Mushroom compost (also called mushroom soil or mushroom culture substrate) is used inmushroom production industry. Mushrooms are cultivated in this substrate under controlled climatic conditions. The substrate primarily serves as a moisture reservoir.
It is readily available (bagged, at nursery suppliers), and its formulation generally consists of a combination ofwheat straw, dried blood,horse manure and ground chalk, composted together. Each ton of horse manure contains an average of:[1][2]
In the mushroom production industry, at the end of the production cycle, the growing medium is no longer useful for producing more mushrooms. What many consider waste, so-called spent mushroom compost (SMC), still contains organic matter, nutrients, and an active microbial community.
It is an excellent source ofhumus, although much of itsnitrogen content will have been used up by the composting and growing mushrooms. It remains, however, a good source of general nutrients (1-2%N, 0.2%P, 1.3%K plus a full range oftrace elements), as well as a usefulsoil conditioner.[3] However, due to itschalk content, it may bealkaline, and should not be used on acid-loving plants, nor should it be applied too frequently, as it will overly raise the soil'spH levels.[4]
Mushroom compost may also containpesticide residues, particularlyorganochlorides used against thefungus gnat. If the compost pile was stored outside, it may contain grubs or other insects attracted to decaying matter. Chemicals may also have been used to treat the straw, and also to sterilize the compost. Therefore, theorganic gardener must be careful regarding the sourcing of mushroom compost; if in doubt, samples can be analyzed for contamination – in theUK, theDepartment for Environment, Food and Rural Affairs is able to advise regarding this issue.
Commercially available 'spent' mushroom compost is not always truly spent. It is sold by mushroom farms when it is no longer producing commercially viable yields of mushrooms. It can be used to grow further smaller crops of mushrooms before final use on the garden.[5]
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