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    RECOGNITION AND PREDICTION OF ACID SULPHATE SOIL CONDITIONS

    R. Brinkman and L . J . Pons Department of SoiZ Science m d Geology University o f Agriculture, Wageningen

    INTRODUCTION

    Time and change in acid sulphate soils

    A s long as sediments or soils containing pyrite remain reduced, they are subject

    to little change apart from possible transformation of other sulphides to pyrite; recrystallization of pyrite, and l o s s of water. Such soils varying little with

    time can be described in terms of relatively few characteristics.

    Soils containing acid sulphates are transients, the acids present being neutrali-

    zed or leached out again within periods of a few years to, at the outside, a hundred years or so . From a geological viewpoint the formation and subsequent

    disappearance of acid sulphate soils is a virtually instantaneous process; it is also very rapid compared with most processes of soil formation. A s in other

    sciences, analysis Of transients covers a broader spectrum than analysis of phenomena that are more constant with time.

    In the study of this transient phenomenon, the initial state, i.e. the nature

    of the supposedly potential acid soil, is only one of the aspects to be consi- dered. Other aspects are the rate of development: the speed of oxidation in one

    place and the downward speed of the oxidation front, for example, as well as the rate at which acids are leached. Since an acid sulphate soil is a soil in

    rapid flux, even the measurement of a similarly low pH at similar times in

    two succeeding years may not mean the same thing twice: the acid formed in the previous year may have leached out and a similar quantity formed with the

    second year's seasonal oxidation, decreasing the reserve of potential acid (pyrite).

    Of great practical importance in reclamation is not only the expected degree of

    acidity, but also the expected time span of its occurrence. Present methods of identifying and predicting acid sulphate soils take into account quantities only,

    and not time aspects.

    Economic and social interest

    The identification and prediction of acid sulphate soils is extremely important

    for economic reasons, but even more so for social reasons.Many times in the past

    as well as in the present - and in many parts of the world - much of the land in

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  • a new polder has proved unsuitable for crops because of acid sulphate conditions.

    This has resulted in a low standard of living or even abandonment of certain areas.

    Similar problems have sometimes arisen in places where drainage was improved.

    The relatively short period of, say, 40 years between the development and disap- pearance of toxic conditions may economically and socially cripple two generations

    of farmers, and prevent repayment of the investments.

    Other factors remaining equal, the physical quality and suitability for agricul-

    ture of clayey soils tend to increase with increasing pyrite contents in the origi-

    nal reduced sediment, up to the point where toxicity begins to play a part. With

    still higher pyrite contents, there is an abrupt drop in suitability from very

    high to virtually useless, at least for dry-land crops.

    Sound identification is therefore necessary before one can decide whether recla-

    mation is justified; if s o , one can work out an appropriate economic management

    system for the expected soil conditions before embarking upon reclamation; if not,

    one can decide u?on alternative development elsewhere.

    This great economic and social interest, besides the obvious attraction of a rap-

    id, explain the long and, venerable list of investigators studying, predicting, and

    identifying acid sulphate soils throughout the last two centuries.

    relatively complicated, natural process occurring right under our eyes, may

    Practical problems

    The immediate causes of failure or of uneconomically low yields of upland crops

    on acid sulphate soils appear to be a combination of some or all of the following:

    a) inhibition of root growth due to aluminium toxicity (e.g. Rees and Sidrak

    1961, Cate and Sukhai 1964);

    b) in severe cases also ferric iron or manganese toxicity;

    c) probably also extreme acidity;

    d) in combination with these three, but also after their elimination by leach-

    ing and mineral dissolution, the - perhaps temporary - low fertility level; e) the generally very low permeability of the subsoil due to inhibited soil

    ripening, causing a low water storage capacity of the soil profile and hazards of

    temporary waterlogging and sometimes irreversible drying of peaty topsoils.

    The low fertility level, which is characteristic for acid sulphate soils but not

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  • restricted to them, is due in some cases (Rhizophora muds) to the high C/N ratio

    of the organic matter, often occurring in high percentages; a low rate of N accu-

    mulation due to the very low microbiological activity; the very high exchange-

    able aluminium percentage; the precipitation of phosphate by soluble aluminium;

    the low contents of micro-nutrients like Cu, Mn; the generally low content of ra-

    pidly weatherable minerals; and, in severe cases, the low cation exchange capa-

    city due to preferential acid dissolution of smectites and illite. Aluminium tox-

    icity may also be induced or aggravated even in old leached acid sulphate soils

    without free (soluble) acids by cation exchange due to fertilizer applications

    without the addition of adequate amounts of lime. Excessive liming may induce other

    micro-nutrient deficiencies.

    The problems for wet rice cultivation are different in kind but similar in sever-

    ity. If an acid sulphate soil is reduced too long before rice is sown, the young

    seedlings may die due to H z S or ferrous iron toxicity. Older plants, e.g. replan-

    ted rice, have more resistance, probably because of the oxygen flow through their

    roots. On the other hand, if rice is sown or transplanted on an acid sulphate soil

    too soon after water saturation, aluminium toxicity inhibits root growth and the

    seedlings may die.

    Although acid sulphate Soils are identified and predicted on the basis of free

    acids present or expected, clearly these acids are only part of the problems

    faced by users of acid sulphate soils.

    Terms and definitions

    Before embarking upon a discussion on identification and prediction of acid sul-

    phate soils, we should delimit the subject by defining some terms.

    An (actual) acid sulphate Soil is a soil with one or more horizons consisting of

    (actual) acid sulphate materials, i.e. materials containing soluble acid alumini-

    um and ferric sulphates in concentrations toxic to most common dry-land crops.

    Such materials have high proportions of exchangeable aluminium, pH (water) below

    4 , and many have characteristically Pale yellow mottles by acid sulphates of iron, potassium-iron or sodium-iron. Some have white mottles of aluminium sulphates.

    The U . S . Soil Conservation Service (USDA 1 9 7 0 ) defines a "sulfuric" horizon as

    a mineral or organic horizon with a pH lower than 3.5 ( I : ] yellow jarosite mottles of hues 2.5Y or yellower and chroma 6 or more (Munsell

    1954 colour notation).

    in water) &with

    A potential acid sulphate soil or material is a soil or a reduced parent material

    1 7 1

  • I .

    which is expected by the person identifying it to become an acid sulphate soil

    or material upon drainage and oxidation under certain defined future field con- ditions. Such soils or materials generally have one or more reduced horizons or

    are reduced, with pyrite contents considerably in excess of any neutralizing amounts of calcium carbonate present. In sandy or peaty materials with low cation

    exchange capacity, the excess pyrite may be small. Whether such a soil will in

    fact become an actual acid sulphate soil depends upon a number of factors besides the contents of S and neutralizing components, as will be discussed below. The

    Soil Conservation Service (USDA 1970) specifically defines "sulfidic" materials as waterlogged mineral or organic s o i l materials that have 0.75 per cent (dry weight) or more total S, mostly in the form of sulphides, and less than 3 times

    as much carbonate (CaC03 equivalent) as sulphur (weight percentages).

    The Soil Map of the World (Dudal

    alluvial soils contain sufficient sulphur compounds to cause acidification of

    the soil upon oxidation to a pH (KC1) less than 3.5 within 100 cm of the surface. This includes both potential and actual acid sulphate soils, and the depth limit

    would seem somewhat deep for detailed maps, but the definition does allow the

    grouping of potential and actual acid sulphate soils as well as shallowly reclaimed

    former acid sulphate soils in one unit: a distinct advantage for soil maps

    at a scale of 1:5 million.

    If the time aspect were also taken into account in the definition and rating of acid sulphate soil, this would require specification of, for example, rainfall

    and its seasonal differences, expected actual evapotranspiration during develop-

    ment, expected soil

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