A common mineral that is used in a variety of products from cement and plaster to a food additive in tofu, gypsum has found its place in gardens due to its natural ability to break up compacted soils. Mined worldwide, gypsum has both proponents and opponents when it comes to its use in agriculture.
Gypsum deposits can be found all over the planet. Domestically, the largest gypsum producing states are Texas, Iowa, California and Nevada. While it can be found in a variety of manufactured products, it is merely ground up and refined for use in agriculture. Applied to compacted soils to create better drainage, the mineral contains two key elements, calcium and sulfur, that are necessary for plant growth.
Gypsum, when added to compacted soils, causes silt, sand and clay as well as other particles to band together. This action creates tiny clods of dirt and removes the "sticky" nature of clay and other thick soils. The process is actually a chemical reaction in which the calcium competes with the salts in the soil, drawing soil particles together and causing clay to loosen.
There is a continuing controversy in the agriculture academic community centered on the unnecessary use of gypsum. The Louisiana State University Agriculture Center reports that most soils within the United States contain a mixture of non-native and native soils and receive little benefit from a gypsum application. The center reports that using core aerification will yield better results with less expense.
Completely nontoxic, gypsum can be handled without any protection. It is safe to apply directly on a lawn or garden and will not burn. In addition, it is child and pet safe. As a natural mineral with no additives, it is completely organic.
Gypsum is sold in a variety of forms, but the flake form is best for a lawn and garden application. The flakes are less likely to be blown by the wind or washed away by overwatering and tend to anchor themselves to the soil. The accepted rate of applications is approximately 40 pounds per 1,000 feet. Simply spread the mineral and water and allow the gypsum to leach into the soil naturally.
While safe to use and safe for your garden, take some time to determine how much of a hard soil problem you have before adding gypsum to your soil. Small pockets of clay are probably not enough to warrant an application. For the most part, hard soils found in the Southwest and along the coasts may qualify for its use. In addition, soils around a new home, while usually mixtures, tend to be intentionally highly compacted for foundation support and can benefit from an application before bringing in topsoil and turf.