What Is Mushroom Compost and Why Use It

How Mushroom Soil Can Benefit Your Plants

Compost and native soil

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Unlike the name suggests, mushroom compost does not a contain any mushrooms—it is a by-product from mushroom farming; the growth medium of mushrooms that 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. It acts as both a plant fertilizer and soil amendment that can benefit your plants when used properly. 

Mushrooms growing in a greenhouse
Mushroom compost is a by-product of mushroom farming

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How Mushroom Compost Is Made

The substrate in which mushrooms are grown is based on a mixture of different agricultural materials: hay, straw, poultry or horse manure, ground corn cobs and hulls, cottonseed or canola meal, cocoa shells, crushed grapes from wineries, soybean mill, peat moss, and other natural organic substances such as gypsum, lime, potash, urea, and ammonium nitrate. The exact composition of the growth substrate varies from grower to grower.

This mixture is composted for about 30 days at a temperature of 160 degrees Fahrenheit, in which any weed seeds, pests, or disease pathogens are killed. Afterwards, the mature compost is steam-pasteurized at about 140 degrees Fahrenheit to kill any pests and pathogens that could cause surface diseases. Then the medium is inoculated with mushroom spawn, or mycelium, and covered with a layer of sphagnum moss and ground limestone to support the mushroom growth.

Mushrooms are grown and harvested for about three to four weeks. At this point, the medium’s ability to sustain mushroom growth is exhausted and replaced with fresh mushroom soil for a new batch of mushrooms.

No longer suitable for mushroom-growing, the material is still full of goodness for gardeners. Some mushroom growers treat it with steam to kill pests, pathogens, and weed seeds. The substrate is then sold as bags or bulk, often labelled as SMC (Spent Mushroom Compost) or SMS (Spent Mushroom Substrate). 

The Pros and Cons of Mushroom Compost 

Mushroom compost is not an 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 boosts the activity of beneficial soil microbes High in soluble salts and other nutrients that can kill germinating seeds, harm young seedlings, and cause damage to salt-sensitive plants 
Improves the structure of clay and compacted soils, which increases drainage  
Suitable for most garden plants, both edibles and ornamentals Pricy and not readily available
High levels of calcium which benefits some crops, such as tomatoes High calcium content can increase the soil pH and make it too alkaline

Mushroom Compost vs. Regular Compost

Mushroom compost and regular compost are not interchangeable due to key differences between the two. 

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 can lead 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.  

Tip

If you are an organic gardener and want to make sure the mushroom source is also fully organic, select mushroom compost from certified organic mushroom farms that practice organic fertilization, pest, and fungal disease management.

When and How to Use Mushroom Compost 

Most mushroom compost sold in the trade has been aged or cured but if you happen to 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. 

Topdressing lawn

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Aged, cured mushroom compost can be applied 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

Make sure to only use well-aged mushroom soil around rhododendrons, azaleas, camellias, and other members of the Heath family, (Ericaceae) because the high salt content can harm the 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, mix about one quarter volume of aged mushroom compost with three-quarters of potting medium.

Article Sources
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  1. What Is Mushroom Compost? Oregon State University Extension Service.

  2. Spent Mushroom Substrate. Penn State Extension.