and determined colorimetrically (Barnhisel

and Bertsch, 1982). The acid-oxalate method of McKeague and

Day (1966) was used to extract amorphous Fe and Al oxides.

Oxalate-Fe in the extracts was analyzed by AAS. Oxalate-AI was

determined colorimetrically by the catechol violet procedure

(Kerven et al., 1989).

9.3.3 Mineralogical analysis

Particle size segregation. Mineral analysis was carried out

on air-dried, sieved « 2 mm) soil samples, which were first

subjected to ultrasonic dispersion and centrifugation to

obtain different particle size classes. The clay fraction «

2 ~m) was further separated into coarse (2 - 0.2 ~m) and fine

« 0.2 ~m) fraction. Only X-ray diffraction patterns of the

coarse clay fraction will be presented and discussed in this

paper.

X-ray diffraction (XRD). Inasmuch as the clay fraction

cracked and curled when placed on a glass slide, the clay was

air dried, ground with a mortar and packed into cavity mounts.

The samples were, therefore, randomly oriented and as much

produced diffraction lines that were proportional to the

180

actual amounts of the minerals present. Random orientation,

however, does not accentuate the basal reflections of the

phyllosilicates. In addition, the untreated soil samples

containing yellow mottles at 90 days after incubation were

handpicked and analyzed by XRD. XRD was carried out with a

Philips Norelco diffractometer with a long fine-focus cobalt

target X-ray tube operated at 40 kV, 25 rnA. A curved graphite

monochrometer was used in place of a K~ filter. XRD

intensities were collected by step scanning the specimens from

4° to 76° 26 with a step size of 0.025° 26 and with a 4 sec

counting time per step. Data were collected by a computer and

stored on disks for further processing.

9.3.4 Soil incubation study

An incubation experiment was conducted, using a split-plot

design with incubation periods as main plots and soil

amendments as subplots. Each treatment was duplicated. The

amendments were: control (no addition); 6 Mg CaC03 ha-1 (3.0 g

kg- 1 soil, the nominal lime requirement); and 40 Mg ha- 1

grounded sesbania tops (20 g kg- 1 soil). These rates were

chosen based on the results of previous experiments (Chapter

8). The CaC03 or sesbania tops (containing in ~ of dry matter

: 3.20 N; 0.25 Pi 2.12 K; 0.94 Ca and 0.93 Mg) were incubated

with 300 g air-dried soils in thin (0.025 mm) polyethylene

bags at 24 ± 1 °c. The soils, CaC03 or sesbania amendments were

well mixed before being brought to 80% of the field water

holding capacity with deionized water. The incubation bags

181

were not sealed but folded over to allow adequate aeration.

Any water loss, which was less than 3% per week, was replaced

weekly. The amended soils were sampled at 2, 14, 42 and 90

days after incubation, and the soil solution was extracted.

Soil solution collection and chemical speciation. The soil

samples from each treatment were rewetted with deionized water

to field capacity and equilibrated for 1 day. Then the soil

solution was extracted by centrifugation (Menzies and Bell,

1988). The solution was filtered through a 0.45 Mm-pore

membrane. The pH and EC were determined immediately on 3 mL

subsamples. The remaining soil solution was kept for chemical

analyses. Calcium, Mg, K, Na, Fe and Mn were determined by

atomic absorption spectrophotometry; N03 - by the phenoldi

sulfonic acid method (Hue and Evans, 1986), P by the ammonium

sulfomolybdate method (Olsen and Sommers, 1982), S04 -2 by the

turbidimetric method (Freeney, 1986), and dissolved organic C

by the pyr-ophosphate-Mn" method of Bartlett and Ross (1988).

The catechol violet method as described by Kerven et al.

(1989) was used to determine the total inorganic Al

concentration in soil solution.

Activities of Al species and other ions in the soil

solutions were calculated by the SOILSOLN computer program of

Wolt (1987). Soil solution pH, and concentrations of AI, Ca,

Mg, Na , K, Mn, P, N03-, S04-2 and soluble organic C were used as

input. Ionic strength of the soil solution was estimated from

the EC (Griffin and Jurinak, 1973). Inorganic monomeric Al

182

species in soil solution considered are Al +3, Al (OH) +2,

Al (OH) 2+, Al (OH) / and Al (S04) + (Blarney et al., 1983). The sum

of the activities of these Al species is termed Al sum •

Ion activities in soil solution were used to construct ion

activity products (IAP) and mineral stability diagrams.

Stability lines for various minerals were drawn based on the

information reported by Lindsay (1979), and Garrels and Christ

(1965) .

9.3.5 Statistical analysis

The main effects of incubation periods and comparisons

among treatments were statistically analyzed following the

split-plot design. Analysis of variance was performed and the

level of significance was set at 5% (p < 0.05) unless

otherwise specified.


9.4.1 Effects of CaC03 and green manure on the soil solid

phase

Effects of the amendments on soil pH (1: 1), EC, ': ,,__ -"ange

able and Al saturation percentage of the effective CEC of the

soils at various incubation periods are shown in Table 9.1. In

general, significant changes (P < 0.05) in some soil-solid

phase preperties were observed by effects of time of

incubation and interaction of the incubation periods with soil

amendments in pH, EC, and Al saturation percentage. The trends

were similar for all two acid sulfate soils and are

183

Table 9.1. Chemical properties of the soil solidphaseas affected by CaC03 (6 Mg ha") and sesbania(40

Mg ha') application at various incubation periods for two acid sulfate soils.

Treatments Incubation pH EC Ex.AI ECEC t AI sat. Ca sat. Alou lv Feox:lI

t/

period (1:1) (dS (%) (%) (%) (%)

(days) m') (cmol, kg:')

Bang Pakong (Bg)

Control 2 3.6 1.4 6.4 15.6 41.0 9.5 1.42 1.39

14 3.3 1.1 7.4 15.4 48.2 6.5 1.42 1.39

42 3.1 1.3 7.6 15.2 50.1 6.0 1.49 lAO

90 2.8 1.0 8.2 14.7 54.7 5.1 1.54 1.42

CaC0 3 2 4.2 2.1 2.8 15.7 17.9 20.7 1.29 1.19

14 4.6 3.1 1.1 16.9 6.9 26.9 1.26 1.19

42 4.9 3.9 0.4 18.3 2.6 32.3 1.24 1.11

90 5.2 3.8 004 19.2 2.1 32.5 1.20 1.01

Sesbania 2 3.5 1.4 5.7 15.3 37.3 12.7 1.40 1.25

14 3.8 2.0 3.5 16.3 21.8 12.7 1.39 1.22

42 4.2 2.2 2.1 16.4 13.1 16.1 1.34 1.15

90 4.5 2.5 1.7 16.5 10.3 16.7 1.30 1.12

LSD (O.O5)§/ 0.2 0.3 0.4 0.6 3.1 1.2 0.03 0.07

Rangsit (Ra)

Control 2 4.0 0.8 4.0 9.0 44.9 15.9 0.58 0.50

14 3.8 0.8 4.3 9.3 46.2 11.0 0.61 0.51

42 3.6 0.7 4.2 8.8 48.6 6.3 0.65 0.53

90 3.2 0.5 4.9 9.2 53.5 8.7 0.69 0.56

CaC03 2 4.6 1.8 1.1 10.1 11.8 43.2 0.35 0.45

14 5.1 2.0 0.3 12.1 2.8 54.8 0.33 0.43

42 5.4 2.5 0.3 13.0 2.7 56.5 0.28 0.39

90 5.6 3.1 0.1 14.5 1.0 61.3 0.25 0.30

Sesbania 2 3.9 1.7 3.0 8.9 33.7 19.9 0.50 0.48

14 4.3 1.8 1.6 9.4 16.9 33.2 0.45 0.46

42 4.8 2.0 0.8 10.5 8.2 44.7 0.38 0.38

90 5.1 2.3 0.6 10.7 5.6 47.6 0.30 0.33LSD (O.O5)§/ 0.2 0.2 0.2 0.7 3.1 1.7 0.02 0.03

t ECEC = Total exchangeable bases + Total exchange acidity

t/ extracted with 0.2 M Ammonium oxalate (pH 3.0)

§/ LSD (0.05) values for differences among treatments

184

illustrated in Table 9.2 using the statistically significant

values of F-test of analysis of variance (ANOVA). The soils

were very acid with the initial pH ranging from 3.5 - 3.6.

Within 90 days of incubation, the pH levels dropped

significantly (P < 0.01) in the unamended soils. Strong

acidification was probably caused by the formation of H2S04 due

to the oxidation of pyrite and ferrous sulfide (van Breemen,

1976) . Acidification was extreme in the Bg soil (pH = 2.8) and

relatively moderate in the Ra soil (pH = 3.2).

Table 9.2. Summary of source, degree of freedom (dF) and F-value of analysis of variance for soil-solid

phase as affected byCaC03 (6 Mgha') andsesbania (40 Mgha") application at various incubation periods

for two acid sulfate soils.

F-value

Source df pH EC Ex.AI AI sat.

(1:1) (dS m') (cmol, kg") (%)

Bang Pakong (Bg)

Block 1 2.46 NS 2.07 NS 0.49 NS 0.69 NSt

Incubation (I) 3 3.92 NS 13.62 • 30.10 •• 16.89·t

Error (a) 3

Treatment (T) 2 88.83 •• 97.76·· 631.29·· 477.38··

IxT 6 7.78· 6.17· 27.40·· 22.78··

Error (b) 8

Rangsit (Ra)

Block 1 3.54 NS 12.11 • 0.25 NS 0.14 NS

Incubation (I) 3 4.23 NS 17.12 • 120.01 •• 75.64 ••

Error (a) 3

Treatment (T) 2 108.29·· 155.47 •• 546.72 •• 548.18 ..

IxT 6 9.84·· 7.10 •• 16.58 •• 17.16 ..

Error (b) 8

t NS: Nonsignificant; •.••: Significant at the0.05 and 0.01% level, respectively.

185

As expected, soil pH and EC increased with CaC03 or sesbania

addition. The application of either amendment increased soil

pH by about 0.8 - 0.9 units in the Bg soil and by about 1.1

1. 3 units in the Ra soil after 90 days of incubation.

Accordingly, KCI-extractable Al was decreased from 6.4 cmole

kg-1 to 0.4 cmol , kg- 1 in the Bg soil and from 4.0 cmol , kg- 1 to

o.1 cmol , kg- 1 in the Ra soil.

Aluminum saturation value of the unamended soils always

exceeded 40%, regardless of the incubation periods (Table

9.1). Such levels have been reported to adversely affect crops

grown in the Bangkok Plain (Osborne, 1985). Adding CaC03 or

sesbania green manure gradually decreased Al saturation which

declined to less than 10% after 90 days of incubation.

Exchangeable bases increased significantly with liming and

sesbania amendments, and also with incubation time. Calcium

was low in both unamended soils where Ca saturation percentage

of the effective CEC was 5.1% and 8.7% (Table 9.1). These Ca

saturation values are much less than the reported values of

12% 13% below which cotton root penetration into acid

Ultisols of Alabama was restricted (Howard and Adams, 1965).

Ca saturation increased with CaC03 addition for each

incubation period. The significant increase (43.2 - 61.3%)

occurred in Ra soil. Liming acid sulfate soils with CaC03 may

take considerable time to be effective. The one- or two-week

waiting period between liming and planting, as recommended for

"regular" acid soils, may not be adequate for acid sulfate

186

soils. Addition of 40 Mg ha" sesbania to both soils also

increased Ca saturation above 12.7 - 19.9% which is higher

than 11% reportedly required for 90% relative root length of

soybean (Bruce et al., 1988).

The oxalate-extractable Al and Fe contents varied

considerably among the treatments and incubation periods. The

acid-oxalate solution presumably extracts the amorphous Fe

oxide and/or amorphous Al hydroxides and oxyhydroxides from

the soils. The NH4 oxalate solution extracted more Al from the

control than from the sesbania amended soils (1.54% vs.

1.30% in the Bg soil, and 0.69% vs. 0.30% in the Ra soil).

Similarly, NH4 oxalate extracted 1.42% Fe in the unamended Bg

soil and 1.12% Fe in the sesbania treatment after 90 days of

incubation.

It is expected that microcrystalline Al oxides would form

as a result of liming of acid sulfate soils. Raising soil pH

by lime converts most of the exchangeable and some amorphous

Al to Al oxides and hydroxides, which are not extracted by NH4

oxalate. Organic matter also forms solid complexes with

amorphous Al (Greenland, 1971), making Al nonextracted by NH4

oxalate at extended periods of incubation.

9.4.2 Effects of CaC03 and green manure on soil-solution

phase

pH, electrical conductivity and ionic strength (I)

The effects of CaC03 and sesbania treatments on soil

solution pH, EC and ionic strength are summarized in Table

187

9.3. At 90 days after incubation, CaC03 application increased

soil solution pH from 3.92 to 5.83 in the Bg series and from

4.35 to 5.92 in the Ra series. A highly significant increase

in pH and EC with 40 Mg ha? sesbania application was also

observed in both soils. The pH was raised from 3.92 to 5.11 in

the Bg soil, and from 4.35 to 5.86 in the Ra soil at 90-day

incubation. By contrast, pH of the control soils decreased

significantly from 3.92 to 3.49 in the Bg series, and varied

slightly from 4.35 to 4.19 in the Ra series.

soil-solution ionic strength of the unamended Bg soils was

< 20.1 mM and decreased with incubation time, whereas the

ionic strength increased from 29.9 - 44.2 mM after 90 days of

incubation in the CaC03 treatment. In general, soil-solution

ionic strength (I) was greater with CaC03 than with the

sesbania treatment. For example, I was 41.2 mM in the Ra soil

incubated with CaC03 and 34.4 mM incubated with sesbania.

Basic cation concentrations

Soil-solution K in the unamended soils was generally low,

ranging from 202 - 301 ~Mdepending on incubation period. For

comparison, Edmeades et al. (1985) found a mean K

concentration in the soil solution for a group of acidic top

soils in New Zealand to be 640 ~M (range 240 to 1030 ~M). The

lack of K-bearing minerals such as feldspar and secondary

micaceous minerals, and the possible formation of sparingly

soluble K bearing sulfate minerals in our acid sulfate soils

can explain for such low K concentrations.

188

Table 9.3. pH, EC and cationic concentrations in the soil solution as affected by CaC03 (6 Mg ha') andsesbania (40 Mg ha'') application at various incubationperiods for two acid sulfate soils.

Treatments/ Incubation pH EC I t Na K Ca Mgperiods (dS m') (mM)

(days) (pM)

Bang Pakong (Bg)Control 2 3.92 1.5 20.1 4447 301 521 915

14 3.65 1.4 18.2 3882 271 592 75442 3.53 1.2 16.2 3748 262 597 76390 3.49 1.0 13.6 3366 249 451 750

CaC03 2 4.46 2.3 29.9 5036 1008 2249 263614 5.10 2.9 38.3 6704 1545 2855 311142 5.78 3.2 42.2 7539 1875 2999 326890 5.83 3.4 44.2 8419 1810 3258 3229

Sesbania 2 4.15 1.7 22.7 4519 952 1191 179114 4.55 2.3 30.5 6505 1953 1826 231142 5.00 2.7 35.1 7718 2057 2097 251390 5.11 2.5 33.1 6805 2024 2401 2505

LSD (0.05) tl 0.19 0.2 3.7 421 87 145 239Rangsit (Ra)

Control 2 4.35 1.4 18.2 1729 262 842 196814 4.22 1.7 22.1 1929 221 803 198642 4.22 1.7 22.3 1626 216 936 189490 4.19 1.6 21.7 1581 202 755 1742

CaC03 2 4.90 2.3 30.5 2579 765 2630 224814 5.98 2.4 31.8 2877 810 3405 297242 6.18 2.8 37.3 2768 886 3872 331590 5.92 3.1 41.2 4143 983 4555 4438

Sesbania 2 4.55 2.0 26.6 2281 855 1856 214714 5.17 2.3 30.1 2755 1157 2442 328742 5.72 2.5 33.1 2472 1115 2901 358490 5.86 2.6 34.4 2659 1267 3211 3706

LSD (0.05) tl 0.22 0.2 3.7 218 96 231 318

t I (roM) = 13*EC(dS mol)

tl LSD (0.05) values for differences among treatments

189

In general, all basic cations in the soil solution of the

CaC03 treatment were at higher concentrations than those in

the unamended soils after 90 days of incubation. Furthermore,

the predominant cation in the soil solution of the unamended

treatment was Na, whereas expectedly it was Ca in the CaC03

treatment. The common notion that Ca is the dominant cation in

soil solutions does not apply to acid sulfate soils. The

dominance of Na ions (in molar concentration terms) in soil

solutions has been reported by Ahmad and Wilson (1992) and

Cisse et al. (1993) for many acid sulfate soils in regions of

the Caribbean and the Republic of Guinea. These investigators

attributed the high Na concentrations in soil solution to the

weak adsorption of Na on the exchange complex. High Na

concentrations in our soils however, may also be related to

the inundation of salt water during the dry season.

Compared with the CaC03 treatment, the application of

sesbania green manure significantly increased the

concentration of Ca and Mg in the soil solution at each

incubation period. Between the two soils, soil-solution Ca

concentration of the sesbania amended Ra series was higher

than that of the Bg series (Table 9.3). For example, at 90

days of incubation the Ca concentration was 3211 ~M in the Ra

soil as compared to 2401 ~M in the Bg soil. The increase in Mg

concentration to 3706 f1M in the Ra soil was also due partly to

the addition of Mg from decomposing sesbania directly to the

soil solution. Thus, green manure application helps improve

190

the availability of Ca and Mg, making these nutrients in acid

sulfate soils more favorable for crop production.

There was no useful relationship found between basic

cations in soil solution and exchangeable cations of the two

acid sulfate soils. By contrast, Nemeth et al. (1970) observed

a close relationship between exchangeable Ca and Ca in soil

solution for a group of temperate soils. Edmeades et al.

(1985) also found a positive relationship between

concentrations of cations in soil solution and amounts of

these nutrients present in exchangeable form. Although

exchangeable cations are often used as indicators of soil

fertility management, it may be inappropriate to emphasize

such parameters in these acid sulfate soils.

Total concentrations of soluble AI, Fe and Mn

Due to the strongly acid nature and the continuous

oxidation in both acid sulfate soils, water soluble AI, Fe and

Mn in the unamended treatments increased significantly

throughout the incubation periods. Concentrations of Fe were

54.8 - 112.4 ~M; Mn, 53.9 - 75.5 ~M; and AI, 111.4 - 238.7 ~M

(Table 9.4).

The high concentrations of Al and Mn in the soil solution

are often regarded as the most limiting factors for crop

production in acid sulfate soils. These Al and/or Mn

toxicities can be alleviated by either CaC03 or sesbania

application (Table 9.4) . There was an exponential relationship

191

Table 9.4. Concentrations of AI, Fe, Mn and anions in the soilsolution as affected by CaC03 (6 Mgha')and sesbania (40 Mgha") application at various incubation periods for two acid sulfate soils.

Treatments Incubation pH AI Fe Mn P SO/ N03-

periods(days) (pM)

Bang Pakong (Bg)Control 2 3.92 180.8 72.1 61.8 9.9 3017 33.8

14 3.65 215.7 80.3 66.0 10.4 2607 36.942 3.53 242.8 92.5 69.5 16.9 2324 25.590 3.49 238.7 112.4 75.5 11.9 2240 24.5

CaC03 2 4.46 98.4 63.2 53.6 22.7 3915 59.014 5.10 39.2 43.6 36.1 48.8 5317 69.642 5.78 15.9 27.6 12.7 62.2 5444 73.990 5.83 9.3 22.4 10.1 50.6 6128 83.3

Sesbania 2 4.15 117.2 68.4 57.7 12.6 3379 47.514 4.55 51.8 52.2 45.7 44.4 3978 51.742 5.00 28.7 43.1 33.6 50.2 4483 64.290 5.11 26.0 41.0 31.3 54.2 5327 95.9

LSD (0.05) v 0.19 15.3 8.0 6.0 7.1 364 9.1Rangsit (Ra)

Control 2 4.35 111.4 54.8 53.9 8.6 4671 84.114 4.22 114.5 62.0 57.1 10.8 4104 80.142 4.22 158.5 64.9 58.6 12.2 4511 69.090 4.19 188.2 76.7 65.1 10.8 4227 59.2

CaC03 2 4.90 92.7 44.5 39.0 29.4 6750 100.214 5.98 11.6 22.9 14.2 52.8 8152 226.642 6.18 15.6 10.1 10.3 82.4 8602 825.9

90 5.92 9.9 20.5 11.0 80.6 9958 2501.0

Sesbania 2 4.55 128.1 52.9 48.1 17.3 4529 107.0

14 5.17 48.4 37.0 25.3 43.3 5348 169.7

42 5.72 19.0 26.4 19.0 71.6 5974 514.2

90 5.86 16.6 14.8 12.2 75.2 6699 1488.0LSD (0.05) tl 0.22 15.0 8.1 6.9 5.3 496 119.0

tl LSD (0.05) values for differences among treatments

192

between soil solution Al concentration and soil solution pH

for all treatments of both soils (Figure 9.1a). When pH was

raised to 5.0 after 90 days of incubation, Al concentration

was reduced to less than 30 ~M, a level considered as nontoxic

b).

3.8 4.3 4.8 5.3 5.8 6.3pH In soil solution

..0

.-.. ..-. ..,... .

~2. 90§ 75~ 60...'E 45~ 30s 15

~ ~.3

a).

3.8 4.3 4.8 5.3 5.8 6.3pH in soil solution

~. -.-..

.____A __•__._.

----~._."-----_...-_... .__._-------.H •••••••• _ ........ ·.··_.·_••• ... ------_..-_.._.....__._---_._--...•._......._. ...•........__._,. •...- .-_._._-.._-_..._-----.,---l-----.---...--a.------. --................ .....•...........-._ .._... ..----.•.•--.'V-:;-.-

~.3

i'2. 250e 200.2-; 150...'E 100Q)

g 50oo

c).

~.-

1---.---. -. -. - -..•.r--.--.-------.-----;a---.--....._----_.-.__.

90

60

30

o3.3 3.8 4.3 4.8 5.3 5.8 6.3

pH in soil solution

~2. 120eo;;lU...-C<IIUCoU<IIu..

- B9 • Ra ,

Figure 9.1. Relationship between Al (a), Mn (b) and Fe (c)

concentrations and pH in the soil solution of two acid sulfate

soils. Regression equation for a). y'= 2. 5*104 (exp·1.2X) , r 2 =

0.92"; b). y'= 164.0 - 25.6X, r 2 = 0.97"; and c). y' = 193.8 -

29 . 7X, r 2 = 0.93".

193

to most plants (Ismail et al., 1993). 8imilar results have

been reported for acid sulfate soils from Malaysia (Edgardo

and 8hamshuddin, 1991) and Indonesia (Konsten et al., 1990).

There were also high linear correlations (r2 = 0.97**,

0.93**, respectively) between Mn or Fe concentration and pH

(Fig. 9.1b and c). For example, when soil pH was raised to

5.11 in the Bg and 5.86 in the Ra soil by the green manure

treatment, Mn concentration was decreased to 30 - 10 ~M, and

Fe concentration to 40 - 15 ~M.

Activities of Al species and basic cations

Both CaC03 and green manure application affected the

calculated activities of AI, Mn, basic cations and 8°4- 2 in the

soil solution (Table 9.5). The production of 8°4-2 is clearly

demonstrated after 90 days of incubation, which shows a rapid

rise in 8°4- 2 activities to values of around 4786 - 5370 ~M. In

contrast to the increasing 8°4- 2 activities, Mn activities

decreased in the liming and sesbania treatments.

Along with exchangeable Al and/or Al saturation, Al

activity in soil solution has been used to predict Al

toxicity. The Al +3 and monomeric Al hydroxy species in solution

are more toxic to plants than soluble polymeric species

(Blarney et al., 1983). Adams and Lund (1966) observed a

critical solution AI+3 activity of 2 ~M for cotton roots. Using

the derived equation of pAl +3 = 1.72pH - 2.22, r 2 = 0.94** from

both acid sulfate soils, 2 ~M Al +3 is attainable when soil

solution pH ~ 4.6.

194

Table 9.S. Effectsof CaCO) (6Mg ha') andsesbania (40 Mg ha") application on pHand activities of ions in the soilsolutionat various incubation periods for twoacidsulfate soils.

Soils! Incubation pH AI+) AIS04+ AI,um t K+ Ca+2 Mg+2 Mn+2 S04·2

Treatments periods(days) Activities (jLM)

Bang Pakong (Bg)Control 2 3.92 26.3 72.5 102.5 269 331 602 39.8 2511

14 3.65 34.9 84.7 121.8 239 380 501 42.6 234442 3.53 42.4 94.2 138.5 234 389 501 44.6 234490 3.49 43.3 97.2 142.8 223 295 501 50.1 2187

CaC03 2 4.46 10.4 33.7 53.9 851 1230 1513 29.5 416814 5.10 1.3 5.4 28.0 1288 1479 1737 19.0 489742 5.78 0.1 0.1 14.5 1548 1548 1778 6.6 524890 5.83 < 0.1 0.1 8.5 1479 1659 1737 5.2 5370

Sesbania 2 4.15 15.4 46.0 66.0 831 707 1096 33.8 331114 4.55 5.1 16.8 28.8 1659 1000 1318 25.1 436542 5.00 1.3 4.7 19.5 1737 1122 1412 18.1 478690 5.11 0.8 3.7 19.1 1698 1288 1412 16.9 4677

Rangsit (Ra)Control 2 4.35 12.2 53.3 72.8 229 537 1288 33.8 2454

14 4.22 13.6 49.7 64.4 194 501 1288 36.3 245442 4.22 17.9 71.8 96.4 190 588 1230 37.1 245490 4.19 22.2 84.0 113.7 177 478 1148 41.6 2290

CaC03 2 4.90 4.9 27.6 63.0 660 1513 1348 22.3 363014 5.98 < 0.1 0.1 10.9 676 1862 1698 7.7 416842 6.18 < 0.1 0.1 14.9 741 2041 1862 5.4 426590 5.92 < 0.1 0.1 9.2 812 2290 2398 5.6 4786

Sesbania 2 4.55 12.3 47.7 76.5 741 1096 1318 28.1 323514 5.17 1.3 5.9 37.2 977 1348 1905 14.1 398142 5.72 0.1 0.1 17.3 933 1584 2041 10.4 407390 5.86 0.1 0.1 15.4 1071 1737 2089 6.6 3981

t A1,um = A1+3 + AI(OH)+2 + AI(OH)/ + AI(OH)3° + A1(S04)+

Most of soil solution Al was in the inorganic monomeric form.

The Al +3 activity in the unamended Bg and Ra soils was 26.3 and

12.2 flM, respectively. In the CaC03 treatment, Al +3 and AISO/

dominated the Al species initially but their activities were

195

reduced significantly as soil solution pH increased with

incubation time (Table 9.5). In the sesbania treatments, AI+3

activity in the soil solution was 0.8 ~M in the Bg soil, and

practically zero « 0.1 ~M) in the Ra soil after 90 days of

incubation. The reduction in AI+3 activity by green manure

application has been reported by Hue and Amien (1989).

Furthermore, high activities of K+, 8°4 -2

, and W may result in

a precipitation of AI-hydroxy sulfates (Hue et al., 1985),

which reduces Al +3 activity .

9.4.3 Mineralogical composition of acid sulfate soils

XRD analysis of the clay fraction

In general, there was no change in the X-ray diffraction

patterns of the clay fractions of both acid sulfate soils by

application of CaC03 or sesbania green manure during the 90

day incubation period (Fig. 9.2 and 9.3). The oriented clay

prepared from the unamended Bg soil samples invariably showed

distinct peaks at 7.15, 10.56 and 14.82 °A (Fig. 9.2),

indicating the presence of kaolinite, illite and high-charge

smectite, respectively. The Ra-soil samples had a similar

clay mineralogy (Fig. 9.3), except that small peaks were found

at 14.0 14.1 °A for all treatments at 90 days after

incubation. For both acid sulfate soils, 2:1 type clay of

smectitic peak appeared at 14.82 °A consistently throughout

the incubation periods. Kaolinite (7.16 °A) was also evident

in all samples. Kaolinite may be considered as a weathering

196

Bang Pakong (Bg)

Ses 40; 90 days

4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64 68 72 76

DEGREES 2-THETA

Figure 9.2. X-ray diffraction patterns of the clay fraction of the Bg soil as affected by CaCO:J and sesbania

application at 2 and 90 days after incubation. Ca 0 = Control;' Ca 6 = 6 Mg ha" CaC03; Ses 40 = 40

Mg ha: sesbania.

197

Rangsit (Ra)

Ses 40; 90 days

Ca 6; 90 days

~~.......Ca 0; 2 days

4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64 68 72 76

DEGREES 2-THETA

Figure 9.3. X-ray diffraction patterns of the clay fraction of the Ra soil as affected by CaC~ and sesbania

application at 2 and 90 days after incubation. Ca 0 = Control; Ca 6 = 6 Mg ha-t CaC03; Ses 40 = 40

Mg ha' sesbania.

198

product of smectites (Bohn et al., 1985). The XRD analysis

showed no evidence of weathering of clay minerals by the

treatments in spite a strong acidification that occurred

during the incubation in the control soils. The acidity

produced from intense pyrite oxidation in the present study

did not induce weathering of clay minerals. It is likely that

an incubation of 90 days was too short for mineral weathering.

XRD analysis of basic ferric sulfate minerals

After 42 days of incubation, a few small straw yellow spots

appeared on the upper surface of the unamended Bg soil. Later

the spots became larger and turned pale yellow (2.5 - 5Y 8/3

8/6). These particles were handpicked at 90 days after

incubation for X-ray analysis (Fig. 9.4). Jarosite was found

to be the only basic ferric sulfate mineral present in the

yellow mottles. Besides jarosite the remaining yellow mottles

had peaks that were attributable to quartz, mica, feldspar and

in one instance pyrite. Neither natroj arosite, ammonium

jarosite, hydronium jarosite, alunite, jurbanite, goethite nor

hematite could be observed in this yellow mottle. Although the

XRD patterns of the basic ferric sulfate minerals differed

only slightly, the position of the (003) reflection at about

4.2 to 4.4 °A and the (021) and (113), double reflection at

3.1 °A, permitted the separation of jarosite from other basic

ferric sulfate minerals. Under the present study jarosite was

identified by its (003) reflection at 4.46 °A and its (021)

199

Bg Yellow 900a..;

500

450

400

350~>- i "'!I- -H •en 300z

~ ~UJl- N -z 250H

..Ji ~< 200 ~l- N N coa

JN -el-

ii!-

oL..-J---J._.1---J---1._L--..L--L..-----I_..L-~__L_.L_~_L_"'___'___J

4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64 68 72 76

DEGREES 2-THETA

Figure 9.4. X-ray diffraction patterns of materials separated

from yellow mottles from the Bg soil after 90 days of

incubation. Only the distinct jarosite peak (at 3.18 °A) is

shown with symbol J.

and (113) doublet at 3.18 and 3.07 °A, respectively. The 4.25

°A and 3.33 °A XRD peaks were the peaks of quartz, which were

present together with the yellowish mottles.

The particles of the yellow mottle collected from the

jarositic horizon in the field conditions of the Bg soil had

200

similar XRD pattern to that of jarosite; no natrojarosite and

other minerals could be detected (data not shown). This was

probably due to the fact that most Na was already leached from

the soil when the jarositic layer began to form, and the

formation of natrojarosite in situ is a slow process. The

above results are in good agreement with synthetic and

naturally occurring basic ferric sUlfate. For example, in the

formation of synthetic basic ferric sulfate (Ivarson et al.,

1982), there was a strong preference for K+ over Na+ and H30+

in the structure. In a survey of acid sulfate soils (> 8000

km") in the Bangkok Plain of Thailand, van Breemen (1976)

showed that K-jarosite was far more abundant than the Na or

H30-form. Our results also confirmed that there was a strong

preference for the formation of jarosite after 90 days of

incubation.

X-ray diffraction analysis, generally supported the

contention that smectite, kaolinite, illite and jarosite are

the four major minerals present in our acid sulfate soils. The

presence of basic aluminum sulfate and/or Al hydroxy sulfate

minerals (e.g., jurbanite, basaluminite and alunite) was not

evident or detected by XRD. Apparently, their low

concentrations and small crystallite size were not detectable

by our X-ray equipment. Identification or verification of

these minerals may require higher resolution instruments (SEM

or electron micrographs) or other more effective techniques.

201

9.4.4 Effects of lime and green manure on metal solubility

and mineral stability diagrams

Aluminum and Fe are two important metals in acid sulfate

soils contributing to the dissolution, precipitation, and

formation of various hydroxy-sulfate minerals. Among these,

Jarosite (KFe3 (80 4 ) 2 (OH) 6) is one of the most important

minerals in acid sulfate soils, and very insoluble with a Ksp

value of 10- 98.6 (Vlek et al., 1974). Ion activity products

(lAP) were calculated for jarosite with the 80IL80LN program.

The results showed that the saturation indices (IAP/Ksp ) were

greater than zero, suggesting that our soil solutions were

supersaturated with respect to jarosite. In other words,

jarosite should be present or is being formed in our acid

sulfate soils; and indeed the XRD data confirmed this.

When the soils were amended with 6 Mg ha! of CaC03 or 40 Mg

ha-1 of sesbania, soil solution pH increased above 5 after 90

days of incubation (Table 9.3). This condition is no longer

conducive for jarosite formation, which requires low pH (2

4) and in an oxidizing environment (Eh > 400 mV) (van Breemen,

1982) .

Ion activity products and mineral stability diagram

The formation of AI-hydroxy sulfate minerals [jurbanite

(AIOH804 ) , basaluminite (A14 (OH) 10804), and alunite

(KAI3{OH)6(804) 2) ] were studied by comparing ion activity and

corresponding Ksp for each mineral. Table 9.6 shows the

possibility of AI-hydroxy sulfate formation with respect to

202

Table 9.6. Soil solutionion activities and ionactivity products of acid sulfate soilsas affected byCaC03

(6 Mg ha") and sesbania (40 Mgha") application at various incubation periods for twoacidsulfate soils.

Treatments/ Incubation pH -log activity (mol L-I) Ion activity productsperiods (AI+3) (K+) (50/) pJurb. pBasai. pAlun. pGibb.

BangPakong (Bg)Control 2 3.92 4.6 3.5 2.6 17.2 121.9 83.1 34.9

14 3.65 4.5 3.6 2.6 17.4 124.2 84.5 35.642 3.53 4.4 3.6 2.6 17.4 125.1 85.1 35.990 3.49 4.4 3.6 2.6 17.5 125.5 85.3 36.0

Mean ± SE 17.3±0.1 124.1±1.6 84.5±0.9 35.6±0.4CaC03 2 4.46 5.2 3.0 2.3 17.2 118.6 80.7 33.8

14 5.10 6.2 2.8 2.3 17.5 116.2 79.6 32.942 5.78 7.8 2.8 2.2 18.3 115.8 80.2 32.590 5.83 8.1 2.8 2.2 18.6 116.7 80.9 32.7

Mean ± SE 17.9±0.6 116.8±1.2 80.3±0.5 32.9±0.5Sesbania 2 4.15 4.9 3.0 2.4 17.3 120.8 82.0 34.5

14 4.55 5.5 2.7 2.3 17.3 119.0 80.8 33.942 5.00 6.2 2.7 2.3 17.5 117.1 80.0 33.290 5.11 6.4 2.7 2.3 17.5 116.8 80.0 33.1

Mean ± SE 17A±0.1 118.4±1.8 80.7±0.9 33.6±0.6Rangsit (Ra)

Control 2 4.35 5.0 3.6 2.6 17.4 119.1 81.8 33.914 4.22 4.9 3.7 2.6 17.2 120.1 82.4 34.342 4.22 4.7 3.7 2.6 17.2 119.5 82.0 34.190 4.19 4.6 3.7 2.6 17.1 119.4 81.9 34.1

Mean ± SE 17.2±0.1 119.5±0.4 82.0±0.2 34.1±0.1CaC03 2 4.90 5.6 3.1 2.4 17.3 116.0 79.6 32.9

14 5.98 8.4 3.1 2.3 18.7 116.2 81.2 32.542 6.18 8.7 3.1 2.3 18.8 115.4 80.9 32.290 5.92 8.3 3.0 2.3 18.9 116.7 81.4 32.6

Mean ± SE 18.4±0.7 116.0±0.5 80.7±0.8 32.5±0.2Sesbania 2 4.55 5.1 3.1 2.4 17.0 117.5 80.2 33.5

14 5.17 6.2 3.0 2.4 17.4 115.8 79.6 32.842 5.72 7.6 3.0 2.3 18.3 115.8 80.4 32.590 5.86 8.0 2.9 2.4 18.6 115.8 80.6 32.4

Mean ± SE 17.8±0.7 116.2±0.8 80.2±0.4 32.8±OA

Mean 17.6 118.5 81.4 33.6

SE 0.6 2.9 1.6 1.1Reference 17.2 t 117.6 t 85.4 t 32.8 §

t van Breemen, 1973; t Adams and Rawajifih, 1977; § Kittrick, 1966;

203

lime and sesbania amendments at various incubation periods.

Since the overall ion activity products were 10-8 1.4 for alunite

(K = 10-85. 4 ) and 10-11 8 .

5 for basaluminite (K = 10-11 7 . 6 ) our~ ~ '

soil solutions were generally oversaturated with respect to

crystalline alunite, but undersaturated with respect to

basaluminite. Eriksson (1993) suggested alunite formation as

the solid phase immobilization of Al in the Me Kong acid

sulfate soils of Vietnam under the different water

managements. On the other hand, the existence of jurbanite

seems to depend on the soil amendments. In the unamended

controls, the soil solutions had -log (ion activity product)

for jurbanite (pJurb.) of 17.3 ± 0.1 for the Bg soil and 17.2

± 0.1 for the Ra soil, which are similar to the reported

solubility range of jurbanite [pKsp = 17.2 (van Breemen, 1973)

or 17.8 (Nordstrom, 1982)]. As lime was added, and the

incubation time was extended, the soil solution became

undersaturated with respect to jurbanite and oversaturated

with respect to alunite. It is possible that jurbanite was

dissolving to form alunite upon OH- (lime) addition as

suggested by the following reaction

For example, the limed Bg soil had pJurb. = 17.2 at 2-day

incubation, which changed to 18.3 after 42 days of incubation.

At the mean time, pAlunite fluctuated about 80.3 in the lime

treatment as compared to 84.5 in the unamended control. Such

shifts in ion activity products also occurred in the sesbania

204

green manure treatment as clearly demonstrated in the Ra soil.

Changes due to green manure addition were more subtle because

the Bg soil was too acid for fast organic decomposition.

Stability diagrams of the AI-hydroxy sulfate minerals were

plotted in Fig. 9.5. Using reference pKsp of 17.2 for jurbanite

(van Breemen, 1973), 117.6 for basaluminite (Adams and

Rawaj ifih, 1977), 85.4 for alunite (Adams and Rawaj ifih,

1977), 32.8 for microcrystalline gibbsite (Kittrick, 1966),

and 14 for water (pKw), the following relationships are

established (parentheses denote ion activities, brackets are

used for grouping) .

For jurbanite, p (AI +3) (OH-) (S04-2) = [pAl + pOH + pS04] 17.23 [6]

For basaluminite, p(AI+3)4(OH-)lO(S04-2) - 6pK"

4 [pAl + pOH + pS04] - 3 [2pH + pS04]

33.6 [7]

which, on rearrangement, yields

[pAl + pOH + pS04] = 8.4 + 3/4 [2pH + pS04] [8]

For alunite, p(K+) (AI+3) 3 (OH-) 6 (S04-2) 2 - 2pKw

3 [pAl + pOH + pS04] + [pK + pOH] - [2pH + pS04]

57.4

which, on rearrangement, yields

[pAl + pOH + pS04] = (19.13 - 1/3 [pK + pOH]) + 1/3 [2pH + pS04] [9]

For gibbsite,

[pAl + pOH + pS04] = 4.77 + [2pH + pS04] [10]

Soil solution ion activities are plotted with [pAl + pOH +

pS04] as a function of [2pH + pS04] , along with stability lines

205

Jurbanite

22 •8gsoil

:+;

*20 ... '*' lit Ra soil~

o(/)c.+J: 18 •o ....a.+--a. 16 »:

11.5 12.5 13.5 14.5

2pH + pS0410.5

14-+---r------,r----r---,---.----~5 1~5

Figure 9.5. Soil solution activities relative to stability

line for AI-hydroxy sulfate minerals and gibbsite in two acid

sulfate soils. Plotted data points above a line indicated

supersaturation, and points below a line indicated

undersaturation. Assumed pK+ + pOH- = 12.3 ± 1.0.

for Al-hydroxy sulfate minerals and gibbsite in a manner

similar to those reported by Hue et al. (1985), and Wolt et

al. (1992). The stability line for alunite is fixed by using

the average value of [pK+ + pOH- = 12.3 ± 1.0 ] for soil

solution from Table 9.5.

206

As seen in Fig. 9.5, all the experimental points were

positioned between the theoretical line for jurbanite and

alunite equilibria. The points for the unamended Bg soil

appeared slightly above the jurbanite isoline (where 2pH +

pS04 < 11.5), whereas those at the higher pH due to lime and

sesbania treatments occurred on or near the alunite isoline.

Thus, as discussed earlier, increasing soil pH by lime or

sesbania application (Table 9.5) may transform jurbanite to

alunite. These observations suggest that soil solution Al +3,

S04-2 and H+ activities as affected by lime and sesbania

treatments (when 2 pH + pS04 > 11.5) closely parallel that of

alunite. If we assume that such a line represents an

equilibrium between an unknown solid phase (alunite-like) and

the solution, then the unknown solid phase has a log Ksp =

-81.4 ± 1.6, which is very close to -80.9 given by Lindsay

(1979), but much larger than -85.6 as suggested by Nordstrom

(1982) or -85.4 reported by Adams and Rawajifih (1977) for

crystalline alunite. Nevertheless, the close association

between the experimental points under the present study and

the alunite phase line of Figure 9.5 suggests the existence of

an Al-controlling phase similar to alunite, but more soluble

than the crystalline form of alunite as described by Adams and

Rawajifih (1977) and Nordstrom (1982). In addition, the X-ray

diffraction pattern of the clays at 90 days after incubation

revealed no sulfate-bearing phase, which is expected if the

phase is noncrystalline.

207


Both potential and actual acid sulfate soils of the Bangkok

Plain were acidified strongly by pyrite oxidation after only

90 days of incubation. The soils were characterized by high

concentrations of AI, Fe, Mn and low concentrations of Ca. For

successful crop production either lime or green manure

applications will be necessary. The use of locally available,

green manure species as a "self-liming material" is a

promising strategy for alleviation of Al toxicity and raising

basic cations in the soil solution.

XRD analysis showed that both soils were similar in their

mineralogical composition. The clay fraction contained mainly

mica, smectite, and kaolinite, with minor amounts of illite

and gibbsite. Liming or green manuring apparently did not

change mineral compositions in spite of a strong and rapid

acidification in the unamended soil. Yellowish jarosite

mottles were a common feature as incubation progressed.

Jarosite may form by chemical precipitation from the soil

solution if 8°4-2 concentration is very high and pH below 5.

Based on the pIAP and stability diagrams, the results suggest

the existence of an alunite-like mineral which has similar

chemical composition, but a higher solubility (pKsp = 81. 4)

than crystalline alunite (pKsp = 85.4). The proposed alunite

is believed to govern AI+3 activities in acid sulfate soils

that were influenced by lime or green manure application.

208

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van Breemen, N. 1973. Dissolved aluminum in acid sulfate soilsand in acid mine waters. Soil Sci. Soc. Am. Proc. 37 :694-697.

van Breemen, N. 1976. Genesis and solution chemistry of acidsulfate soils in Thailand. Agric. Res. Rep. No. 848.Center for Agric. Publ. and Document. Wageningen, TheNetherlands.

van Breemen, N. 1982. Genesis, morphology, and classificationof acid sulfate soils in coastal plains. In J.A. Kittricket ale (eds.) Acid Sulfate Weathering, pp. 95-108. SSSASpec. Publ. No. 10. Madison, WI.

Vlek, P.L.G., J.M. Bloom. J. Beek, and W.L. Lindsay. 1974.Determination of solubility product of various ironhydroxides and jarosite by chelation methods. Soil Sci.Soc. Am. Proc. 38 : 429-432.

Wolt, J.D. 1987. Soil solution. Documentation, source code,and program key. Version 1.4 ed. In Univ. of Tenn. Agric.Exp. Stn. Res. Rep., pp. 87-19.

Wolt, J.D., N.V. Hue, and R.L. Fox. 1992. Solution sulfatechemistry in three sulfur-retentive Hydrandepts. SoilSci. Soc. Am. J. 56 : 89-95.

XXXXXXXXXXXXXXXXXX

212

APPENDIX A

1. Soil Infor.mation and Profile Descriptions of Bang Pakong

Series.

Soil infor.mation

Soil Name : Bang Pakong Series Field symbol Bg

Taxonomy

Location

Typic Sulfaquents

Bang Pakong Soil Conservation Center, Chachoengsao

Province

Physiography : Formerly mangrove forest

Topography : Flat

Slope : 0 - 1%

Elevation Approximately 1 - 2 m above mean sea-level

Air-temperature : 27°C

Water table : Close to the surface

Drainage

Land use

Poorly drained ; Permeability

Transplanted and broadcast rice

Slow

Parent materials : Brackish water deposits

Described by : Soil Survey Staff, Dept. of Land Development

Profile descriptions

Horizons Depth (cm) Description

A 0 - 10 50% dark grayish brown (10YR 4/2) and50% brown to dark brown (10YR 4/3) silty clay; many, coarse,faint, diffuse, dark reddish brown (5YR 4/3) mottles; weakcoarse, angular blocky; sticky, non-plastic; few mediumvertical, inped and exped, continuous, tubular pores; manyvery fine roots, pores frequently filled with fecal pellets;clear, smooth boundary.

213

B1 10 - 18 70% dark grayish brown {10YR 4/2} and30% brown to dark brown {10YR 4/3} silty clay; many coarse,distinct, clear to diffuse, reddish brown {5YR 4/4 - 5YR 4/3}mottles and glossy coatings on pore walls; locally manycoarse, distinct, clear, strong brown {7.5 YR 5/6} mottles;weak, coarse angular blocky; sticky, non-plastic; common, fineto medium, vertical, inped and exped, continuous, tubularpores; many very fine roots; many pores filled with fecalpellets; clear, smooth boundary.

B2g 18 - 30 grayish brown {10YR 4/2} and locallyvery dark gray (10YR 3/1) silty clay; many, coarse, distinct,clear to diffuse, reddish brown {10YR 4/4 - 10YR 4/3} glossycoatings on pore walls; only at a few spots in the wall of thepit; many, coarse, prominent, sharp to clear, yellow {2.5Y8/6} jarosite on ped faces and in layers {appro 3 mm thick}sandwiched in between the matrix and the reddish browncoatings on pore walls; jarosite is sometimes associated withhalf decayed wood {root} fragments; weak, coarse, angularblocky; slightly sticky, non-plastic; common, fine to coarse,continuous, vertical inped, tubular, mostly open pores; poresoften filled with fecal pellets and root remnants; few roots,locally pieces of soft half decayed wood {up to 10 cm indiameter}; moderate to very rapid permeability; gradual,irregular boundary.

C1g 30 - 60 greenish gray {5GY 4/1} clay;unmottled; weak, coarse prismatic; slightly sticky, nonplastic; common, fine to coarse, continuous vertical, inped,tubular pores, sometimes filled with fecal pellets or halfdecayed roots; locally remnants of Xylocarpus and Nipa roots;moderate to rapid permeability.

C2g 60- greenish gray {5GY 4/1} clay;unmottled; locally many soft shell fragment and black pocketsof organic matters; rapid to very rapid permeability; at manyspots in the wall of the pit FeS suspension seeps from thesepockets of organic matter into the pit.

214

2. Soil information and Profile Descriptions of Rangsit Very

Acid-Phase Series.

Soil information

Soil Name : Rangsit ; Field symbol : Ra

Taxonomy

Location

Sulfic Tropaquepts

Ongkharak Acid Sulfate Soil Experiment Station,

Nakhon Nayok Province

Physiography : Former tidal flat

Topography Flat

Slope 1% ; Elevation : 2 m above mean sea-level

Air-temperature : 28 DC

Water table : 80 cm

Drainage

Land use

Poorly drained ; Permeability

Paddy field, broadcast rice

slow

Parent materials : Fresh and brackish water deposits

Described by : Soil Survey Staff; Dept. of Land Development

Profile descriptions

Horizons Depth (em) Descriptions

Ap 0 - 30 black (10YR 2/1) clay; common, fine,distinct, sharp, dark yellowish brown (10YR 3/4) rootrust;common, medium, faint, diffuse, dark yellowish brown (10YR3/4) mottles on ped faces; common, medium, distinct, diffuse,yellowish brown (10YR 5/6) mottles on ped faces (mainly in thelower part of the horizon); weak, coarse, and moderate, fineto medium, angular to sub-angular blocky; slightly sticky,plastic to slightly plastic; few, fine, mainly vertical,mainly inped tubular pores; abundant fine roots, mainly alongped faces; very slow permeability; clear, wavy boundary.

215

B1g 30 - 60 grayish brown (10YR 5/2) clay; many,coarse, prominent, clear, dark yellowish brown (lOYR 4/6)mottles; few, medium, distinct, clear yellowish brown (10YR5/6) mottles; weak, coarse, and moderate, fine, angularblocky; slightly sticky, slightly plastic; common, continuousslickensides; few to common, fine, vertical, inped and expedtubular pores; few fine to medium pores filled with black (10YR 2/1) clay, very few roots, mainly in the upper part of thehorizon; slow permeability; clear, wavy boundary.

B2g 60 -85 grayish brown (10YR 5/2) clay; mainlyin the upper part of the horizon common, coarse, prominent,clear, red (lOR 4/6) mottles; common, medium to coarse,distinct, diffuse, yellowish red (5YR 4/6) mottles, surroundedby medium to coarse, distinct, diffuse, yellowish brown (10YR5/6) mottles, both in the matrix, along pores and along pedfaces; common, medium to coarse, prominent, sharp and clear,yellow (5Y 8/6) jarosite, mainly as pore-fillings andsurrounded by yellowish brown (10YR 5/6) mottles; moderate,coarse and medium, angular blocky; slightly sticky, plastic;common, broken slickensides; few to common, fine to medium,continuous, vertical, mainlyexped, tubular pores; moderatelyslow to moderate permeability; no roots; gradual, smoothboundary.

B3g 85 - 115 grayish brown (10YR 5/2) clay; few tocommon, medium to coarse, distinct, diffuse, yellowish red(5YR 4/6) mottles and common, medium to coarse, distinct,diffuse, yellowish brown (10YR 5/6) mottles along verticalpores; common, medium to coarse, distinct, sharp, yellow (5Y8/6) jarosite; weak, coarse prismatic; sticky, non-plastic;patchy slickensides; common, fine to coarse, continuous,vertical, inped and exped, tubular pores; very few roots;moderate to moderately rapid permeability; gradual, smoothboundary.

B4g 115 - 170 grayish brown (10YR 5/2) clay; few,medium to coarse, distinct, diffuse, yellowish brown (10YR5/6) mottles; mainly along vertical pores; few, medium tocoarse, distinct, sharp, yellow (5Y 8/6) jarosite mottles;

216

locally broken coatings of jarosite and yellowish brown (10YR5/6) rust on prism faces; common, distinct, hard, yellowishbrown (10YR 5/6) coatings (1 - 2 mm thick) along fine tomedium, vertical pores, with a very thin dark yellowish brown(10YR 3/4) coating inside; weak, coarse prismatic; sticky,non-plastic; common, fine to coarse, continuous, vertical,inped and exped pores; very few roots; moderately rapidpermeability; diffuse, smooth boundary.

Cg 170+ gray (5Y 5/1) clay, unmottled.

A review

217

APPENDIX B

Aluminum phytotoxicity in acid tropical soils

ABSTRACT

Aluminum (AI) toxicity is a major factor limiting plant

growth in acid soils. Thus, knowledge of Al reactions in soils

and plants are essential for increasing soil productivity and

crop production. This review discusses (i) soil parameters

(e. g., pH, soluble Al concentration and activity, exchangeable

AI, and Al saturation percentage) that have been used to

indicate Al phytotoxicity, (ii) estimation of lime quantities

required to correct soil acidity and Al toxicity, (iii) Al

species in the soil solution and their differential

toxicities, and (iv) Al tolerance mechanisms of various

plants. Future work on Al research is suggested.

Key words Al activities; Al phytotoxicity indices; Al

tolerance mechanisms; monomeric hydrolytic species;

Introduction

Aluminum toxicity is probably the most important growth

limiting factor to plants in acid soils of the tropics (Foy,

1988). The problem is particularly severe below pH 5.0, but

has been reported to occur at pH as high as 5.5 in some soils

(Foy, 1984). The critical soil pH at which soluble or

exchangeable Al becomes toxic depends upon various factors,

including clay minerals, organic matter levels, concentrations

218

of other cations, anions and total salts, and particularly the

plant species or cultivars (Kamprath and Foy, 1985).

Generally, Al toxicity adversely affects roots more than

shoots. The AI-affected roots often appear coralloid, with

many stubby lateral roots and no fine branching (Foy, 1984).

Aluminum-damaged roots can explore only a limited volume of

soil and are inefficient in absorbing nutrients and water.

Furthermore, excess soil Al interferes with uptake, transport

and utilization of essential nutrients (Ca, Mg, K, P and Fe)

and may inhibit microbial processes that supply nutrients to

plants (Foy, 1988).

Tests for Aluminum Toxicity by Acid Soils

Conventional tests for Al toxicity in soils include pH,

exchangeable AI, Al saturation percentage of CEC, dilute acid

extractable Al (Wright, 1989), and soil-solution Al (Bruce et

al., 1988). For soils having similar parent materials and clay

mineralogy, pH alone or absolute levels of Al extracted by KCI

or other unbuffered salts may be adequately indicative of Al

toxicity to a given plant (McCormick and Amendale, 1983). In

general, a more useful predictor of Al toxicity is the

percentage of the cation exchange capacity (CEC) occupied by

Al (Evans and Kamprath, 1970; Kamprath and Foy, 1985).

Aluminum saturation is determined by displacing soil Al with

a neutral, unbuffered salt (such as 1M KCI), and expressing

the Al as a percentage of the CEC (measured by 1M NH40Ac, pH

219

7.0) or as a percentage of the effective CEC (Sum of Ca, Mg,

K, and Na extracted with NH40Ac and KCI-extractable AI). To be

effective, the Al saturation percentage must be applied within

rather narrowly defined set of conditions because the critical

Al saturation associated with toxicity varies with soil type,

plant species and even with genotype (Foy, 1987). Critical

soil exchangeable Al levels for alfalfa reportedly ranged from

0.2 - 0.9 cmol., kg-lor 4 to 19% Al saturation (Foy, 1964).

Unfortunately, neither exchangeable Al nor Al saturation

remains constant when applied to a range of different soils in

defining critical soil Al levels.

Evans and Kamprath (1970) found that when the Al saturation

reached 60% of the effective CEC, the Al concentration in the

soil solution was generally greater than 1 tJ.g s' which is

toxic to most plants. The relationship was fairly constant for

kaolinitic, and mixed montmorillonitic mineralogies. This

finding led Fox (1979) to suggest that Al saturation greater

than 60% would be considered as detrimental effect to most

crops and 10 - 20% to highly sensitive crops (Figure 1).

Exchangeable Al has been used to calculate lime

requirements for soils with low permanent charge and

relatively high pH-dependent charge (Reeve and Sumner, 1970).

In this method, lime quantities are added as multiples of the

amount of exchangeable Al on a chemically equivalent basis.

After the lime has had sufficient time to react, the amount of

220

-?fl. 100-"'0 90-Q)

):: 80

C'.a 70~

o 60

<V50>.-

~ 40<V

0::: 30

20

I

8 CIt ·• a a00• ••F- O· ·

•fo • 0 -•

·

·0 Wharton 1976 0

• Whorton1977f-a Murrill 1976

•fo. Murrill 1977

~ ·~

10 20 30 40 50 60 70

Al Saturation (%)

Figure 1 Relationship between AI saturation of two Ultisols

In Pennsylvania and relative corn grain yield

Source: Fox (1979)

Figure 1. Relationship between A) saturation of two Ultisols

in Pennsylvania and relative corn grain yield (Fox, 1979).

exchangeable Al neutralized is determined (Kamprath, 1970).

various studies have shown that lime rates chemically

equivalent to 1.5 to 3 times the exchangeable Al must be added

to completely neutralize Al (Table 1). The greater lime rates

can be explained by the bUffering properties of acid soils.

221

Exchangeable Al is responsible for the buffering in the pH

range of 4.0 to 5.6, while hydroxy-aluminum and aluminum

organo complexes control the buffering pH in the range of 5.6

to 7.6 (Jackson, 1963). Studies on Ultisols showed that when

soil pH < 5.3, lime reacted with both exchangeable Al and pH

dependent sources of acidity (Kamprath, 1970). On average,

lime must be added at a rate chemically equivalent to twice

the exchangeable Al content to eliminate Al toxicity.

Table 1. Liming factor (AI in cmol, kg" x factor) required to give equivalent of calcium carbonate to

reduce aluminum saturation to less than 10 %

Location Soil pH AI AI Factor Final

(surface 15 ern) (cmol, kg") saturation (%) pH

Brazil Red-Yellow Latosol 4.0 0.7 70 3 4.9

Red-Yellow Latosol 4.4 0.9 75 2 5.5

Dark-Red Latosol 4.0 1.9 86 2 5.0

Columbia Oxisol 4.3 3.5 78 2 5.3

Panama Latosol 5.1 1.2 53 1.5 5.9

Latosol 5.0 3.0 64 1.5 6.0

United States Ultisol 4.5 0.9 82 2.0 5.9

Ultisol 4.7 1.0 78 2.0 6.0

UItisol 4.5 2.3 73 1.5 5.7

Ultisol 4.7 4.2 54 1.5 5.6

India < 5.0 2.0 5.3

Natal Oxisol < 5.0 3.3

Aluminum Speciation and Al Tolerance of Plants

It is generally agreed that some measure of Al activity in

222

soil solution is more indicative of potential toxicity than

concentrations of water soluble Al or exchangeable Al or Al

saturation percentage (Adams, 1981). Adams and Lund (1966)

found that the critical Al activity which limited cotton root

growth in soil solution was the same as in nutrient solution

(> 0.15 X 10-5 M) (Fig. 2). Toxicity of water soluble Al is

further reduced by chelation with organic acids or other

ligands (Hue et al., 1986); and this process must be taken

into account when calculating the activity of the AI+3 ion.

Many investigators (Parker et al., 1988; Kinraide and Parker,

1989) have reported that the calculated activity of the AI+3

ion is the best indicator of Al toxicity. However, recent

evidence also indicates toxicity from monomeric hydrolytic

species as Al (OH) +2, Al (OH) 2+ and from polymeric hydrolytic

species of Al (Parker et al., 1989). At low OH:AI, and P:AI

molar ratios, most Al is present as monomeric species; at high

OH:AI and P:AI ratios, soluble Al polymers may develop. These

polymers tend to precipitate with time but often remain

soluble in dilute solutions (White et al., 1976). The AIS04+

species is generally considered as non-toxic or much less

toxic than AI+3 (Cameron et al., 1986).

The total concentration of monomeric Al in acid soil

solutions is the sum of various monomeric species : Al +3,

Al (OH) +2, Al (OH) /, and Al (OH) 30. Complex ions of Al with S04=

and F- also exist when these anions are presented (Tanaka et

al., 1987). Thus, the total soluble Al concentration over the

223

o

1'0

~

t;0·8Z

~-J

6 0'6

0a::0'4

~

>~et 0·2-J1LIa::

00

o

1·0 2'0

x - CaS~ solution

o - Norfolk subsoil solution

o - Dickson subsoil solution

Cl - Bladen subsoil solution

o

o

3·0

o

4·0

MOLAR ACTIVITY OF AL lC10-S

Figure 2 Effect of molar activity of AI in subsoil solutions in situ and

in sUbsurface nUfrient solufions on cotton primary root growth.

Source: Adams and Lund (1966)

Figure 2. Effect of molar activity of Al in subsoil solutions

in situ and in subsurface nutrient solutions on cotton primary

root growth (Adams and Lund, 1966).

pH range of 4-6 is described by :

[1 ]

If the total concentration of monomeric Al in soil solution,

pH, ionic strength, 804= and F' concentration, are known, the

concentrations, activity coefficients, and activities of

224

individual species in Eg. (1) can be calculated. Blarney et al.

(1983) reported a good correlation of soybean root elongation

with the sum of activities of all monomeric hydrolytic species

O:aA1 mono) • Alva et al. (1986a) reported a better prediction of

Al phytotoxicity by using EaA1 mono over the use of the measured

total soluble Al concentration, or the measured individual Al

species in solution.

Solution cultures may provide well controlled environments

for studying effects of Al on plant growth (Bell and Edwards,

1987). Such experiments often use Al concentrations greatly in

excess of those commonly found in acid soil solutions, coupled

with high phosphate concentrations (Blarney et al., 1983).

Those studies also correlated plant growth with the nominal

(added) Al concentration, or the total measured Al

concentration (Alva et al., 1986a).

On the other hand, when plants are grown in nutrient

solutions in which the activity of monomeric Al species can be

measured and controlled, comparisons between plant species or

growth phases are possible. For example, Alva et al. (1987)

showed that the critical EaA1 mono for growth of roots and shoots

of soybean cultivar IIFitzroyll were 5 and 9 p.M, respectively,

whereas the critical EaA1 mono for nodulation was 0.4 p.M. Other

studies have also shown that nodulation is more sensitive to

Al than the host plant growth (Suthipradit, 1989). By

contrast, Franco and Munns (1982) showed that the primary Al

limitation was on host plant and not on nodulation.

225

Reports on toxic Al concentrations associated with yield

reduction are extensive for a wide range of plant species.

Asher (1981) reported that toxic Al concentrations for various

species grown in flowing solution culture were : maize, > 20

~M; cassava, > 20 - 30 ~M; soybean> 40 ~M; sweet potato, > 40

- 80 ~M; ginger and taro > 80 ~M. While comparisons can be

made among species of plants grown in a common solution or

similar solutions, comparisons between different studies are

difficult. Differences in solution pH, ionic strength,

phosphate concentration, and calcium concentration make such

comparisons nearly impossible. For example, Al va et al.

(1986b) showed that the value of EaAl mono necessary to reduce

root elongation by 50% varied with increasing Ca

concentrations (0.5 to 15 mM cai , this Al varied from 12 to 17

~M for soybean, < 8 to 16 ~M for sunflower, < 7 to 15 ~M for

subterranean clover, and 5 to 10 ~M for alfalfa.

Plant species and cultivars within species vary greatly in

their ability to cope with AI-toxic soils as shown in Figure

3 for six rice cultivars (Fageria et al., 1987). The

physiological processes by which certain plants tolerate high

levels of Al are still unclear. Fageria et al. (1988) have

summarized various proposed mechanisms responsible for

aluminum tolerance in plants. These mechanisms include pH

increases, Al trapping in nonmetabolic sites within plant

cells, greater P, Ca, and Mg uptake and transport and use

226

o IACII3J Y. 107,92- 0'17AI3+ R2•

0'88

D. FERNANDES y. III' 65- CXl8A ,3+ R2.O·60

o MATAO y. 97'60-0'IGAr rf.0·99

• IPEAC5623+ 2

Y ·r08·03rO· OIAI R ·0·98

A lRAT2 Y • 97.3 leoOO04A13+ R2c 0-96

.IPEACOI623+

R2.O·82

Y • 76·57.0.004AI-~ A-I-

100:I:o 90W~ 80

I- 700 600:I: 50(f)

40W> 30

~ 20

.....l 10W~

0 100 200 300 400

0AI+! IN NUTRIENT SOLUTION· (}J M)

Figure 3. Influences of aluminum activities on relative shoot

growth of six rice cultivars (Fageria et al., 1987).

efficiency, lower root CEC, greater root phosphate activity,

higher internal concentrations of si, and higher organic acid

contents. Aluminum tolerance in higher plants appears to be

due to a combination of both exclusion and internal tolerance

mechanisms (Taylor, 1988). The exclusion mechanism refers to

the immobilization of Al at the root-soil interface. Binding

of Al by cell walls, selective permeability of plasma

membranes, plant induced pH barriers, and eXUdation of

227

chelating ligands are known to play a role in Al exclusion

mechanisms. Chelation of Al by carboxylic acids or AI-binding

proteins of the cytosol, compartmentation of Al in the

vacuole, and evolution of an AI-tolerant enzyme systems are

possible internal Al tolerance mechanisms.

Future Knowledge Needs

Activities of Al species in solution probably give the best

indicator of soil acidity. Soil solution extraction is

however, too time-consuming to be used as a routine test. A

relatively quick but reliable chemical test to identify

potentially AI-toxic soils is desirable. The indirect methods

currently used to calculate phytotoxic Al in soil solution

need to be strengthened by direct measurements. A better

understanding of the role of soluble and solid phase organic

components in controlling Al phytotoxicity is also needed.

Answers should be sought regarding the relative toxicity of

Al +3, Al (OH) +2 and Al (OH) / to different plant species as well

as the toxicity of Al polymers. Research efforts should be

focussed on the rhizosphere rather than on the bulk soils.

Aluminum tolerant plants are a partial answer to soil Al

toxicity. Some combinations of agronomic practices to minimize

soil acidification, such as additions of lime or other soil

amendments to reduce Al toxicity, are needed to overcome soil

Al toxicity and to increase crop production in the long term.

228

References

Adams, F., and Z.F. Lund. 1966. Effect of chemical activity ofsoils solution aluminum on cotton root penetration onacid subsoils. Soil Sci. 101 : 193-198.

Adams, F. 1981. Nutritional imbalances and constraints toplant growth in acid soils. J. Plant Nutr. 4 : 81-89.

Alva, A.K., F.P.C. Blarney, D.G. Edwards, and C.J. Asher.1986a. An evaluation of aluminum indices to predictaluminum toxicity to plants grown in nutrient solutions.Commun. Soil Sci. Plant Anal. 17 : 1271-1280.

Alva, A.K., D.G. Edwards, C.J. Asher, and F.P.C. Blarney.1986b. Effect of phosphorus/aluminum molar ratio andcalcium concentration on plant response to aluminumtoxicity. Soil Sci. Soc. Am. 50 : 133-137.

Alva, A.K., D.G. Edwards, C.J. Asher, and S. Suthipradit.1987. Effect of acid soil infertility on growth andnodulation of soybean. Agron. J. 79 : 302-306.

Asher, C.J. 1981. Limiting external concentrations of traceelements for plant growth : Use of flowing solutionculture techniques. J. Plant Nutr. 3 : 163-180.

Bell, L.C., and D.G. Edwards. 1987. The role of aluminum inacid soil infertility. In Soil Management under HumidConditions in Asia (ASIALAND), pp. 201-223. IBSRAM Proc.No.5, Bangkok.

Blarney, F.P.C., D.G. Edwards, and C.J. Asher. 1983. Effects ofaluminum, OR : Al and P : Al molar ratios, and ionicstrength on soybean root elongation in solution culture.Soil Sci. 136 : 197-207.

Bruce, R.C., L.A. Warrell, D.G. Edwards, and L.C. Bell. 1988.Effects of aluminum and calcium in the soil solution ofacid soils on root elongation of Glycine max cv. Forrest.Aust. J. Agric. Res. 39 : 319-338.

Cameron, R.S., G.S.P. Ritchie, and D.A. Robson. 1986. Relativetoxicities of inorganic aluminum complexes to barley.Soil Sci. Soc. Am. J. 1231-1236.

Evans, C. E., and E.J. Kamprath. 1970. Lime response asrelated to percent Al saturation, solution aluminum andorganic matter content. Soil Sci. Soc. Am. Proc. 34 :893-896 .

229

Fageria, N.K., V.C. Baligar, and R.J. Wright. 1987. The effectof aluminum on growth and uptake Al and P by two ricecultivars. Agron. Abstr., pp. 201.

Fageria, N.K., V.C. Baligar, and R.J. Wright. 1988. Aluminumtoxicity in crop plants. J. Plant Nutr. 11 : 303-319.

Fox, R.H. 1979. Soil pH, aluminum saturation, and corn grainyield. Soil Sci. 127 : 330-334.

Foy, C.D. 1964. Toxic factors in acid soils of the southernUnited States as related to the response of alfalfa tolime. USDA Res. Rep. No. 80, U.S. Govt. Printing Office,Washington, D.C.

Foy, C.D. 1984. Physiological effects of hydrogen, aluminum,and manganese toxicities in acid soils. In F. Adams (ed.)Soil Acidity and Liming. 2nd ed. Agronomy 12 : 57-97. Am.Soc. Agron., Madison, WI.

Foy, C.D. 1987. Acid soil tolerance of two wheat cultivarsrelated to soil pH, KCl extractable aluminum and aluminumsaturation. J. Plant Nutr. 10 : 609-623.

Foy, C.D. 1988. Plant adaptation to acid,soils. Commun. Soil Sci. Plant Anal. 19

aluminum toxic959-987.

Franco, A.A., and D.N. Munns. 1982. Acidity and aluminumrestraints on nodulation, nitrogen fixation, and growthof Phaseolus vulgaris in solution culture. Soil Sci. Soc.Am. J. 46 : 296-301.

Hue, N.V., G. R. Craddock, and F. Adams. 1986. Effect oforganic acids on aluminum toxicity in subsoils. Soil Sci.Soc. Am. J. 50 : 28-34.

Jackson, M.L. 1963. Aluminum bonding in soils: a unifyingprinciple in soil science. Soil Sci. Soc. Am. Proc. 27 :1-10.

Kamprath, E.J. 1970. Exchangeable aluminum as a criterion forliming leached mineral soils. Soil Sci. Soc. Am. Proc. 34: 252-254.

Kamprath, E.J., and C.D. Foy. 1985. Lime-fertilizer-plantinteractions in acid soils. In O.P. Engelstad (ed.)Fertilizer Technology and Use, 3rd ed., pp. 91-151. SSSA.Madison, WI.

Kinraide, T.B., and D.R. Parker. 1989. Apparent phytotoxicityof mononuclear hydroxy-aluminum to four dicotyledonousspecies. Plant, Cell and Environment 12 : 909-918.

230

McCormick, L.H., and F.A. Amendale. 1983. Soil pH, extractablealuminum and tree growth on mine soils. Commun. Soil Sci.Plant Anal. 14 : 249-262.

Parker, D.R., T.B. Kinraide, and L.W. Zelazney. 1988. Aluminumspeciation and phytotoxicity in dilute hydroxy-aluminumsolution. Soil Sci. Soc. Am. J. 52 : 438-444.

Parker, D.R., T.B. Kinraide, and L.W. Zelazny. 1989. On thephytotoxicity of polynuclear hydroxy-aluminum complexes.Soil Sci. Soc. Am. J. 53 : 789-796.

Reeve, N.G., and M.E. Sumner. 1970. Lime requirement of NatalOxisols based on exchangeable aluminum. Soil Sci. Soc.Am. Proc. 34 : 595-598.

Suthipradit, S. 1989. Effects of aluminum on growth andnodulation of some tropical crop legumes. Ph. D. Thesis,Univ. of Queensland, Brisbane, Australia.

Tanaka, A., T. Tadano, K. Yamamoto, and N. Kanamura. 1987.Comparison of toxicity to plants Al+3

, AlS04+ and Al-Fcomplex ions. Soil Sci Plant Nutr. 33 : 43-55.

Taylor, G.J. 1988. The physiology of aluminum tolerance inhigher plants. Commun. Soil Sci. Plant Anal. 19 : 11791194.

White, R.E., L.O. Tiffin, and A.W. Taylor. 1976. The existenceof polymeric complexes in soil solutions of aluminum andorthophosphate. Plant Soil 45 : 521-529.

Wright, R.J. 1989. Soil aluminum toxicity and plant growth.Commun. Soil Sci. Plant Anal. 20 : 1479-1497.

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