Abstract

Potential acid sulphate soils, i.e. anaerobic soils that contain pyrlte, FeS₂, occur extensively in the coastal plains of Malaysia and elsewhere.In the natural waterlogged state the pH of these soils is usually close to 7 but when they are drained oxidation of the pyrite causes them to become very acidic, pH values of 3 or less being common. A simple method that requires only a pH meter or soil pH indicator solutions is proposed for the routine identitication of these soils during soil surveys. This method, termed moist atmospheric oxidation, involves the storage of unmixed auger samples of soil in open polythene bags for about 8 weeks. During this period the samples are kept moist and aerobic to facilitate maximum oxidation of pyrite and acidification. If necessary, water is added occasionally to prevent the soils from drying, but they are not allowed to become waterlogged. The initial pH at the centre of the sample (the least oxidised part) and the final pH at the outside (the most oxidised part) are measured.In the samples of potential acid sulphate soils tested the pH decreased from between 6.6 and 7.0 in the fresh soils to between 2.3 and 3.4 after moist atmospheric oxidation. Such large decreases in pH make it possible to identify these soils easily and unequivocally. Alternate drying and rewetting of samples, which is often practised, offers no advantage as it involves more effort and gives a smaller decrease in pH. Air-drying in the sun and oven-drying both give much decreases in pH than moist atmospheric oxidation, so that it is more difficult to identify potential acid sulphate soils by these methods. Furthermore, irreproducible results can be obtained because the degree of acidification varies with the conditions of drying, particularly with air-drying. Oxidation of samples with hydrogen peroxide produces a large decrease of pH in pyritic soils and is a good method of identification if the results are required quickly, but as similar decreases can also be obtained with certain non-pyritic soils. This method could lead to confusion.

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