By continued use of good quality irrigation water, good irrigation methods and cropping practices, further removal of adsorbed sodium, especially in the subsoil, usually takes place. In some cases, it may be necessary to reclaim to greater depths to obtain adequate drainage and root penetration. The purpose of an amendment is to provide soluble calcium to replace exchangeable sodium adsorbed on clay surfaces. Gypsum is moderately soluble in water. It requires about 1 acre-foot of water per acre to dissolve 1 ton gypsum per acre.
Calcium chloride is highly water soluble and fast-acting, but it generally is too expensive to use.
Sulfuric acid reacts immediately with the soil calcium carbonate to release soluble calcium for exchange with sodium. Elemental sulfur must be oxidized by soil bacteria and react with water to form sulfuric acid. The formation of appreciable amounts of sulfuric acid from elemental sulfur may take several months to several years. Calcium carbonate must be present in the soil when acid or acid-forming amendments are added. Choose the amendment mainly on the basis of the cost of the soluble calcium furnished directly or indirectly by the amendment and the speed of the reaction.