Spent mushroom soil is a readily available resource in Delaware as the mushroom industry is nearby. The following is information on the characteristics of spent mushroom soil.
Characteristics of SMS
Spent mushroom substrate is the composted organic material remaining after a crop of mushrooms is harvested. Mushrooms are grown in a mixture of natural products, including horse-bedded straw (straw from horse stables), hay, poultry manure, ground corn cobs, cottonseed hulls, gypsum, and other substances. This mixture is composted in piles or ricks, creating a dark brown, fibrous, and pliable organic growing media. When the composting process is complete, the media is brought into mushroom houses where it is placed into beds or trays and used as a substrate for growing mushrooms. After the mushrooms are harvested, the "spent" substrate is removed from the houses and pasteurized with steam to kill insects, pathogens, and mushroom remnants.
Spent mushroom substrate is sometimes sold immediately after it is removed from mushroom houses; in this case it is referred to as "fresh SMS". Alternatively, the SMS can be placed in windrows and further composted for several weeks or several months. This material is often called "weathered SMS" and differs in composition and appearance from fresh SMS. Some producers blend SMS with soil to produce a ready-to-use growing medium for turfgrasses and other plants.
Information from "USING SPENT MUSHROOM SUBSTRATE (MUSHROOM SOIL) AS A SOIL AMENDMENT TO IMPROVE TURF" from Penn State University.