Horticultural grade granular (quick-release) gypsum. Gypsum is commonly used as an ingredient in mushroom substrates and casing mixtures. The primary role of gypsum is to act as a mineral supplement, as it's presence promotes mycelial growth by providing substrates with calcium and sulfur, both essential elements for good fruit body formation. It kind of acts as a mushroom "fertilizer" so to speak. Gypsum is also commonly used to hold the pH of a substrate stable by preventing drastic swings in pH.
Sold in ½ gallon increments.