The varying levels of purity in calcium sulfate sources gave rise to a standardized way of comparing the nutrient value of products. The concept of Gypsum Equivalence compares a given product to theoretically pure calcium sulfate dihydrate. If 100 lbs. of Product X contains 79 lbs. of calcium sulfate, it is “100% gypsum equivalent.” Thus, a gypsum product that is 95% pure gypsum is considered 95% gypsum equivalent, or about 75% calcium sulfate.

Let’s look at a specific example to help illustrate the point. Anhydrite gypsum mined from the deposit near Carson City, NV can be about 85%-90% pure anhydrite gypsum, and much of the impurity is dihydrate gypsum, in the range of 5%-7%. (The other 5%-7% is limestone.) Since anhydrite gypsum contains no water, the Carson City anhydrite gypsum is at least 90% calcium sulfate, and thus about 114% dihydrate gypsum equivalent . (Theoretically pure anhydrite gypsum would be 127% dihydrate gypsum equivalent.)

Dihydrate Gypsum Equivalence of theoretically pure calcium sulfate minerals and typical real-world minerals sources.

A product containing mostly anhydrite gypsum and small amounts of dihydrate gypsum, such as Art Wilson CalCM+, can be proportioned to be 100% dihydrate gypsum equivalent. (Ironically, that is a boast that very few “gypsum” products can make.)

Calcium soil amendment to improve
When selecting a calcium sulfate product, it is important to check the assay or the “gypsum equivalence” to know that you are actually getting the proper amount of calcium and sulfur.