Can you tell me what mushroom compost is and explain the advantages of using it in the garden?
Rita Klang, Duluth, MN
Paul J. Wuest, Ph.D., professor at Pennsylvania State University and mushroom specialist, replies: Mushroom compost, sometimes called mushroom soil, is the spent growing medium used by commercial mushroom farmers. Commercial mushrooms grow in a specially formulated and processed compost made from wheat straw, hay, corn cobs, cottonseed hulls, gypsum, and chicken manure. These ingredients are composted for three to four weeks, and the process is closely supervised to ensure that composting temperatures exceed 160°F for a few days. Then the mixture is steam-pasteurized. A layer of sphagnum peat moss mixed with ground limestone is top-dressed onto the compost, and mushrooms grow on the peat.
When the mushroom harvest is finished, farmers again steam-pasteurize everything in the growing room and dispose of the spent growing medium, often selling it to the gardening public. Mushroom compost is great for gardens as a slow-release fertilizer when mixed into soil, or as a mulch. It has an NPK ratio of 2-1-1 and a pH of 6.8. The compost ingredients have very low levels of heavy metals and mushroom farmers have used integrated pest management practices for decades, so they rarely use pesticides on mushroom crops. With the two steam-pasteurizations, all weed seeds are killed in the mushroom compost, as are any insects or other pests that might be present.
Many garden centers have mushroom compost available by the truckload or the bushel. A list of sources for truckload quantities can be obtained from the American Mushroom Institute.