Mushroom compost is a totally organic rich, dark, moist mixture of wheat straw, peat moss, cottonseed meal, gypsum, lime, and chicken litter. This combination of ingredients is used in commercial mushroom farms to grow mushrooms. These materials are composted for many weeks and then placed into a huge room where it is completely sterilized and then the mushroom growing cycle begins. Strangely enough, mushrooms will only grow in this mixture for a very short time, usually 18 to 20 days. At this time the compost has to be removed and a brand new batch will already have been prepared for the next crop.