Gypsum penetrates the clay particles in heavy soils, helps loosen the soil structure and adds calcium and sulfur for overall plant health. Pure gypsum is 23 percent calcium and 19 percent sulfate (CaSO4-2H2O)
Clay soils in commercial agricultural sites are often treated with gypsum to help break up the clay and enhance calcium, which breaks up excess sodium. The effects are short lived but serve to soften the soil enough for plowing and sowing. One of the main uses of gypsum is to remove excess sodium from the soil and adding calcium. Because it reduces salt levels in soil, it is effective in coastal and arid regions. However, it doesn’t work in sandy soils and it can deposit an excess of calcium in regions where the mineral is already abundant. Additionally, in areas with poor salinity, it pulls out too much sodium, leaving the location deficient in salt. Considering the cost of a few bags of the mineral, using gypsum for garden tilth is uneconomical.
Apply 10 pounds per 1,000 square feet of garden area up to three times each growing season to correct calcium deficiencies. Over-application of gypsum is generally not a problem because it has low solubility and the increase in salinity is low.