Powdered gypsum is used to increase the pH in mushroom growing substrates such as coco choir, sawdust, coffee grounds and straw. It is a yield enhancer as it supplies minerals and trace elements to the mycelium.
It is also used when preparing grain spawn to stop the grain from clumping together.
Gypsum should be mixed at a rate of 2.5% of the dry weight of the bulk substrate and 1 tablespoon per litre of grain spawn.
The product of powdering brown rice is a popular substrate for growing mushrooms. Its use as a substrate was made popular by the use of cake spawns in the PF Tek method of growing. It is most often powdered into a flour and combined with vermiculite.