Spent mushroom compost is the residual compost waste generated by the mushroom production industry. It is easily available, and its formulation consists of a combination of wheat straw, dried blood, horse manure and ground chalk, composted together. It is an excellent source of humus, although much of its nitrogen content will have been used up by the growing mushrooms. It remains, however, a good source of general nutrients (0.7% N, 0.3% P, 0.3% K plus a full range of trace elements), as well as a useful soil conditioner. However, due to its chalk content, it is highly alkaline, and should not be used on acid-loving plants, nor should it be applied too frequently, as it will overly raise the soil's pH levels.
Mushroom compost may also contain pesticide residues, particularly organochlorides used against the fungus gnat. Chemicals may also have been used to treat the straw, and also to sterilise the compost. Therefore, the organic gardener must be careful regarding the sourcing of mushroom compost; if in doubt, samples can be analysed for contamination – in the UK the Department for Environment, Food & Rural Affairs (DEFRA) is able to advise regarding this issue.