Recently there has been some misleading information concerning various gypsum sources. According to this information, it would lead one to believe that their gypsum is the only truly calcium sulfate dihydrite. This is not true. First dihydrite simply means that with each molecule of calcium sulfate there are two molecules of water. To truly be gypsum it must have this chemical makeup.
There also is calcium sulfate anhydrite. Anhydrite means it doesn’t have the two molecules of bound water. This form of calcium sulfate is usually not used in crop production and its water solubility is much lower. The misleading information would lead the reader to believe that all other sources are calcium sulfate anhydrite. At least in the Midwest and plains state, calcium sulfate dihydrite or gypsum is the primary form that is marketed as a soil amendment.
PRO CAL 40 is calcium sulfate dihydrite and is a superior form of gypsum because it is all crystalline and does not contain any mineral coating like some of the mined gypsum does. It will all pass a 100 mesh screen. This means it has greater solubility and that it will react quickly in the soil.