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Selected Papers of the Dakar Symposium on Acid Sulphate Soils

Dakar, Senegal, January, 1986

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Selected Papers of the Dakar Symposium on Acid Sulphate Soils Dakar, Senegal, January, 1986

Edited by H.Dost

Publication 44

International Institute for Land Reclamation and Improvement/ILRI P.O. Box 45,6700 AA Wageningen, The Netherlands, 1988.

0 International Institute for Land Reclamation and Improvement/ILRI Wageningen, The Netherlands 1988. This book or any part there of must not be reproduced in any form without written permission of ILRI.

ISBN 90 70754 13 4

Printed in the Netherlands

Preface

This selection of papers reflects the current developments in the study of Acid Sulphate Soils, which were discussed at the Third International Symposium on Acid Sulphate Soils, held in Dakar, Senegal, from 6 to 11 January 1986.

Most of the information that was presented at the Dakar Symposium elaborated on subjects that had been broached at the earlier symposia held in Wageningen (1972) and Bangkok (1981). The conclusions and recommendations of Dakar demonstrate the persistence of the problems that are encountered in investigating and using acid sulphate soils.

A matter of special concern to the editor was the diversity of criteria and terminology that was used by the symposium’s participants. The semantic confusion this caused in the papers exceeded the editor’s capacities in some cases. In part, this imperfection is a symptom of an - in other respects favourable - innovative development, i.e. the tendancy to consider acid sulphate soils as part of soil-water-plant systems and, conse- quently, to widen the disciplinary scope of the subject matter.

In Dakar, the influence of this development became apparent in a shifting of the focus to fresh aspects of conventional subjects. In this context, we mention the presen- tation of several cases of acid sulphate soil phenomena from upland areas. The need to study the variety and variability of essential soil properties over a wide range of temporal and spatial orders of magnitude was emphasized in several presentations. Corresponding innovations were the introduction of geostatistical analysis and im- proved methods of identifying active and potential acidity. The refinement of mathe- matical models that simulate essential processes, and the renewed attention for micro- biological and plant physiological processes, are sustaining this multidisciplinary ap- proach.

In adapted research, the same holds for various papers on pisciculture and forestry as alternatives to the conventional use of acid sulphate soils for rice cultivation and also for papers on the environmental and socio-economic aspects of reclamation pro- jects.

The present volume contains most of the papers that demonstrate this widening diciplinary scope. Most other papers will be published in a complementary volume.

For the editing and translating of the papers into French, the editor is indebted to Dr. Nadia Pons-Ghitulescu.

The Editor.

Avant-Propos

La présente sélection des articles reflète le développement actuel des études des sols sulfatés acides, comme ceci a été mis en évidence durant le 3e Symposium International sur les Sols Sulfatés Acides, tenu à Dakar, du 6 au 1 1 Janvier 1986.

La plupart des informations présentées au Symposium de Dakar concernent des sujets qui ont déjà été traités durant les précédents Symposias de Wageningen (1972) et Bangkok (1981). Les conclusions et les recommandations de Dakar montrent la persistance des problèmes rencontrés dans la recherche concernant la mise en valeur des sols sulfatés acides.

Un problème d’un souci particulier pour le rédacteur a été la diversité des critères et la terminologie variée utilisées par les participants au Symposium. La confusion sémantique dont certaines des communications ont fait preuve a dépassé parfois les capacités du rédacteur. Mais comme partiellement cette imperfection est un sympt6me du développement innovatif de la tendance, entre autre, de considérer les sols sulfatés acides comme partie intégrante du système sol-eau-plante et par conséquence d’élargir la champ disciplinaire du sujet, on peut la considérer comme étant une imperfection favorable.

A Dakar, l’influence de ce développement s’est manifesté par un déplacement du centre d’intérêt vers de nouveaux aspects des sujets conventionnaux. La présentation de plusieurs cas des sols sulfatés acides sur terres élevées peut être un exemple dans ce contexte. La nécessité d’étudier la variété et la variabilité des principales propriétés du sol sur grande échelle dans le temps et dans l’espace a été soulignée dans différentes publications. Des innovations analogues sont entre autre I’introduction d’analyse géostatistique et des méthodes améliorées potentielles de discernement et d’identifica- tion de l’acidité actuelle. Le perfectionnement des modèles mathématiques simulant les processus essentiels, ainsi que l’attention renouvelée pour les processus de microbio- logie et de physiologie des plantes, soutienent I’approche multi-disciplinaire du sujet.

Dans le domaine de la recherche adaptée, le Symposium de Dakar a fourni des articles divers sur la pisciculture et la foresterie comme utilisation alternative à c6té de l’utilisation rizicole. L‘élargissement du champ disciplinaire a été démontré égale- ment par les articles concernant l’étude de l’environnement et des aspects socio- économiques des projets d’amélioration.

La publication présente contient les articles qui soulignent les développements inno- vateurs mentionnés. Les autres contriebution ont été publiés dans un volume complé- mentaire et dans une forme plus économique.

Pour la rédaction des communications présentées en français, ainsi que pour les traductions en langue française, le rédacteur reste obligé à la Dr. Nadia Pons-Ghitules- CU.

Le Rédacteur.

Contents

Page Preface

k Papers on geographic, genetic and systematic aspects of typical Acid Sulphate Soils of coastal lowlands

M.E.F. van Mensvoort and Le Quang Tri Morphology and genesis of actual acid sulphate soils without jarosite in the Ha Tien Plain, Mekong Delta, Viet Nam 1 1

J.Y. Loyer, P. Boivin, J.Y. le Brusq et P. Zante Les sols du domaine fluvio-marin de Casamance (Sénégal): Evolution récente et réevaluation des contraintes majeures pour leur mise en valeur

H. Wada and Bongkot Seisuwan The process of pyrite formation in mangrove soils ' 24

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Papers on Acid Sulphate Soils of uplandareas, pyritic mine spoils and odd lowland sites

D.S. Fanning, P.A. Snow, M.C. Rabenhorst and M.A. EI Desoky Evidence of eluviation-illuviation of sulphur and heavy metals in Sulfaquepts in recent Baltimore harbor (MD) dredged materials 38

N. Prokopovich Engineering impacts of acid sulphate deposits in California

I.D. Pulford, C.A. Backes and H.J. Duncan Inhibition of Pyrite oxidation in coal mine waste

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Papers on field identification, diagnostic criteria and classification of Acid Sul- phate Soils

P.A. Burrough, M.E.F. van Mensvoort and J. Bos Spatial analysis as a reconnaissance survey technique: an example from acid sulphate soil regions of the Mekong Delta, Viet Nam 68

L.J. Pons Should acid sulphate soils be classified among the inceptisols or the entisols? 90

Papers on analysing and monitoring dynamic processes in Acid Sulphate Soill waterlplant systems. Physico-chemical, microbiological and plant physiological parameters. Models

J. Bouma Using morphological data for the simulation of water regimes in clay soils 97

I C.J.M. Konsten, W. Andriesse and R. Brinkman A field laboratory method to determine total potential and actual acidity in acid sulphate soils 106

Le Ngoc Sen The evaporation and acidification process in an acid sulphate soil 135

K. Prade, J.C.G. Ottow and V. Jacq Excessive iron uptake (iron toxicity) by wetland rice (Oryza sativa L.) on an acid sulphate soil in the CasamanceISenegal 150

Papers on improvement of Acid Sulphate Soils and the selection oftolerant culti- vars

M. Agyen-Sampong, K. Prakah-Asante and S.N. Fomba Rice improvement in the mangrove swamps of West Africa 163

Papers on alternative approaches to the utilization of land and water with Acid Sulphate restrictions. Environmental aspects

A. Doyen La mangrove à usages multiples de l’estuaire du Saloum (Sénégal)

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176

L. Le Reste Conséquences sur l’environnement aquatique et la pèche d’un barrage-écluse anti-sel en Casamance (Sénégal) 202

V.P. Singh, A.T. Poernomo and R. Brinkman Reclamation and management of brackish water fish ponds in acid sulphate soils: Philippine experience 214

Papers on socio-economic aspects of reclamation projects in Acid Sulphate Soil areas

J. van der Klei Soil reclamation: a technical or a social-economic problem? Reclaiming the acid- sulphate soils in the tidal swamps of Basse Casamance, Senegal 229

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1 D.L. Ligue and V.P. Singh Social and economic status of farmers in acid sulphate soil areas in The Philip- pines 238

Morphology and genesis of actual acid sulfate soils without Jarosite in the Ha Tien Plain,

I Mekong Delta, Viet Nam M.E.F. van Mensvoort Agricultural University, Wageningen, The Netherlands

Le Quang Tri Can Tho University, Viet Nam

1 Summary

Actual acid sulfate soils without jarosite in the sulfuric horizon occur widely in the north-western part of the Mekong Delta. They differ morphologically from acid sul- fate soils with jarosite in the following aspects: they have peaty clay parent material; have a dark brown subsoil; show low porosity and saturated hydraulic conductivity; pyrite is intensively mixed in the matrix of the subsoil; the sulfuric horizon still contains pyrite; they are potentially acid to great depth.

The absence of jarosite is probably due to the low redox potential in the sulfuric horizon. Redox potential is kept low by the organic matter. Under such conditions, pyrite only oxidizes to ferrous sulfate, not to jarosite.

Résumé

Des sols sulfaté-acides sans jarosite, mais présentant toutes les autres caractéristiques de ces sols (pH < 3,5 dans l’horizon oxydé, teneur élevée en sulfures, teneur élevée en Al et Fe dans la solution de sol, saturation élevée en Al), apparaissent sur de grandes superficies dans la partie nord-ouest du Delta du Mekong, dans la Plaine de Ha Tien. Morphologiquement ils diffèrent des sols sulfaté-acides à taches de jarosite par: - un materiau origine1 tourbeux; - un sous-sol brun foncé (10 YR 4/3) a brun gris (10 YR 4/1) qui apr& avoir été

exposé à l’air, devient en quelques minutes gris très foncé (N 3/0); - une porosité et une conductivité hydraulique en milieu saturé, basse; - un horizon sulfurique contenant encore de la pyrite; - une teneur en pyrite assez importante, intimement melangé dans la matrice du sous-

sol.

L’analyse de I’acidité totale et potentielle (Mé!hode Konsten et al 1986), montre que le sous-sol de ces sols présente une acidité actuelle et potentielle beau’coup plus élevée que les sols sulfaté-acides à taches de jarosite.

L’absence du jarosite est probablement due au faible potentiel redox de l’horizon sulfurique, qui est une conséquence de la haute teneur en matière organique. Dans telles conditions, la pyrite est oxydée en sulfate ferreux et non pas en jarosite.

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2 Introduction

Field studies of the acid sulfate soil areas of the Mekong delta revealed the occurrence of extensive tracts of acid sulfate soils without the conspicuous straw-yellow jarosite mottles in the oxidized zone, but having all other properties associated with acid sulfate soils: a very low pH < 3.5 in the oxidized horizon; high sulfate, aluminum and iron contents in the soil solution; a high Al saturation percentage. Detailed soil profile descriptions of such profiles were made in the field. Samples for thin sections were taken from actual acid sulfate soils without and with jarosite, for micromorphological study.

3 Morphological observations

The morphological studies showed differences in several characteristics between pro- files with and profiles without jarosite.

Profiles withoutjarosite generally have a dark brown ( 1 OYR 4/3) to dark grey (IOYR 4/1) subsoil. Upon exposure to the air, the colour of the subsoil of these profiles with- out jarosite changes to very dark grey (N 3/0) within a few minutes. In many cases profiles with jarosite have grey, blueish grey or greenish grey subsoils with a value higher than 4, and in variably a chroma of 1. The colour does not change upon exposure.

The subsoils of profiles without jarosite have a high organic matter content (10-15 mass percent). The organic matter is intensively mixed in the soil matrix, fibrous, and densely packed. The subsoils of profiles with jarosite have a low organic matter con- tent. Organic matter in these soils is usually found as partly decomposed root or leaf remnants.

The subsoils of profiles without jarosite have few very fine pores, and a low saturated hydraulic conductivity. The porosity in the subsoils of profiles with jarosite is high: medium sized or coarse continuous vertical pores are common and there are many fine pores. The saturated hydraulic conductivity is high.

Micromorphological observations confirmed the finely fragmented nature of the organic matter and the intensive mixing with the matrix clay in the suboils of the profiles without jarosite. A part of the organic matter could be recognized as having cellular structure. Most of these parts were only fragments of leaves or roots. Pyrite, too, was found intensively mixed into the clay matrix. It did not only occur associated with organic matter, but also in the clay, separated from organic matter.

In acid sulfate soils with jarosite, little or no finely fragmented organic matter is found mixed with the matrix clay. Clearly recognizable half decomposed roots and leaves form the majority of the organic matter in these subsoils. The pyrite is concen- trated in the root channels and associated with the organic matter.

The lower part of the sulfuric horizon of profiles without jarosite still contains pyrite, seen as isolated pyrite framboids. In sulfuric horizons of acid sulfate soils normally pyrite and jarosite are spatially separated with the exception of cases in which the initial supply of oxygen is abundant and sudden, for example upon artificial drainage (Miedema et al. 1974). The presence of small remnants of pyrite evenly distributed over the lower part of the sulfuric horizon in acid sulfate soils without jarosite may

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be an indication of the slowness of the pyrite oxidation in these soils. In a few places pyrite centres are surrounded by iron (hydr)oxydes but the structure

of the framboids is still visible. This phenomenon may have to be attributed to quick oxidation during preparation of the thin sections. No pyrite was seen in the lower part of the sulfuric horizon of profiles with jarosite.

Thin sections of the sulfuric horizon of profiles without macroscopically visible jaro- site do show some small traces ofjarosite along the few macropores present. However,

nels, as is the case in the acid sulfate soils with jarosite mottles. Analysis of total and potential acid by the method of Konsten et al. (1986) has

shown that the subsoils and substrata of soils without jarosite have much higher actual + potential acid contents then soils with jarosite (Brinkman et al. 1986). Pyrite con- tents up to 4% using the method of Begheyn et al. (1978) have been found in the pyritic subsoil of the profiles without jarosite.

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these are incorporated in the soil and do not occur as pure jarositans along root chan-

4 Discussion: genesis of acid sulfate soils without jarosite

4.1 Sedimentation of the parent material

The high organic matter content of the sediment can be explained by the age of the sediment: it was formed more than 5500 years ago in a period of rising sealevel and slow sedimentation affectinglarge areas (Pons 1986; Brinkman et al. 1986). The frag- mented and fibrous nature of the organic matter and the intensive mixing of organic matter and pyrite with the matrix clay indicate that originally clayey sediments and peat must have been eroded, reworked, mixed and redeposited by action of the sea. The low macroporosity of the sediment indicates that there was no significant period of mangrove forest vegetation after the redeposition because mangrove roots normally produce common coarse and medium tubular pores.

4.2 Drainage and acidification

When such a dense, highly organic sediment with low porosity is drained, it remains water-saturated due to its spongy structure, and oxidation is very slow. Oxygen dif- fuses slowly into the water-saturated sediment; microbial decomposition of organic matter keeps the Eh low. Therefore it is likely that upon drainage of the sediment the Eh rises to a level which permits the oxidation of sulfide to sulfate, but not the oxidation of Fe (11):

Fes, + 7/2 O2 + H 2 0 -+ Fe2+ + 2SO:- + 2H+

Jarosite is formed at low pH (less than 4.0) and at Eh higher than about 400 mV (van Breemen 1976). At lower Eh values, pyrite can still be oxidized, but only to dis- solved ferrous sulfate, pH values measured in the sulfuric horizon of acid sulfate soils without jarosite are in the range of 2.4 - 2.6; Eh values are between 300 and 400 mV, measured by pushing a Pt electrode directly into the mud, after standardization by comparison with a ferrous/ferric solution with an Eh of 430 mV.

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The presence of high concentrations of ferrous iron in the Vietnamese acid sulfate soils without jarosite was confirmed by a field test. An aqueous solution of 5% potas- sium ferric cyanide was added to the soil solution from sulfuric horizons in profiles without jarosite, and showed a strong colour change to dark blue (ferrous ferric cyan- ide), indicating high concentrations of dissolved ferrous iron. Laboratory analysis showed ferrous iron concentrations in the vicinity of 500 to 1000 ppm Fe, or pFe between 1.5 and 2.

A stability diagram (Figure 1) of jarosite and dissolved ferrous sulphate was calcu- lated for various levels of pFe. Lower valuks of pFe (meaning high concentrations of ferrous iron) move the boundary between the stability zone of jarosite and dissolved ferrous iron sulfate downwards considerably. However, the above mentioned values of Eh and pH show stability of dissolved ferrous sulfate.

Micromorphological observations confirmed the slow progress of the oxidation process by the presence of partly oxidized pyrite in the lower part of the sulfuric hori- zon.

80C

600 Eh

millivolts

400

200

O

-200

O I i 3 4

PH

pFe=4

pFe=3

pFE= I . 5 p m = 2

Figure 1 Stability diigram of jarosite and dissolved sulfate. LineS indicate solubility isotherms for jarosite at different levels of Fe2+. Shaded area indicates pH and Eh measurements in Vietnamese acid sulfate soils without jarosite.

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Acknowledgements

The authors would like to thank Dr. N. van Breemen and Dr. R. Brinkman for critical comments on the paper.

1 References

Begheijn, L. Th., N. van Breemen, and E.J. Velthorst 1978. Analysis of sulfur compounds in acid sulphate soils and other recent marine soils. Commun. in Soil Science and Plant Analysis, 9 (9), 873-882.

Brinkman, R., Nguyen Bao Ve, Tran Kim Tinh, Do Phuoc Hau, and M.E.F. van Mensvoort 1986. Acid sulfate materials in the Western Mekong Delta, Viet Nam. This symposium.

Konsten, C.J.M., W. Andriesse, and R. Brinkman 1986. A field laboratory method to determine actual and potential soil acidity in areas with acid sulphate soils. This symposium.

Miedema, R., A.G. Jongmans, and S. Slager 1984. Micromorphological observations on pyrite and its oxidation products in four Holocene alluvial soils in the Netherlands. In: G.K. Rutherford (ed.). Soil Microscopy. Proceedings of the 4th international meeting on soil microscopy. Limestone press, Kingston, Ontario. p. 772-794.

Pons, L.J. 1986. Wetland soils with high sulfide contents. This symposium. Van Breemen, N. 1976. Genesis and solution chemistry of acid sulphate soils. Agric. Research Reports

848, Centre for Agric. Publishing and Documentation, Wageningen, The Netherlands.

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