Spent mushroom compost

Spent mushroom compost is the residual compost waste generated by the mushroom production industry. It is easily available, and it's 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 it's 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 + full range of trace elements) as well as a useful soil conditioner[?]. However, due to it's 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) are able to advise regarding this issue.