Mushroom Compost Frequently Asked Questions

Field

Mike Fidanza, Ph.D.
Associate Professor of Horticulture
The Pennsylvania State University
Reading, PA
Email: maf100@psu.edu

Mushroom compost (also referred to as “spent mushroom substrate” or “mushroom soil”) has become a popular organic soil amendment for the establishment and maintenance of lawns and sports fields, gardens, agricultural and horticultural crops and with land reclamation projects. Here are some commonly asked questions and answers about mushroom compost.

What is mushroom compost?
How does compost improve the soil?
What beneficial properties are found in mushroom compost?
How much mushroom compost should be used for establishing roadside vegetation?
What about salts?
What about weeds?

What is mushroom compost?

Mushroom compost is a viable and useful by-product of mushroom farming. Those edible mushrooms found in the produce section of your grocery store are grown in a specific medium. This growth media is a mixture of agricultural materials, such as straw from horse stables, hay, poultry litter, ground corn cobs, cottonseed hulls, cocoa shells, peat moss, and other natural organic substances. These products are formed into a rich organic media that serves as the nutrient source for mushrooms. After the mushroom crop is harvested, this organic material is removed from the production house, where it is processed into a consistent homogeneous by-product called “mushroom compost.”

How does compost improve the soil?

In general, a good, organic compost, if used properly, can improve plant growth in poor or marginal soils. This is because compost amended into those soils will improve the structure of clay soils, reduce surface crusting and compaction and therefore improve drainage, increase beneficial soil microbial activity, and provide nutrients to plants which can reduce the need for fertilizer. Overall, compost can be very beneficial to the soil, and mushroom compost is no exception.

What beneficial properties are found in mushroom compost?

The visual appearance of a good quality, thoroughly processed mushroom compost typically resembles a dark topsoil, has a loose crumbly structure, and has an “earthy” aroma. Recent research conducted at the Pennsylvania State University showed that mushroom compost contains an average of 25 percent organic matter and 58 percent moisture on a wet volume basis. Where uniform application and good mixing with soil is required, this amount of organic matter and moisture in mushroom compost is ideal for handling and making surface applications or incorporating into the soil. Mushroom compost contains an average of 1.12 percent nitrogen in a mostly organic form that slowly is available to plants. Also, mushroom compost contains an average of 0.67 percent phosphate (phosphorous) and 1.24 percent potash (potassium), as well as other plant nutrients such as calcium (2.29 percent) magnesium (0.35 percent) and iron (1.07 percent). The average pH of mushroom compost is 6.6 (6.0 to 7.0 is an ideal range for most plants). The amount of carbon relative to nitrogen is an important indicator of nitrogen availability for plant growth, and an ideal compost should have a ratio of 30:1 or lower. Mushroom compost has an excellent 13:1 ratio, indicating outstanding nutrient availability and mature and stable organic compost.

How much mushroom compost should be used for establishing roadside vegetation?

The best approach would be to apply mushroom compost uniformly and evenly at a one to three inch thickness (three to nine cubic yards per 1,000 square feet) on the surface of the intended site, and then incorporate into the existing soil below. Next, seed or transplant the desired vegetation. For example, with turfgrass, sow the grass seed uniformly at a rate recommended for the turfgrass species used, and water thoroughly. For sites with steep slopes, erosion blankets or netting may be helpful to reduce further the possibility of soil erosion.

What about salts?

Excessive amounts of soluble salts (for example, calcium, magnesium, potassium and others) in the soil can cause injury to turfgrasses and other groundcover-type plants. However, research at Penn State shows that good quality mushroom compost does not contain soluble salt concentrations high enough to impede turfgrass seed germination or cause damage to an existing turf stand. Also, when mushroom compost is tilled or incorporated into the soil, the salt concentration is diluted greatly, and irrigation or natural rainfall will further reduce salt concentrations by leaching those salts from the root zone. Ask your mushroom compost supplier to provide you with a detailed laboratory analysis to ensure you’re getting good quality and a reliable compost product.

What about weeds?

Prior to removing the compost from a mushroom production house, the entire inside of the house is steam-treated (i.e., pasteurized) to eliminate any pests, pathogens or weed seeds. In the past, lawn and garden centers often advertised “mushroom soil,” which essentially had been sitting outside in a pile for one or two years and had accumulated air-borne weed seeds (such as lambsquaters and velvetleaf). Then, when this compost was tilled into a garden, those weed seeds would emerge with a vengeance. Again, insist on a uniform and quality mushroom compost that has been pasteurized, processed, and properly stored to ensure no contamination from weed seeds.


Web site produced with a grant from the Pennsylvania Department of Agriculture.