Gypsum (CaSO4) is considered both a soil ammendment and a source of plant nutrients.
When soils have been exposed to too much sodium (usually in the coastal areas), two concerns arise. One is that sodium on the exchange sites of clays and organic matter tends to make them disperse and the soil becomes impermeable as the clays clog up the pore spaces. This is not really a problem in the coastal region where the soils are very sandy and have essentially no clay. The second concern with sodium is that excessive amounts in the soil can exclude the uptake of other nutrients such as calci um and magnesium. Too much uptake of sodium by a plant can cause toxicity problems usually manifest by necrosis of leaf edges. An application of gypsum to the soil will help to exclude the uptake of so much sodium by the roots just by the fact that the calcium ions will vastly out-number the sodium ions.
Often when the soil pH tests less than 5.8, the calcium level in the soil will also test only "low" or "medium". There are instances, however, when the soil pH value will be adequate (between 5.8 and 6.5) yet the calcium level will still test less tha n the optimum "high". In this case, the addition of lime to provide the needed calcium will raise the pH value too high and likely cause other problems such as a micronutrient deficiency. Gypsum can be added to the soil to supply the needed calcium with out altering the soil pH value. The alternative method of adding more calcium without raising the pH value is to use fertilizers which contain calcium.
Gypsum also is a good source of sulfur but this is only a side benefit from its use. In the rare instances where sulfur is needed in the soil, most people will add elemental sulfur or use a fertilizer with some form of sulfur in it.
There are no easily accessible guidelines regarding the application rate of gypsum in a homeowner situation. It is sparingly soluble and so it is nearly impossible to over-apply. In the Western part of the United States, many crops grow just fine in soils that have naturally occurring, undissolved accumulations of gypsum throughout the soil. Generally, a homeowner can just sprinkle a fine layer over the soil surface and work it in. A general application rate is 100 to 150 pounds per 1,000 square feet. Gypsum is a must for tomato growers in our South Carolina to prevent "blossom end rot". In an agricultural situation, enough gypsum should be applied to supply 100 pounds of calcium per acre when the soil test report does not call for lime but the calcium level tests "medium" or "low".