Calcium sulfate is available for agricultural use in two naturally occurring chemical forms: anhydrite gypsum and dihydrate gypsum.

Anhydrite gypsum is calcium sulfate, pure andsimple, CaSO4.

Dihydrate gypsum is calcium sulfate plus about 21% water in crystalline form.

If you bought 100 pounds of pure anhydrite gypsum, you would get 100 pounds of calcium sulfate. If you bought 100 pounds of pure dihydrate gypsum, you would get 79 pounds of calcium sulfate and 21 pounds of water. It is, in effect, partially diluted.

Mineral Purity
In the real world, neither of these materials is pure in its natural form. There are always small percentages of other minerals, which may include quartz, dolomite,11 or limestone. Often, mined anhydrite gypsum contains small amounts of dihydrate, and mined dihydrate contains small quantities of anhydrite gypsum. Both are sedimentary rocks, often found in layers that are not always perfectly separated, and mining operations may penetrate through one layer into another.

Some sources are purer than others. There are large, relatively pure (in the range of 95-98%) natural deposits of both anhydrite gypsum and dihydrate gypsum located close to California growers. These include the anhydrite gypsum deposit near Carson City, NV—the only significant US source of anhydrite gypsum—that is mined for Art Wilson CalCM+.

Recycled Gypsum
Recycled gypsum—such as flue gas desulfurization gypsum that results from scrubbing emissions of coal-burning power plants—is also available for agricultural use in some parts of the country, largely in the eastern US. Recycled gypsum from drywall could, theoretically be ground up and used for agricultural applications, but there are additives in it that make it a controversial material. Boron is often added as a fire retardant, and excessive boron can be toxic to plants. Waste drywall from new construction is thought to have fewer additives and is currently in trials for soil amendment, but the State of California does not recommend using demolition