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Gypsum will improve the physical condition and drainage of most clay soils allowing better root development by breaking up clay deposits. Its sulphur content helps neutralize alkaline soils.
Gypsum works 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 (“electrolyte” means salt solution). This effect is based on the fact that swelling and clay dispersion decrease as the salinity of water infiltrating the soil increases. It occurs with all types of salts, not only gypsum. It is short-term in nature because it ceases when all the applied gypsum has disappeared.
The second effect is specific to calcium salts, including gypsum. It is based on the fact that cations in the soil, such as calcium (Ca2+), Magnesium (Mg2+), sodium (Na+) and potassium (K+), are bound to clay particles by electrical forces and are exchangeable. This means that if a high concentration of calcium cations are introduced to the soil solution (by adding gypsum) they will exchange for other cations, particularly sodium, on the clay. By this process a sodic clay is changed to a calcic clay (that is, one dominated by exchangeable calcium), thereby reducing the swelling and clay dispersion. At the same time, the sodium cations are released into the soil solution, but are leached below the root zone where their presence is less important than nearer the surface. This second effect is a long-term benefit, lasting well after all the applied gypsum has disappeared from the root zone.