INTRODUCTION
Calcium Sulfate is one of the most useful and widely used agricultural supplements. It is a fertilizer providing two essential plant nutrients, calcium and sulfur. It is also a soil amendment that can improve both soil chemistry and soil structure, leading to better crops and more efficient water usage.

Calcium sulfate’s history in US agriculture is older than the nation itself. As gypsum, it has been spread by North American farmers for 250 years. It is used extensively in California, especially for crops such as nuts, grapes, and tomatoes. Yet, despite how widely-used it is, many growers are aware of it only as a fertilizer, and do not know of its importance for amending soil problems such as those widely found in California.

Agriculture can utilize either of two naturally-occurring forms of calcium sulfate. One is anhydrite gypsum, which is simply pure calcium sulfate. The other is dihydrate gypsum, because it is calcium sulfate plus two water molecules. Both anhydrite gypsum and dihydrate gypsum are available to California growers in products suitable for spreading or injection into irrigation. They are similar in agricultural benefits but may differ significantly in cost effectiveness.

The similarities and differences between agricultural anhydrite gypsum and dihydrate gypsum are simple, but they are not widely known, even among agronomists. This was evident during a session at the Western Plant Nutrition conference in Reno, NV in March 2019, attended by numerous industry and government experts. In regards to a question about the two materials, no one in the room could answer concerning their relative solubility, dissolution rate, or their comparative agronomic effectiveness. The facts about calcium sulfate are valuable information for anyone growing crops in California.