Available bagged and ready to use. (£3.45-100g DISCOUNTED on larger amounts)
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Agricultural Gypsum. Gypsum is the common name for calcium sulfate (CaSO4). It is a greyish powder used for making cement and drywall. Gypsum is used in in grain spawn in order to keep kernels from becoming stuck together and as a pH-buffer and lightening agent in bulk substrate and casing material preparation. 
Used at a guide rate of 2-5% dry substrate weight. 
Using a PH-Buffer such as gypsum will allow for more flushes from your substrate.