Mushroom soil is the substrata used in commercial mushroom growing. It is sold after the mushroom crop is harvested. Mushroom soil can contain a variety of materials and varies from lot to lot.
It is usually steamed, pasteurized and partially composted, and may contain hardwood dust, coconut coir, vermiculite, peat moss, rice, cottonseed hulls, soybean and canola meal. Also, wheat straw, gypsum, and manure, usually horse or chicken. It is inoculated with the fungal spores and allowed to grow mushrooms.
 
Mushroom soil has a variety of good uses in the garden:
It makes a good soil amendment, adding organic material to the soil that improves the texture and water retention of the soil. It is a good addition to heavy soils. Note that this ability can be a problem, as the soil may retain too much water, so use with restraint — 25% mushroom soil to 75% soil or potting medium.
The soil is also a good top dressing for lawns, providing some slow-release nutrients.
Mushroom soil can be used as a mulch. Its low nutrient content dissuades weed growth and can be scratched into the soil later to improve structure. Used as a mulch, it may also discourage birds from eating newly sown seeds. The moisture retentive tendency will keep the seeds moist until they germinate.
Some sources indicated that mushroom soil can be used as bedding for vermiculture containers.