Unlike the name suggests, mushroom compost does not contain any mushrooms—it is a by-product from mushroom farming; the growth medium of mushrooms is removed after the mushrooms are harvested. The more apt name for it is mushroom soil.
Just like regular compost, mushroom compost is often touted as black gold. When used properly it acts as a plant fertilizer and soil amendment to benefit your plants.
How Mushroom Compost Is Made
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When the substrate in which mushrooms are grown is no longer suitable, it is still valuable for gardeners.
The mushroom substrate contains hay, straw, poultry or horse manure, and crop residues such as corn cobs and cocoa shells. The exact composition of the growth substrate varies from grower to grower. The substrate is composted for about 30 days at 160 degrees to kill weed seeds, pests, or disease pathogens. Additionally,it is steam-pasteurized at about 140 degrees F to kill any pests and pathogens that could cause surface diseases. Then the medium is inoculated with mushroom spawn, or mycelium, and covered with sphagnum moss and ground limestone to support the mushroom growth.
After mushrooms have been harvested for about three to four weeks, the medium’s ability to sustain mushroom growth is exhausted and replaced with fresh mushroom soil for a new batch of
mushrooms. The substrate is then sold in bags or bulk, often labeled as SMC (Spent Mushroom Compost) or SMS (Spent Mushroom Substrate).
The Pros and Cons of Mushroom Compost
Mushroom compost is not a cure-all for your garden soil. It has benefits and disadvantages.
| Pros | Cons |
| Supplies macronutrients (nitrogen, phosphorous, potassium), and micronutrients (calcium, magnesium, iron) to the soil | Nutrient composition varies depending on the materials used |
| High in organic matter, which increases the water-holding capacity of the soil, and boosts the activity of beneficial soil microbes | High in soluble salts and other nutrients potentially kills germinating seeds, harms young seedlings, and causes damage to salt-sensitive plants |
| Improves the structure of clay and compacted soils, which increases drainage | Pricey and not readily available |
| Suitable for most garden plants, both edibles and ornamentals | High calcium content increases the soil pH and makes it too alkaline |
| High levels of calcium which benefit some crops, such as tomatoes |
Mushroom Compost vs. Regular Compost
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Mushroom compost and regular compost from your compost heap or composter are not interchangeable due to key differences.
All compost varies in composition, unlike commercial fertilizer, where the label gives a detailed analysis of the nutrients. Mushroom compost has already been used as a growing medium, which means it generally contains less nitrogen than regular compost. It may be a good choice for soil already rich in nitrogen since excess nitrogen leads to overgrowth of foliage and stunted roots.
Mushroom soil also contains more calcium than regular compost due to the addition of chalk. Plants like tomatoes do well in a calcium-rich soil which helps tamp down blossom-end rot. However, calcium increases the soil pH level which is problematic for plants like blueberries that need acidic soil.
Mushroom Compost vs. Manure
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Mushroom compost and manure are two different soil amendments, not only in their composition but also in their use.
Mushroom compost contains a wide range of organic materials, including horse manure, which has been sanitized before the mixture is used for growing mushrooms. It is less dense, lighter than manure, and ready to be spread in the garden. Manure, on the other hand, regardless of the type (cow, horse, or poultry), only contains animal dung and straw or sawdust from the animal’s bedding or litter areas. Fresh manure is very high in nitrogen and needs to mature before being used, otherwise it will burn the plants.
Fresh mushroom compost has an average of 6.6 so it falls right into the ideal pH range for most crops between 6.0 and 7.0. Manure has a higher pH (above 7.0), and makes soil more alkaline.
Tip
If you are an organic gardener and want to ensure the mushroom source is fully organic, select mushroom compost from certified organic mushroom farms that practice organic fertilization, in addition to pest, and fungal disease management.
When and How to Use Mushroom Compost
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Most mushroom compost sold in the trade has been aged or cured but if you buy mushroom compost that is still fresh, it needs curing before you add it to your soil. Purchase fresh material in fall, winter, or early spring to give it ample time to sit and cure before planting or spreading it around plants. Rain and snowfall leach excess salts from the substrate and the already low pesticide residue that might be in the substrate from the mushroom growing has time to decompose.
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Apply aged, cured mushroom compost any time in the spring or summer. Application is most effective when the soil is fairly dry, which avoids compacting the soil while tilling in amendments.
Aged mushroom compost, either worked into the soil or spread around the base as mulch, can be added to a wide range of plants, including:
- Lawns, also as a top-dressing for newly seeded lawns
- Vegetable gardens
- Orchards
- Perennials, trees, and shrubs
- Container plants
For flower beds and vegetable gardens, evenly spread one to three inches of mushroom compost over the surface and till it into the top six inches of soil. For container plants, blend a ratio of 1/4 of aged mushroom compost with 3/4 of a potting medium.
FAQ
What plants like mushroom compost?
Most garden crops benefit from mushroom compost as a soil amendment, especially plants that tolerate higher salt content and benefit from additional calcium, such as tomatoes.
Can you plant directly in mushroom compost?
Do not use mushroom compost as your only growth medium, but work it into the soil. However, tender seedlings and seeds are sensitive to the high salt levels in mushroom compost so it’s better to wait until the plants are established before working some mushroom soil into the soil around the plants.
Can you use too much mushroom compost?
As with all soil amendments, overdoing it is harmful. Don’t exceed this recommendation: 1- to 3-inch layer or 3 to 9 cubic yards per 1,000 square feet.
What plants should you not use mushroom compost with?
Do not use mushroom compost on salt-sensitive plants such as azaleas, rhododendrons, camellias, and other members of the Ericaceae family because the high salt content can harm the plant. Also, keep mushroom compost away from seedlings, and don't add it to soil where you start plants from seed because the high salt in mushroom soil impacts germination.
The Spruce uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
Growing Mushrooms - Microbial Activity in Substrate. Penn State Extension.
Does Adding Manure Lower or Raise Soil pH? University of Florida.
Spent Mushroom Substrate. Penn State Extension.
Mushroom Compost FAQ. American Mushroom Institute.
