FACTS ABOUT MUSHROOM COMPOST: Commercial mushrooms grow in a specially formulated and processed compost made from wheat straw, hay, corn cobs, cotton seed hulls, gypsum and chicken manure. The 3 to 4 week long composting period is closely supervised and managed to assure that the composting temperatures exceed 160°F for a few days in addition to a steam pasteurization which occurs about one week before mushroom spawn is mixed with the compost. Finally, a layer of sphagnum peat moss mixed with ground limestone is top dressed onto the compost, and mushrooms grow on the peat.

When the harvest if finished, farmers steam pasteurize everything in the growing room and dispose of the peat moss and compost that remain. This product is sold as mushroom soil, spent mushroom compost (SMC) or spent mushroom substrate (SMS). Mushroom soil is great for gardens as a slow release organic fertilizer (2-1-1, pH 6.8) when mixed into soil or as a mulch one year and a soil amendment the next. With SMC there need be no concern about heavy metals or pesticides since the compost ingredients have very low levels of heavy metals. Mushroom farmers have used integrated pest management practices for decades and pesticides are rarely used on mushroom crops. With steam pasteurization, all weed seeds are dead as are any insects and other pests that might be present. It is best not to plant or transplant directly into SMC, mix the SMC with soil at 50-50.