What Is Mushroom Compost?

Last Updated on March 31, 2022 by Grow with Bovees

We all know that compost is great to use in any garden, to enrich the soil and make for healthy plant growth.

But, have you ever come across or heard of mushroom compost?

Mixing mushroom compost into your soil is great to add to your gardening routine. Not only is it great at breaking down dense and compact soil or clay, but it is also great at retaining moisture and building soil.

To follow in this article, we shall answer the question “what is mushroom compost?”, and also discuss some mushroom compost benefits, and how you might be able to use it, whether you can make it yourself and more gardening tips!

What Is Mushroom Compost?

Mushroom compost is a nutrient rich organic compost. Its texture is of a crumbly nature, and it is dark brown, letting off a lovely earthy smell.

Along with potassium, nitrogen and phosphate, this compost is also made up of iron, calcium and magnesium. These components are all very important nutrients to a wide variety of plants.

It is often used as a light fertilizer and aids in retaining moisture when mixed into the soil.

One could say that mushroom compost comes in two types. One is known as mushroom substrate. This type is mainly made to plant mushrooms in. And then you have spent mushroom compost. This is a secondary product that is produced post mushroom growth.

Mushroom Compost

Mushroom substrate is most commonly made of a mixture of horse manure/chicken manure, gypsum and straw and is frequently used by mushroom growers.

The process of making mushroom substrate starts with bales of straw being saturated with water after which they get pushed through a wood chipper in order to break them down into loads of tiny pieces.

After the wood chipper is done chopping, the little pieces of saturated straw then get combined with manure and gypsum, after which this mixture is arranged in warm compost piles. To start the process of composting, water is added to the piles.

Two weeks later, the hot compost piles are lined up and left alone for a further two weeks to continue composting.

The next step is to pasteurize these piles of mushroom compost. This is done to sterilize it, removing any unwanted weed seeds and microorganisms.

The mushroom substrate is now a mushroom-growing medium ready to be planted together with spores of mushrooms.

One can also add the mushroom substrate to the soil just as it is. It will enrich the soil, adding a great amount of vital nutrients.

Spent Mushroom Compost

A mushroom substrate is most commonly, however, used to cultivate mushrooms. And when it is used as such, the result is the compost breaking down to become a substance resembling soil. Mushrooms grow followed by a steam treatment. This is a pasteurization method to remove harmful bacteria and pests.

The compost that is left over after this process — the waste compost — gets filled into bags after which it can be purchased and sold to landscape supply firms as spent mushroom substrate.

Generally speaking, one can say that spent mushroom substrate is recycled mushroom compost from mushroom farming.

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Spent mushroom fertilizer is also the type of mushroom compost that one would most commonly find in garden centers.

It does have less nutritional value to it than non-spent fertilizer. This is due to the mushrooms already having consumed a lot of the nutrients, including nitrogen.

What Is It Made Of?

Many people assume that mushroom compost is made up of mushrooms — this is thanks to the name I suppose. This is, however, not the case.

Mushroom compost, in fact, consists of organic materials that you would use or have already used in the compost that you would use to cultivate mushrooms.

The most common organic matter or compost ingredients would be hardwood dust, coconut coir, cottonseed hulls/cottonseed meal, canola meal, straw, hay, animal manure, gypsum, chalk, sphagnum moss, peat moss, soybean meal and lime.

According to a study conducted by the Pennsylvania State University, mushroom compost has a moisture content of roughly 58%, meaning that it indeed contains great amounts of water.

Benefits Of Organic Mushroom Compost

Like with all things green, this one, too, has a great number of benefits. Continue on as we discuss a few of them below.

It Holds Moisture

A briefly mentioned above, mushroom compost is great at holding onto moisture. Which also means that it aids in the transportation of water through the soil, keeping it nice and moist.

So, if you have plants in your garden that require a high amount of moisture, but you do not have the time to water all that often, adding mushroom compost to your soil is a great and helpful addition. It can in fact cut down the water needed by half.

The high moisture content of mushroom compost also helps the environment by saving water. And seeing as it is a recycled by-product of the mushroom growing industry which does not contain any other added materials, it makes a lovely eco-friendly product to use in your garden.

Improves Soil Structure

Commercial mushroom composts often have a straw base, making them great for amending dense clay-like soils over time. Add it to fairly dry garden soil and over time, it will loosen up and break down the dense soil, improving its structure and ability to drain. This in turn makes it satisfactory to grow plants.


As plant lovers, we should know that nitrogen is important for the growth of foliage and healthy plant growth. Seeing as the nutrient content of mushroom compost is not too dense and contains only a small percentage of nitrogen, it is suitable to use as a light fertilizer. It slowly enriches the soil without encouraging weed growth.

Can You Plant With Mushroom Compost And No Garden Soil?

It is not a good idea to replace planting soil with mushroom compost when planting plant seeds. This type of compost contains large amounts of salt which may be harmful to some plants.

Due to its ability to retain moisture, it may become waterlogged, which could lead to rotting and fungal infections.

Rather, mix the mushroom compost with garden soil. Mix it at a ratio of 1:3 — 1 part compost and 3 parts soil — and the salt content will be diluted.

This way, the mushroom compost will also behave like a slow-release fertilizer. Planting seeds into this mixture will provide them with a lovely steady dose of nutrients.

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Which Plants Benefit If You Use Mushroom Compost?

As the name reveals, growing mushrooms benefit greatly from this type of compost. But, seeing as it works as a light fertilizer, to use mushroom compost on other garden plants such as fruits, fruit bushes, fruit trees, vegetable gardens, vegetable beds and different potted house plants will also be beneficial.

The large amounts of calcium in mushroom compost is quite helpful when growing tomatoes, for example. They often die from blossom-end rot due to a lack of calcium.

Hibiscus and other tropical plants will also benefit greatly from mushroom compost as these types of plants like and need quite a lot of moisture and hydration.

Another great use for mushroom compost, is mixing it into the soil when planting grass seeds or laying new turf. It will encourage its growth and aid in a beautiful green appearance.

When Not To Use Mushroom Compost

Like with everything that has benefits, mushroom compost can also come with disadvantages when not used correctly. Let us have a look at a few points on when not to use this type of compost.

Water Retention

The ability to retain water is great, but it could, in some situations, be quite the problem. Do not mix mushroom compost into the soil of plants that does not require high amounts of moisture. Succulents, for example, need good drainage and limited amounts of water.


As mentioned before, mushroom compost gets sterilized before it can be purchased. This means that it does not contain beneficial microorganisms that naturally would protect your plants against diseases and infections.

If you add mushroom compost to your plants to protect them from harmful diseases, we suggest that you blend in another compost — which re-adds microorganisms — before using it in your plants soil.

Salt Levels

Due to the great amount of soluble salts in mushroom compost, it should not be used for germinating seeds. It will harm and possibly kill germinating seeds and young seedlings.

It will especially harm salt sensitive plants such as Camellias, blueberries and azaleas.

Compost pH Levels

Generally, mushroom compost has quite a neutral pH. If it contains chalk, however, the pH might be alkaline. In this case, mushroom compost would be bad to use for plants such as azaleas or rhododendrons that thrive in acidic soil, but it could be useful to change the color of hydrangea blooms from blue to pink.

Making Fresh Mushroom Compost At Home

Yes, you can make a batch of mushroom compost in the comfort of your own home. The first thing you need to decide is which organic matter you are going to use in the compost process. Some popular organic materials would be coffee grounds, horse manure and straw.

If you choose to use bales of wheat straw or wheat seeds and are only in need of a small amount, follow the steps below.

Making Mushroom Compost Using Straw

Step 1: Use a wood chipper or something similar to break down your straw into smaller pieces.

Step 2: Make a mixture of water and dishwashing liquid and clean your straw, getting rid of weed seeds and microorganisms that might be present on the straw. Rinse the soapy water off with clean fresh water.

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Step 3: Submerge the straw into boiling water to sterilize it. Tip: Placing the straw into a strainer before placing it into the hot water, will make it easier to remove once pasteurization is done.

Step 4: When the straw is completely submerged in the boiling water, decrease the heat. The temperature of the water should be at a temperature of around 70-80 degrees Celsius. Let the straw pieces soak and rest for about 1-2 hours.

Step 5: Prepare a contaminant-free and clean area or surface and lay out the straw to drain and cool.

Once the straw has cooled down, the organic mushroom compost can be used to plant with mushroom spores.

Making A Mock Mushroom Compost Pile

If you feel that making your own mushroom compost requires too much effort, simply make mock mushroom compost. This is one of the alternative mushroom composts, and it carries many of the same benefits.

Start by combining manure — chicken or cow manure is usually used — chopped straw and soil. The ratio of this mixture of organic material should be roughly 20% manure, 40% straw and 40% soil.

Thoroughly combine these ingredients and make a compost pile.

Every two weeks, go about turning the compost heap, ensuring that the deep parts also get mixed well. Don’t forget to check the moisture level of the soil. you don’t want it to be soaking, but rather moist. .

If you notice excessive moisture, spread it out and let it dry. Make sure to cover the pile if there’s a chance of rain. You want moisture levels to stay consistent.

If you feel that the mixture is too dry, simply add a bit of water and mix it well, keeping the soil moist.

After having turned the compost heap 5 times at 2-week intervals, cover the compost material with a tarp and let it age for about one month or longer.

Don’t Want To Make It? Then Buy It!

Not in the mood or do not have the time to make mushroom compost at home. Do not stress. Go out and buy mushroom compost at landscape suppliers and garden centers.

Look out for labelling that reads either SMS — abbreviated for spent mushroom substrate — or SMC — short for spent mushroom compost.


So, to sum up the above information, mushroom compost that you buy in the store is basically left over material from the mushroom farming industry. It is most commonly found under the name of spent mushroom compost.

This compost type is inexpensive and rich in nutrients. We recommend you buy it or make your own if you wish to improve the quality of your garden soil. It is suitable for most environments and conditions.

Using mushroom compost as a soil amendment for your vegetable garden and flower beds and containerized plants, is an excellent way to improve soil quality.

Beneficial soil worms love it, making it the perfect addition for healthy and prosperous garden soil, helping plants to thrive and to mature healthily.