One of the by products of mushroom farming is mushroom compost. More precisely, the compost that the mushrooms were grown in that no longer has enough nutrients for mushrooms but plenty of nutrients left for the garden and landscape plants.
Mushroom compost is made specifically for mushroom farming and is made of any number of organic material but generally can include peat moss, straw, chicken manure, grape crushings, soybean meal, ground corn cobs, cottonseed hulls, cocoa shells, potash, gypsum, ammonium nitrate, and lime. The mushroom compost is pasteurized at high temperatures to ensure it is weed and pest free and then used for several weeks to produce several crops of mushrooms.
Although the used mushroom compost still has plenty of nutrients it is more economically for mushroom farmers to replace the compost with new substrate. Some of the mushroom compost may be retained at the mushroom farm to be mixed in with new batches of fresh compost but most of it is repasteurized and is resold as Spent Mushroom Compost (SMC), Mushroom Soil, or Spent Mushroom Substrate (SMS).
The spent mushroom substrate is low in heavy metals and certified weed free; however, it is high in soluble salts such as calcium, magnesium, and potassium. Because of the high salt content mushroom compost should not be used on young plants or members of the Ericaceae Family (heather family). Plants from the Ericaceae family, which thrive in acidic soils, include rhododendrons, azaleas, camellias, huckleberry, cranberry, and blueberry.
When using mushroom compost it is best to mix the compost with garden soil and then spread around the plants. This will provide a slow release of organic fertilizer (2-1-1, pH 6.8) and will assist with moisture retention. The specific composition of spent mushroom compost will vary slightly from each mushroom farm since each farmer will have a different recipe to create the compost.
The American Mushroom Institute estimates that the amount of spent mushroom compos exceeds 1.2 billion cubic feet per year in the United States and about 50% of that is generated in Pennsylvania. Consequently, consumers and backyard garden enthusiasts are more likely to find local sources of spent mushroom compost.
Good quality spent mushroom compost should have an earthy aroma and should not have a strong ammonia or other offensive odor. Strange odors may indicate that the compost is not fully mature and may have harmful effects on plants and garden. -KATHY FAIRCHILD