Gypsum is a common mineral, hydrated calcium sulphate, used in Plaster of Paris, sheetrock and other household applications. Gypsum is a useful agricultural soil amendment for certain types of clay soils, because it slows the soil dispersal rate. Gypsum is also helpful when added to soils with excess salts, as it will leach the salts from the soil. Add good quality gypsum to the soil when testing indicates its suitability to increase soil structure. This will, in turn, increase crop yields and decrease soil maintenance and enhancement costs.
Test your soil dispersion rate for gypsum suitability. Fill a few small bowls half way with distilled water or rain water. Obtain soil samples--about a half-cup each--from various locations in your fields. Squeeze each soil sample into a ball and place a single sample in each bowl of water.
Let each sample sit for 24 hours and observe the rate of soil dispersal into the water. If all or most of the soil has dispersed into the water, your soil may benefit from the addition of gypsum.
Have a local agricultural extension service test your soil for a wide variety of elements. If your soil is low in iron, manganese and magnesium, do not add gypsum because it will further reduce these elements in the soil. If the tests reveal high soil salt concentrations, gypsum may benefit soil composition by helping to leach soil salts for dispersal in irrigation or rainwater.
Determine the appropriate quantity of gypsum for application. Consult a reputable academic source for recommended application quantities.
Buy pure calcium sulphate from a agricultural chemical supplier. Do not use gypsum-based wallboard or other construction products, which are likely to contain significant quantities of other materials, some of which may be harmful or even toxic to your soil.
Turn the gypsum in to the top 6 to 12 inches of soil using a disc tiller for large fields and a rototiller for market or home garden applications. Many garden-advice programs indicate that gypsum should just be spread on the soil surface, but the University of Arizona College of Life and Agricultural Sciences recommends physical incorporation for more rapid and effective soil structure alteration.
The University of Arizona publishes a gypsum application chart on a per-acre or per-thousand-square-foot basis for soils with different particle sizes. The West Australia Department of Agriculture and Food recommends 2.5 tons per hectare.