Acid Sulfate Soils

Acid sulfate soils have been described as the "nastiest soils in the world" (Dent and Pons, 1995). They contain high concentrations of sulfide, mainly in the form of pyrite (FeS2), which may be oxidized to yield free and adsorbed sulfates, resulting in extremely acid soils (pH's as low as 2). Dent and Pons (1995) have described the pedologic formation of these soils. In the first step, bacteria decomposing the abundant organic matter in tidal swamps and marshes reduce SO243 from the tidewater and Fe3+ oxides from the sediment. The main end product is pyrite, which remains reduced in the tidal zone and when buried by peat or freshwater alluvium. Extensive areas of unripe sulfidic clays are still found in Indonesia, West Africa, and northern Australia. Some sulfidic sediments have been drained naturally by tectonic uplift or through changes in the hydrology of deltas. However, there has been an incentive to reclaim swamplands in an attempt to exploit the expected fertility. Problems arise whenever the rate of acid production from oxidation of sulfides exceeds the buffering capacity of the soil. Acid sulfate soils produced by drainage become severely acid within weeks or months. Usually, they can be identified by straw yellow mottles of jarosite, KFe3(SO4)2(OH)6, that develop on ped surfaces and by acidic, red drainage water. The major limitation to soil productivity in many cases is not the acidity of the soil itself, but increased aluminum and manganese toxicity to plants caused by the release of these elements due to the acidification.

The acidification of these soils results from the abiotic and microbial oxidation of pyrite when reducing conditions no longer exist. The overall reaction is

This summarizing equation is somewhat misleading in that the primary oxidant involved in pyrite oxidation in most situations is ferric iron rather than molecular oxygen, and pyrite oxidation is a multistep process involving an oxygen-independent reaction and oxygen-dependent reactions. The proper sequence is

2 FeS2 + 7 O2 + 2 H2O ^ 2 Fe2+ + 4 H+ + 4 SO4~, 2 Fe2+ + O2 + 4 H+ ^ 2 Fe3+ + 2 H2O,

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