Gypsum Powder Calcium Sulphate for Mushroom Substrate Grain Spawn Mycology. Our calcium sulphate is CE fertiliser grade gypsum obtained from naturally occurring minerals. Powder form, gypsum is used in mushroom growing to adjust the pH in media such as grain, sawdust, straw and coffee grounds. It provides minerals that are essential for fruiting. Add between 2 to 5 % per weight of substrate.
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