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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
8.2 Introduction 145
8.3 Materials and methods 147
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
9.2 Introduction 175
9.3 Materials and methods 177
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
9.4.1 Effects of CaC03 and green manure
on the soil solid phase 182
9.4.2 Effects of CaC03 and green manure
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
Attanandana, T. 1982. Fertility problem of acid sulfate soilof Thailand. Ph.D. Thesis. Kyoto Univ., Kyoto, Japan.
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. 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.
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
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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.
Patel, S. 1966. Indian field crops. Agul prakashan, Gujarat,India.
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.
Singhabutra, N., S. Arunin, and Y. Anuluxtipan. 1987.Experiment on two rhizobium strains for inoculation intoSesbania spp. Use as a green manure on the reclamation ofsaline soil. In 25th Annual Technical Meeting, KasetsartUniv., Bangkok. 25 p.
Sratongkam, S. 1976. Technical report on green manure and soilimproving crops. In Proc. of the National Seminar onCropping Systems. Fac. of Agriculture, Chiang MaiUniversity.
Suhayda, C.G., and A. Haug. 1986. Organic acids reducealuminum toxicity in maize root membranes. Physiol. Plant68 : 189-195.
Sukthumrong, A., S. Chotechaungrnanirat, J. Chancharoensook,and V. Verasan. 1986. Effect of green manure-chemicalfertilizer combinations on soil fertility and yield of
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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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Jones, D., and M.J. Wilson. 1985. Chemical activity of lichenson mineral surfaces. Int. Biodeterior. 21 : 99-104.
Kinraide, T.B., and D.R. Parker. 1987a. Nonphytotoxicityofthe aluminum sulfate ion, A1SO/. Plant Physiol. 71 :207-212.
105
Kinraide, T.B., and D.P. Parker. 1987b. Cation amelioration ofaluminum toxicity in wheat. Plant Physiol. 83 : 546-551.
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106
McColl, J.G., and A.A. Pohlman. 1986. Soluble organic acidsand their chelating influence on Al and other metalsdissolution from forest soils. Soil Air Water Poll. 31 :917-927.
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109
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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 Mg (cmol, kg")
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.
Jugsujinda, A., Y. Tadashi, and N. van Breemen. 1978. Aluminumtoxicity and phosphorus deficiency in acid sulfate soilsof Thailand. IRRI Newsl. 3 : 1.
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 Materials and methods
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 Mg (cmol, kg")
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 Results and discussion
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
t Application rates of CaC03 : 0, 4, 8 Mg ha'
*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
t Application rates of CaC03 : 0, 4, 8 Mg ha'
~ 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
8.5 Summary and conclusions
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., G.R. Craddock, and F. Adams. 1986. Effect oforganic acids on aluminum toxicity in subsoils. Soil Sci.Soc. Am. J. 50 : 28-34.
Hue, N.V., and I. Amien. 1989. Aluminum detoxification withgreen manures. Commun. Soil Sci. Plant Anal. 20 : 14991511.
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) .
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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 Materials and methods
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 Results and discussion
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
9.5 Summary and conclusions
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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Nordstrom, D.K. 1982. The effect of sulfate on aluminumconcentrations in natural waters Some stabilityrelations in the system A1203-S03-H20 at 298 "K. GeochimCosmochim. Acta 46 : 681-692.
Olsen, S.R., and L.E. Sommers. 1982. Phosphorus. In A.L. Pageet al. (eds.) Methods of Soil Analysis, Part II, 2nd ed.,pp. 403-430. Agronomy Monog. 9. ASA-SSSA. Madison, WI.
Osborne, J. F.sulfate
1985. End of assignment report to the acidsoils improvement project. Dept. of Land
Development, Bangkok and OverseasAdministration, London. Vol. I and II.
211
Development
Panichapong, S. 1990. Water management for upland crops afterrice on lowland clayey soils : The Thai experience. InProc. 1st Int. Workshop on the Manaqernent; of LowlandClayed Soils for Upland Crops after Rice in Asia, pp.109-120. IBSRAM Proc. No 11. Bangkok, Thailand.
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Shamshuddin, J., S. Paramananthan, and W. Nik Mokhtar. 1986.Mineralogy and surface charge properties of two acidsulfate soils from peninsular Malaysia. Pertanika 9 :167-176.
Soil Survey Staff. 1990. Keys to Soil Taxonomy (4th ed.) SMSSTech. Monogr. No. 19. Cornell Univ., Ithaca.
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.
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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.
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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
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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.
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