Characteristics of Spent Mushroom Substrate

The typical composition of spent mushroom substrate fresh from a mushroom house will vary slightly. Since raw materials and other cultural practices change, each load of fresh spent substrate has a slightly different element and mineral analysis. Therefore the characteristics shown in Table I indicate a range of values for each component. Sometimes, fresh substrate is placed in fields for at least one winter season and then marketed as "weathered" mushroom soil. This aged material has slightly different characteristics because the microbial activity in the field will change the composition and texture. The salt content may change during the aging period. If you have any specific questions concerning characteristics of either fresh or aged spent substrate, please contact your local Cooperative Extension agent.