Gypsum works on improving spoil structure in two ways. Both depend upon the gypsum being dissolved by rain or irrigation water and entering the soil solution.
The first is called the electrolyte effect which is based on the fact that swelling and clay dispersion of sodic clay soils decrease as the salinity of water infiltrating the soil increases. It It is short-term in nature because it ceases when all the applied gypsum has disappeared.
The second is related to calcium salts, including gypsum. It is based on the fact that cations are bound to clay by electrical forces and are exchangeable. Through this process a sodic clay can be changed to a calcic clay. This effect is a long term effect lasting well after the gypsum has disappeared.
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