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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
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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
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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
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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 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.
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References
Coulter, J.K. 1952. Gelam Soils. Malay. Agric. Journal, Volume 35,
No. 1.
Dennet, J.H. 1933. The classification and properties of Malayan Soils.
Malay. Agric. Journal 21, No. 8.
Gopinathan, B. 1973. The semi-detailed soil survey of the West Johore
coastal plain. Unpublished report, Department of Agriculture, Peninsular
Malaysia.
Joseph, K.T. and N. Maarof 1975. 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. 383-
391.
Paramananthan, S. 1976. Soils of the West Johore area. Soil Correlation
Report No. 111976, Ministry of Agriculture and Rural Development,
Malaysia.
Powlson, D.S. 1976. The identification of Potential Acid Sulphate Soils.
MARDI Res. Bulletin 4, 1: 34-43.
USDA 1974. Soil Taxonomy. Agriculture Handbook No. 463.
Wilshaw, R.G.H. 1940. Notes on the development of high acidity in certain
coastal clay soils of Malaya. Malay. Agric. Journal, Volume 28,
No. 8.
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