Mushroom compost is a type of slow-release, organic plant fertilizer. The compost is made by mushroom growers using organic materials such as hay, straw, corn cobs and hulls, and poultry or horse manure. Since the mushroom growing process varies slightly between individual growers, mushroom compost recipes may differ here and there. For instance, additional materials like gypsum, peat moss, lime, soybean meal, and various other organic items may be added to the compost as well. Once the mushroom spawn are mixed into the compost, it is steam pasteurized to kill weed seeds and any other harmful agents. A mixed layer of sphagnum moss and lime is top-dressed onto the top of the pile for the growth of mushrooms.