Short Notes on Acid Sulphate Soils

This article provides short notes on acid sulphate soils.

Acid sulphate soils are extremely acid soils. They have a pH which is less than 4.0 in some layer (in the upper 50 cm) of the profile. The acidity is due to presence of sulphuric acid and iron and aluminium sulphates. They are derived from marine sediments high in pyrites and poor in bases. Sulphur oxidation is an acidifying process.

The reactions are as follows:

During the submerged period, sulphates (SO₄-) in the water are reduced to sulphides (S- -), in which form they are stabilized generally as iron sulphides (FeS). When these areas are drained, the sulphides or elemental sulphur (S) are oxidised forming sulphuric acid.

The soil pH may drop to level as low as 1 or 2 (highly acidic condition). Obviously plant growth cannot occur under these conditions. Sizeable areas of these kinds of soils, cat-clays, are found, in South-east Asia. Most soils will retain sulphate (this is the form taken up by plants). The mechanism of adsorbing sulphate is simple. Sulphate or acid sulphate ions replace hydroxyl (OH) held by aluminium ions (Al).

Acid sulphate soils are usually clay. The organic carbon content varies from 1.5 to 18%. The cation exchange capacity is 10-25 meq/100 gm. Available nitrogen and phosphorus are low. There may be toxicity of iron, aluminium, H₂S in these soils. These soils when submerged, are nearly neutral in reaction but when these are drained and dried they become extremely acidic and lethal to crop plants.

These soils can be managed for growing of crops. Reclamation measures depend on the degree of water control. If water is available, the soils are kept continuously flooded or saturated to prevent acidification. Where that is not possible, the soils are drained and leached to remove the acid materials. The soils are then limed, fertilised with nitrogen, phosphorus and crops grown in the wet season.

Rice is the most important crop which can tolerate flooding and water logged condition. Under flooded condition, acidity is low (pH is higher).

Broadcasting of rice seed is avoided under dry land condition because aluminium toxicity may affect the seedlings. Transplanting should be preferred under puddle wet land condition.

In temperate regions with at least 20 cm top soil (pH 4.5) grassland development may be encouraged. Rye grass and Bermuda grass are tolerant to moderately acidic condition and slightly saline condition.

In situation where water table control is impractical or fresh water is not sufficient for crop production, forestry remains an important option.