Gypsum, no matter how it is used, must be applied with heavy irrigation so that the sodium released from the exchange surfaces leaves the root zone along with the irrigation water. The important point is that the volume of water required for dissolving a specific amount of gypsum is usually calculated based on the solubility of the gypsum in free water. In practice, after gypsum is mixed with water, its solubility increases and much less water will be needed. However, sufficient amount of water must be used after applying gypsum to remove the excess sodium from the root zone. That is why gypsum is used in winter in arid and semi-arid regions when there is access to good amount of water.
To be suitable for soil amendment, gypsum must have these properties:
– Its salinity must be less than 3dS/m (the salinity of pure gypsum solution is about 2.5 dS/m). The type of impurity in gypsum is especially important for the final salinity of the solution because presence of soluble salts such as sodium chloride at amounts less than 2% causes severe salinity even if the gypsum has high purity. However, although impurities like sand particles reduce purity of gypsum, they do not increase salinity and are tolerable. In many cases, application of gypsum mixed with sand can be cost-effective due to its low salinity and low costs, and it serves as a better amendment in heavy soils.
– Its magnesium content must not be high ( the calcium/magnesium ratio must be at least 4).
– particles should preferably be smaller than 2 mm (the suitable size depends on economic factors and method of application.