STUDIESON

MACROMESOAND MICROMORPHOLOGYAND CLAYMINERALOGYOFTHE ACID SULPHATESOILSOF KERAlA

BY

M. SUBRAMONtAIYER

TH ESIS

Submitted in partial fulfilment of therequirements for the degree

DOCTOR OF PHILOSOPHY IN AGRICULTURE

Faculty of AgricultureKerala Agricultural University

l'

DEPARTMENT OF SOIL SCIENCE AND AGRICULTURAL CHEMISTRYCOLLEGE OF AGRICULTUREVEllAYANI - TRIVANDRUM

1989

CERTIFICATE

~~-

Certified that this thesis entitled "Studies on macro, meso and

that i t has nC?t previously formed the basis for the award of any

degree, fellowship or associateship to him.

Chairman.Advisory CommitteeProfessor and Head

Department of Soil Science and Agricultural Chemistry

Vellayani

29th April, 1989~

micromorp hology and clay mineralogy of the acid sulphate soils of

KeralaII. is a record of research work done independently by

Sri. M. Subramonia Iyer, under my guidance and supervislOn and

ABSTRACT

The acid sulphate soils of Kerala cover an area of approxi-

mately 0.2 million hectares on the West coast of Kerala.

A well integrated study on the genesis, morphology, mineralogy

and certain physico-chemical properties of these soils was conducted.

Aspects of its genesis, position in the global system of classification,

macro, meso and micro morphology, mineralogy both macro and micro

as well. as primary and secondary, physical and physico-chemical

properties relevant to classifica tion and management have recei ved

. attention. The salient points of the study are highlighted emphasising-

conclusions pertinent to the expanding frontiers of our knowledge on

these soils.

The acid sulphate soils occurring along the West coast of

Kerala based on morphological observations as well as stage by stage

microscopical study are indicated to have been formed by sedimentation

of finer material overlying and often impregnating wood. fossils, faunal

and floral relics in the recent geological past. The extent of alluvia-

tion, sta,lge of degradation of the fossilised wood and incorporation.it

of sediments and formation of secondary products vary from location

to location as indicated in the study.

In almost all the acid sulphate soils around the globe, pyrite

is the major mineral component contributing to acidity. They have/

framboidal micro structure with size ranging from 15-50 u, However,

in the acid sulphate soils of Kerala pyrroh ti te ( FeS ) is the

predominant sulphide mineral with a size range < Z /J along with small

amounts of pyrite (FeSZ) which also are <.. Zp. These minerals have

been formed in the recent geological past, under the anaerobic

environment releasing ferrous iron from the alluviated soil materials

esp~ciapy laterite falls and the sulphur from the sulphates added

by the ingress of sea water, fossilised wood and decaying organic

matter.

The p yri te (FeSz) and pyrroh ti te (FeS) undergo oxidation

especially when the aeration is encouraged by tidal influences, and

acidi ty conditions. As has been demonstrated in the present study

they are oxidised initially to ferric hydroxides and then to

jarosi tes. The end product of oxidation of pyrite, however, varies

from situation to situation in Kerala. Thus it may be j arosi tes as

encountered in the surface soils of all the locations while the

oxidation may be only to the stage of fe"ric hydroxide as observed

in the fourth horizon of Mathikayal and Kattampally where the pyrite

occurs overlying a bed of lime shells.

The ripeness of the acid sulphate soils generally are decided

by the extent of acidity generated on oxidation with HZOZ and also

the' n I value of the soil which is related to the pH as well as the

organic matter and clay content. On this basis all the acid sulphate

soils of Kerala vary from half ripe to fully ripe. The ripe Soils

have been encountered only at Kattukambal in the kole area. Another

factor. is the depth of occurrence of the jarosite mottle laden layer.

In the acid sulphate soils of the present study. the jarosites have

been located. within 50 em. Below 50 em pyrite and pyrrohtite are

the dominant sulphur containing minerals. Among the oxidation products.

the mineral lepidocrocite (Fe-O-OH) a variant of goethite has been

found to be associated with pyrrohti te ~specially at the Karumadi

location. The occurrence of lepidocrocite in acid sulphate soils has

not been reported earlier. It is possible that it is the intermediate

stage in the oxidation of pyrrohtite to jarosite.

The lower layers. of the acid sulphate soils have lime shells

in some locations such as Mathikayal t other kayal areas of Kuttanad

and some of the Qokkali and kaipad soils. The pyrites are found

to be closely overlying the lime shells without suffering major alter-

ations to either of them.. Thus the pyrite fram boides have been

transformed partly in a few of i ts micr~ystals to the oxidised form

of ferric hydroxides. but the acidity of the embedding soils has

nei ther dissolved nor reacted with the lime shells. .

X-ray diffraction. thermogravimetric as well as chemical

studies conducted with the clay indicate kaolinite as the dominant

mineral in these soils. Minor quantities of '. mineral viz.. smectite.

chlori te and illite have' been detected. Mica and quartz have been

found in quantities equal to that of kaolinite. It is possible that

like laterite falls. fine quartz also is alluviated into these soils

from the midland regions of Kerala.

Soils which attain pH of Z. 5 by oxidation with 100 per cent

HZOZ have been considered to be dangerously acid sulphate soils.

The soils in the present study attain pH values less than Z. 5 even

with 30 per cent HZOZ' Though the upper layers are half to fully

ripe, they are still found to be dangerously acid. The lower layers

with more of reserve pyrites are much more dangerously acid. These

observations on the acid sulphate soils are a pointer to the cropping

patterns and water management to be pursued in these areas in the

foreseeable future. Thus the r:ice crop in a waterlogged si tua tion

is possibly the only crop that may throw up lesser problems in

soil management. The dangerous nature of the potential acidity

especially in the lower layers have to be considered in ruling out

all propositions of tree crop alternatives such as rubber and oil

palm.

From the present studies it has ~en found that these soils

have only a sulphidic enrichment within 50 cm from the surface,

instead of a sulphuric horizon. Further the sulphur content is also

not sufficient enough to include them in the category of soils with

sulphuric horizon. Soils with sulphuric horizon only are considered

to attain a pH of 4 by airdrying and Z. 5 by HZOZ treatment.

soils in the present study despite an enrichment, only with sulphidic

The

materials attain a pH of Z. 5 to 3.0 by mere airdrying and pH as

low as Z.O by HZOZ oxidation. Thus the lack of a sulphuric horizon

but in its place a mere sulphidic enrichment makes them almost

dangerously acid. Evidently this has to be attributed to the

pyrrohtitic (FeS) nature and the smaller size «. ZfU) of the framboid

conferring it to be placed as a class separate from typical acid

sulphate soils reported from the rest of the world. Thus these soils

from the present study are found to be more dangerously acid sulphate

than others though they do not satisfy the requirement of either

the total sulphur content or the presence of a typical sulphuric

horizon. Further the pyrite framboids have a size range of only 1/25th

of that of the framboids reported elsewhere from the globe. This

has made them more dangerously acid. All the same. they require

to be placed as soils with sulphidic materials rather than the soils

with sulphuric horizon as per Soil Taxonomy. (USDA, 1975) .

Nevertheless. the fact remains that these soils are highly acidic

with considerable amounts of reserve pyrites and hence reserve

acidi ty . These soils call for utmost care in their management. The

acid sulphate soils of Kerala are thus to be continuously managed

under a waterlogge~ milieu to enable OPtim~m productivity with minimum

problems due to acidity and related aspects. The possibility of

growing perennial crops such as rubber and. oil palm require partial

to fully aerobic situations. These conditions are likely to result in

oxidation of the p'yrite laden layer noticed up to 90 cm in the present

study. This can generate an enormous quantity of free sulphuric acid.

The pyrite laden layer extends up to 90cm in the present study.

The acid sulphate soils of Kerala are defini tely different

from Malaysian acid sulphate soils in that the surface layer is not

completely free of pyrites. In view of the fact that the surface soil

contains only jarosite and no pyrites> the Malaysian peats and acid

St4tphate soils have been subjected. to cultivation with oil palm and

rubber. However the Malaysian experience cannot be transplanted as

such to Kerala inview of the very serious initial problems likely

to be thrown off by the generation of acidity by the oxidation of

pyrites under aerobic si'b&ations.

Being half to full ripe. acid sulphate soils of Kerala are

still dangerously acid to warrant the continua tion of the existing

management practices and cropping systems.