39% SO3, 27% CaO
In Powder form, gypsum is used to adjust the pH in mushroom growing media such as grain, sawdust, straw and coffee grounds.
The main reason is to add calcium carbonate and sulfur, which helps to stabilize PH levels to near neutral so it acts as a PH buffer as well as adding minerals which are vital for fruiting. Add between 2 to 5 %.
Also was in Morel cultivation.
It also helps prevent grains from sticking and clumping.