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.
Mushroom compost, sometimes called spent mushroom substrate, has become a popular soil amendment for home gardeners and farmers. As the name suggests, this material is left over after mushrooms have been commercially grown. While it has some benefits for enriching garden soil, mushroom compost also has some drawbacks to consider. Let’s look at the pros and cons of using mushroom compost in your garden.
What is Mushroom Compost?
Mushroom compost is the leftover growing medium from commercial mushroom farms once the mushrooms have been harvested. It is composed of wheat straw, hay, or other agricultural wastes along with nitrogen sources like chicken manure, horse manure, cottonseed meal, and corncobs.
To prepare mushroom compost the straw is soaked and chopped into small pieces. The manure and other amendments are added in specific ratios. This mixture is put into piles watered, and turned frequently to heat up to at least 160°F through microbial activity. This pasteurization process kills any pathogens, viruses, weed seeds, and insect larvae in the compost.
After 2-3 weeks, the dark brown, fertile compost is inoculated with mushroom spawn. The mushrooms grow quickly under controlled conditions. Once harvested, the spent compost is sold off rather than dumped as waste This gives mushroom farms an additional revenue stream while supplying gardeners with an organic soil enrichment
The Advantages of Using Mushroom Compost
There are several reasons why mushroom compost has become a gardening staple
1. Improves Soil Structure
The biggest benefit of mushroom compost is improving the physical structure of soil. The fine particles and fiber add organic matter that helps bind minerals and enhances moisture retention. The improved aeration and drainage prevent densely packed, waterlogged soil.
2. Provides Organic Nutrients
Mushroom compost contains partially broken down organic matter and microorganisms that release nutrients for plants as it further decomposes. This organic fertilizer adds key macronutrients nitrogen, phosphorus, and potassium, as well as a range of micronutrients.
3. Increases Microbial Activity
The microbiome of beneficial bacteria and fungi in mushroom compost colonizes garden soil and boosts microbial activity. These microbes help plants access nutrients while decomposing organic matter.
4. May Suppress Diseases
Some research indicates mushroom compost enhances populations of disease-suppressing microorganisms like pseudomonads and actinomycetes. It may help prevent certain fungal and bacterial plant diseases.
5. Is Widely Available
Mushroom compost is commercially produced on a large scale and sold bagged at garden stores or in bulk. It’s generally affordable and accessible for gardeners.
6. Odorless and Easy to Apply
Unlike manure-based composts, mushroom compost has an earthy, soil-like scent when moist instead of being foul smelling. It can be spread and mixed into garden beds without an unpleasant odor or dust.
The Disadvantages of Using Mushroom Compost
On the other hand, there are some potential drawbacks to using mushroom compost that should be kept in mind:
1. Low in Nitrogen
Fresh mushroom compost is depleted in nitrogen since the mushrooms growing in it absorb most of this nutrient. The carbon to nitrogen ratio is wide, and nitrogen levels may be too low for good plant growth.
2. May Contain Pesticides
Mushroom farms sometimes use fungicides or insecticides that could persist in the spent compost sold to gardeners. These should biodegrade over time, but chemical residues are a concern.
3. High Salinity
Salt content can be high in some mushroom composts, whether from manures, fungicide drenches, or supplements added during production. Excess salts cause osmotic stress for plants.
4. Can Burn Plants
Uncomposted fresh mushroom substrate direct from farms can actually damage plants due to high ammonia levels. Mushroom compost needs curing time to finish the composting process and reduce phytotoxicity risks.
5. May Spread Diseases
Mushroom farms aim to produce disease-free compost, but some pathogens could potentially survive pasteurization. Examples are mushroom viruses, bacteria like Pseudomonas, or fungal diseases.
6. Weed Seeds May Persist
While the heating phase kills most weed seeds, some may recontaminate the mushroom compost after pasteurization or overwinter and sprout later.
7. Can Be Variable
Since formulations vary between mushroom farms, the quality and characteristics of mushroom compost products also varies quite a bit from source to source.
How to Use Mushroom Compost Effectively
Follow these tips to maximize benefits and avoid potential issues when using mushroom compost in your garden:
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Look for compost that has been cured for 1-2 months after harvest for safety and stability. Avoid fresh, hot compost direct from farms.
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Test the pH and salt content before applying.Neutral to slightly alkaline pH around 6.5-8.0 is best.
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Mix mushroom compost into soil instead of using as a surface mulch to dilute salts and avoid burning plants. Thoroughly incorporate compost.
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Limit applications to a thin layer about 1-2 inches deep mixed into the top 6 inches of native soil.
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Supplement with a nitrogen fertilizer or extra manure if plants show signs of nitrogen deficiency.
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Avoid mushroom compost for salt-sensitive plants like azaleas, blueberries, and conifers. Stick to well-composted manure mixes instead.
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Discontinue use if compost seems to spread disease. Sterilize tools after use in contaminated beds.
An Organic Gardening Amendment with Some Caveats
Mushroom compost can be a good addition to improve soil structure, nutrition, and microbial activity if sourced carefully and applied correctly. However, gardeners should be aware it has some potential downsides to manage. Test compost before use, blend into soil, and supplement with fertilizer as needed for the best results from mushroom compost. With a bit of care, it can be a sustainable organic amendment for healthy gardens and crops.
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.
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).
Is Mushroom Compost Good For A Garden Or Should You Avoid It?
FAQ
When should you not use mushroom compost?
Mushroom compost use should be avoided where ericaceous plants such as rhododendrons, camellias, azaleas and heathers are being grown, as these plants need acidic growing conditions and are chalk-hating.
What plants do not like mushroom compost?
It makes sense that alkaline-loving plants like brassicas will thrive in soil amended with mushroom compost. Using the same logic, we also know that it’s best to avoid using mushroom compost with ericaceous (acid-loving) plants.
Can you have too much mushroom compost?
A: Yes, but depending on your base soil (clay, sand) and your water-table level, you may experience a build-up in soluble salts.
Can you grow plants directly in mushroom compost?
You don’t want to plant directly into the compost, since it is much too rich for this type of application.