Gypsum is a mineral whose chemical structure consists of calcium sulfate with two water molecules in its structure (CaSO4 ⸳ 2H2O). This mineral has been used in agriculture as a fertilizer for centuries, mainly as a source of calcium and sulfur. There are three main sources of gypsum available today for agricultural use: mined, recycled wallboard, and flue-gas desulfurization (FGD) gypsum. Chemically these sources are identical, with the exception of recycled wallboard gypsum, which might contain pieces of paper within the material. Currently there is considerable interest in FGD gypsum for agricultural use as it is readily available. Flue-gas desulfurization gypsum is generated in air scrubbers engineered to remove sulfur from exhaust gases in coal-burning electric power plants. This type of gypsum typically has a smaller particle size than mined sources; thus it dissolves and reacts more readily.