Mushroom compost is a type of organic plant fertilizer and soil amendment that is made by mushroom growers using organic materials such as hay, straw, corn cobs, and hulls, and poultry or horse manure. It is a by-product of mushroom farming that is removed after the mushrooms are harvested. Mushroom compost is readily available at nursery suppliers and garden centers, and it is an excellent source of humus and general nutrients. However, much of its nitrogen content will have been used up by the composting and growing mushrooms. Mushroom compost may also contain pesticide residues, particularly organochlorides used against the fungus gnat, and chemicals may have been used to treat the straw and sterilize the compost. Therefore, the organic gardener must be careful regarding the sourcing of mushroom compost, and if in doubt, samples can be analyzed for contamination.

Mushroom compost is generally sold in bags labeled as spent mushroom compost or spent mushroom substrate (SMS or SMC) . It can be used as a soil amendment for lawns, gardens, and container plants, and it enriches the soil and supplies nutrients for the healthy growth of plants. Mushroom compost also increases the water-holding capacity of soil and helps to amend clay-like soils over time. However, mushroom compost should be used with caution due to its high soluble salt levels, which can kill germinating seeds, harm young seedlings, and cause damage to salt-sensitive plants, like azaleas and rhododendrons. Mushroom compost is a slow-release fertilizer at about a 2-1-1 level, and it’s low in heavy metals but can be higher in soluble salts than other fertilizer choices. Mushroom compost pH levels are around 6.6, putting it in the neutral range.

Aged, cured mushroom compost can be applied any time in the spring or summer, and it can be worked into the soil or spread around the base as mulch. Mushroom compost can also be used as a light top-dress layer for lawns, and it will slowly work its way down into the garden soil, improving it and helping to give a little boost to the lawn. Mushroom compost is a versatile and useful soil amendment that can benefit many types of plants, but it should be used with care and attention to its composition and potential contaminants.