PROBLEMS OF CLASSIFYING SOILS WITH SULFIDIC HORIZONS IN PENINSULAR MALAYS IA

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1 PROBLEMS OF CLASSIFYING SOILS WITH SULFIDIC HORIZONS IN PENINSULAR MALAYS IA S. Paramananthan Department of Agriculture Peninsular Malaysia, and B. Gopinathan Malaysian Agricultural Research and Development Institute Serdang, Selangor, Malaysia 1 Summary An acid sulfate soil sequence developed on coastal clay soils in Peninsular Malaysia is described. Soil ripening after the construction of bunds and drains has resulted in the development of sulfuric and cambic horizons. The underlying material is commonly sulfidic. Two similar sequences are recognized, one on marine alluvium, and the other on brackish water deposits. The latter is characterized by higher organic matter content. The problems associated with mapping and classification are noted, and changes in the Soil Taxonomy to cater or the anomalies are proposed. 2 Introduction Peninsular Malaysia is situated between latitudes lo 13" and Ï'ON and longitudes 99'E and 104' 20'E. The physical relief is dominated by the Central Mountain Range which runs nearly through the middle of the Peninsula. The west coast is dominated by clayey deposits of marine and brackish water origin, while the east coast is dominated by sandy beach ridges. This difference in geomorphology is attributed to the calm seas of the Straits of Malacca off the west coast and the relatively turbulent South China Sea off the east coast. The coastal clays extend as an almost continuous block along the west coast and is only broken by occasional islands of sedimentary and 96

2 igneous rocks. The coastal plain varies in width from about 10 km to 50 km. The eastern boundary often grades into peat swamps. The coastal plain which averages in height from 0-15 m above mean sea level is extensively used for agriculture. The agricultural development which began in the early part of this century has been made possible by the construction of a coastal bund to keep out inundation by sea water and the reclamation of these areas by drainage. Rice, rubber, oil palm, coconut, cocoa and coffee are the main crops cultivated on these reclaimed areas. This paper deals mainly with the soils which were developed when large parts of this coastal plain were bunded and drained for the cultivation of perennial crops. 3 Literature Review Prior to the reconnaissance soil survey of Peninsular Malaysia, very little was known about the coastal clay soils. Dennet (1933) recognized that soils with high concentrations of acid were associated with the nipah palm, and Wilshaw (1940) reports the occurrence of acid conditions on draining coastal soils. Coulter (1952) referred to soils containing considerable amounts of pyrite as gelam (MeZaZeuca leucodendron) soils. A mixture of circumstantial and intrinsic soil properties was originally used to distinguish potential and actual acid sulfate soils from soils not showing or not expected to develop extreme acidity on drainage and cultivation. Systematic development of morphometric criteria started around 1967 on the basis of experience in comparable tropical countries when the term 'cat clay' or acid sulfate layer as a diagnostic horizon definei tentatively as one with a ph of about 3.3 or less on the air drysoil and a soluble sulfate content in the air dried soil exceeding 0.1 percent was introduced. The morphometric criteria proposed by Soil Taxonomy (USDA 1974) for sulfidic materials and sulfuric horizons were tested for Malaysian conditions since 1973 using the earlier drafts of Soil Taxonomy when systematic detailed mapping of the coastal plain was initiated by the Soil Survey Section of the Department of Agriculture in West Johore. These studies (Gopinathan 1973, Joseph and Maarof 1975, Paramananthan 1976) made it possible to recognize sequences of 97

4 4 Profile Development On ripening, the alluvial deposits give rise to the development of cambic or sulfuric horizons with a moderately developed structure and In1 values < 0.7. Straw-yellow jarositic mottles with hue 2.5 or yellower and chroma of 6 or more are often seen together with strong brown (7.5YR 5/6-5/8) and brownish yellow (IOYR 6/6-6/8) mottles, particularly on brackish water deposits, which normally have a high organic matter content. The brown mottles tend to dominate in the upper part of the diagnostic horizon, the straw-yellow in the lower part, the latter often being associated with old root channels. This distribution pattern is most obvious in Sulfic Tropaquepts like Tongkang A (Figure 1). The cambic or sulfuric horizon is normally underlain by a sulfidic C-horizon with Inr values > 0.7, greenish gray (5GY 6/1) OK dark blue (5B 5/1) in colour and with many decomposing pieces of wood and old root channels, through which water flows easily. Depth of the sulfidic C-horizon ranges from less than 50 cm to more than 1 meter. 5 Mapping and Classification Identification of cambic horizons in the field is normally no problem; neither is the determination of their ph OK the recognition of jarositic mottles. Identification of sulfidic soil material, however, remains a practical difficulty as it requires laboratory determinations of at least ph of fresh and incubated samples (Powlson 1976) and preferably, determination of total sulfur. This difficulty complicates identification of superficially developed soils in all clayey alluvial parent material but it can be overcome by organizational methods, and identification and delineation of mapping units henceforth can proceed as a routine operation, once locally applicable field criteria have been defined for the recognition of potential acid sulfate soil horizons. Difficulties of a more fundamental nature need to be overcome when one tries to correlate mapping units based on local and practical distinguishing criteria, with a world-wide soil classification system, viz. Soil Taxonomy, which strives to express distinguishing criteria of general genetic significance. Acid sulfate soils are anomalous in the sense 99

5 that their intensive and rapid chemical dynamics may overrule the more slowly developing properties which are normally accepted as parameters for soil development. In Soil Taxonomy the cambic horizon, which normally is the main diagnostic horizon for the Inceptisols has been overruled in acid sulfate soils by the sulfidic subsoil where its upper limit occurs within 50 cm. The sulfidic material derives its importance from its chemical acidifying potential, to which the genetic significance of the cambic horizon has been sacrificed in this case. In Malaysia several soil series are recognized with sulfidic materials within 50 cm but overlain by well developed cambic and/or sulfuric horizons e.g. Sekat series, Sedu A series (Figure I). With the present definitions these soil series would partly fit into Typic Sulfaquents, and partly into the Typic Sulfaquepts, although for practical and genetic reasons they are similar, and very different from Typic Sulfaquents without a cambic horizon such as the Linau series. Therefore it does not appear logical to separate them at the order level. A better solution would be to recognize the sulfidic soil material as an indicator of potential acidity, ds a secondary diagnostic horizon for the distinction of classes at the subgroup level analogous to the sulfuric horizon, which is subordinate to the cambic horizon in Soil Taxonomy. With this status also the presence of sulfidic material between 50 cm and 100 cm in Inceptisols without a clear sulfuric horizon, could be given due recognition. With the present criteria such soils are classified as Typic Tropaquepts, e.g. Jawa B and Tongkang B in Figure I. Hence the authors propose that a sulfidic subgroup be established to indicate soils having a cambic horizon overlying sulfidic material within 1 meter. The above mentioned Jawa B and Tongkang B series would thus be classified as Sulfidic Tropaquept. Soils with sulfidic material within 50 cm but overlain by a well developed cambic and/or sulfuric horizons would then fall into the Sulfidic Tropaquepts and Typic Sulfaquepts respectively (Figure 2). 1 O0

A A 0 0 C C Taxonomy Typic Sulfaquent Typic Tropoquept Typic

Sulfidic Tropaquept 1 I I I NOTE:- 11//1 Jarosite Mottles ure 2.

6 A A 0 0 C C Taxonomy Typic Sulfaquent Typic Tropoquept Typic Sulfaquent posed Changes Typic Sulfaquept Sulfidic Tropaquept Sulfidic Tropaquept 1 I I I NOTE:- 11//1 Jarosite Mottles ure 2. Profile morphology to illustrate proposed changes in the Soil onomy Acknowledgement! authors wish to thank the Director Generals of the Department of,iculture and MAl2DI for their permission to present this paper. 101

7 References Coulter, J.K Gelam Soils. Malay. Agric. Journal, Volume 35, No. 1. Dennet, J.H The classification and properties of Malayan Soils. Malay. Agric. Journal 21, No. 8. Gopinathan, B The semi-detailed soil survey of the West Johore coastal plain. Unpublished report, Department of Agriculture, Peninsular Malaysia. Joseph, K.T. and N. Maarof A field study showing classical changes in acidity over large areas in the Parit Jawa area of Johore during the last decade. Proceedings Third ASEAN Soils Conference, p Paramananthan, S Soils of the West Johore area. Soil Correlation Report No , Ministry of Agriculture and Rural Development, Malaysia. Powlson, D.S The identification of Potential Acid Sulphate Soils. MARDI Res. Bulletin 4, 1: USDA Soil Taxonomy. Agriculture Handbook No Wilshaw, R.G.H Notes on the development of high acidity in certain coastal clay soils of Malaya. Malay. Agric. Journal, Volume 28, No o2

Pyrite in acid sulfate soils: transformation and inhibition of its oxidation by application of natural materials

Symposium no. 28 Paper no. 97 Presentation: oral Pyrite in acid sulfate soils: transformation and inhibition of its oxidation by application of natural materials SHAMSHUDDIN Jusop and SARWANI Muhrizal