Gypsum is a naturally occurring mineral in many parts of the United States and the world. Gypsum is also often available as a by-product material. Gypsum is hydrated calcium sulfate. The chemical name of gypsum is calcium sulfate dihydrate. The chemical formula for pure gypsum is Ca(SO4)•2(H2O). In the chemically pure form, gypsum contains 23.28% calcium (Ca) and 18.62% sulfur (S) in the readily available sulfate form (SO4). However, the typical gypsum sources that are commercially available for agricultural often contain impurities which result in a Ca level between 18%-23% Ca and 15%-19%. Gypsum is also available as a byproduct material. Some of the more common byproduct gypsum sources are flue gas desulfurization (FGD) gypsum and waste wallboard (also known as sheetrock or drywall). A few locations may have access to byproduct phosphogypsum from the production of P fertilizer, and some locations may have access to byproduct gypsum sources related to other industrial processes..