Gypsum, a natural sulphate of calcium is found in large deposits in various parts of Rajasthan in India. It reacts with exchangeable sodium and gets converted into sodium sulphate, which is soluble in water. Sodium sulphate is leached down from the soil to reduce the soil pH. In accordance with the Indian standard specification, the agricultural grade gypsum must have 70 per cent purity. Gypsum contains two essential plant nutrients i.e. Calcium and sulphur to the tune of 16-19 per cent and 13-15 percent, respectively. Phospho-gypsum is another form of mineral gypsum which has the same formula but contains 21 per cent calcium and 16 per cent sulphur. The phospho-gypsum has relatively high purity than commercial gypsum varying between 90 and 95 percent. These two forms of gypsum are very important in agriculture for changing the part of the caustic alkali carbonates into sulphates. Several tons of gypsum per acre is usually required for reclamation. The exact dose of gypsum is determined on soil test basis. Soil must be kept moist to hasten the reaction and the gypsum should be mixed in the soil surface.
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