Hydrated lime or calcium hydroxide Ca (OH) is a white dry powder used to act as a buffering agent (balance pH levels) in substrates as well as casing layers.

Many will add 15% crushed oyster shells and 8% hydrated lime by volume, to an overall mix of 50% vermiculite and 50% peat moss (as known as 50/50 plus mix) and use this as their casing layer. The hydrated lime will help in buffering the pH level as well as fight off some contaminates.