Gypsum is hydrated calcium sulfate (CaSO4·2H2O) that is sometimes used as a soil amendment to improve the tilth, or ability to be tilled, of clay and other heavy soils. In addition to its use as a soil amendment, it is widely used to make plaster of Paris, chalk and drywall for the construction industry.
Gypsum comes in the form of flattened crystals. Natural gypsum is mined from geologic deposits; synthetic gypsum is produced by some coal-fired power plants as a by-product of pollution-control processes.
Gypsum is especially beneficial as an amendment to heavy, clay soil to improve its texture and ability to drain. It makes the soil less compact, easier to cultivate and better able to accept air and nutrients needed for plant growth.
Gypsum removes sodium from saline soils and replaces it with calcium.
Gypsum has no effect on the fertility or structure of any soil type other than heavy clay.
Scientific research is mixed on whether gypsum changes the acidity of soil. Some say it increases acidity; others say it decreases it.
Gypsum may be added to soils to that are deficient in calcium. The best way to determine if your soil needs calcium is to have it analyzed; contact your agricultural extension service for details on how to get this done in your area. The cost of having a laboratory test your soil is usually cheaper than buying a single bag of gypsum.
Some forms of gypsum do have the benefit of adding lime to the soil. These varieties of gypsum contain calcium carbonate (agricultural lime), calcium hydroxide (hydrated lime) or calcium oxide (burned lime). Check the label on a gypsum bag to see whether it contains lime.
Gypsum may cause the leaching of micronutrients from soil, including aluminum, iron and manganese. Aluminum can be toxic to plants, but leaching it into the watershed contaminates the environment. The leaching of iron can cause chlorosis or yellowing of the leaves, in some plants. Adding gypsum to acid soils can result in a deficiency of magnesium. Adding gypsum to sandy soils can depress the transport of copper, phosphorus and zinc.
Researchers at the USDA Agricultural Research concluded that gypsum can be used to control soil erosion without fearing negative effects on plants caused by nutrient deficiencies.
Dr. Linda Chalker-Scott, professor of horticulture at Washington State University, says on the school's website that gypsum amendment is most useful in arid and coastal regions that are high in soil salts; it is also useful in the southeastern U.S. for heavy clay soils. Chalker-Scott says gypsum amendment is less useful in urban soils that are mixes of native and non-native topsoils and in home landscapes that contain high levels of organic and non-organic chemical additives.
While gypsum is useful in reducing soil compaction in heavy clay soils, using organic mulch as an amendment is cheaper and more environmentally sustainable in other soils.