Mushroom Compost: What It Is, What It Does, And How to Make It

Mushroom Compost

You’ve probably seen bags of mushroom compost in your local nursery but never knew what they were for.

In this article, I’ll explain what is mushroom compost, how to use it and what is it made from. This way, when the right situation comes up, you can use this wonderful product to improve your garden soil.

Let’s get started.

What Is Mushroom Compost?

Mushroom compost is an organic fertilizer used for plants. It is commonly sold in nurseries and garden centers, sometimes being labeled as mushroom soil.

What makes it different from traditional garden fertilizer is that is a slow release. And, it is compost that’s been used to grow mushrooms.

That said, it is worth noting that the commercial mushroom compost you find store shelves are not necessarily made from compost from growing mushrooms.


Mushroom Compost for Gardening

Mushroom Compost

To better understand mushroom compost it is a good idea to see the packages they come in.

In most stores, you’ll see them labeled as either

  • Spent mushroom compost (SMC) or
  • Spent mushroom substrate (SMS)

But they mean just the same thing.

Depending on how much you need, you can get them in bags or for larger landscaping operations by the bushel or truckloads.

Mushroom compost is most often used as soil amendment in gardening. This makes it useful for lawns, flower beds, gardens and even potted plants.

However, like fertilizer and other amendments you do want to use it properly. That’s because mushroom compost is high is soluble salts.

Thus, using too much of it can make your soil similar to that near the beach where plants have a hard time growing. High salt levels also tend to be problematic to seedlings as well as kill seed that are germinating.

As such, you want to limit the amount you use during these stages as well or on salt-sensitive plants.


Mushroom Compost/Substrate

To avoid confusion, I’d like to take a step back and explain the difference between:

  • Mushroom compost or mushroom substrate
  • Spent mushroom substrate

In general, mushroom compost is the medium that the mushrooms have been grown in. Again, combination of ingredients can vary. But, in many cases, they will include poultry manure, straw and other ingredients.

For commercial purposes, large quantities need to be used. As such, you’ll see bales of straw being blended in with manure and gypsum.

The ingredients will be used to make a compost pile similar to those you see in backyards, albeit with different ingredients and recipes.

From there, both water and air are used in balanced amounts that will eventually cause the compost pile to heat up.

The process will take a few weeks after which, you’ll see the piles of compost turn into dark brown, soil-like material.

In order to make sure that there are no pathogens and weed seeds, the compost is then steam pasteurized. This ensures that the product you get is sterile with no potential of causing soil diseases.

To help you better understand the scale of production and how mushroom compost is made, here’s a video of the production process.


Spent Mushroom Substrate

In most cases, the product you pick up or see in store shelves is this one, spent mushroom substrate. That is, it is mushroom compost that has already been used to grow a batch of mushrooms.

As such, it is not too useful for growing a new batch of mushrooms. Instead, it is beneficial to gardeners as a soil amendment.

While it does not contain a lot of nutrients (since the mushrooms have already consumed most of it), spent mushroom substrate is useful for improving soil quality by adding density. It also helps break up heavy soil like clay.

In some cases, spent mushroom substrate is also sold as turf conditioner. You can use this to improve the quality of the soil in your lawn.


How is Mushroom Compost Made?

Mushroom Compost

As such, the mushroom compost you actually get is the leftover soil that mushroom growers sell after they harvest their crop of mushrooms.

While the nutrients have been depleted from growing the mushrooms, the compost is still a valuable soil amendment or soil conditioner as it contains high organic matter.

Unlike plants, mushrooms are not green in color. That’s because they do not have or produce chlorophyll. The downside of this is that they cannot produce their own food (carbohydrates) via photosynthesis like other plants do.

Thus, the medium in which the mushrooms are grown in need to supply them will all their nutritional requirements. This means the medium (or substate) has to contain all the essential nutrients the mushrooms need.

And as you would guess, each grower will have their own recipe for mushroom substrate. Therefore, they’ll likewise be using different ingredients. However, many will have a few similar ingredients, including straw, poultry or other manure, gypsum and water.

After the mushrooms are harvested, the leftover soil is then sold by the farmers or growers. These are what you find in stores in the form of spent mushroom compost (SPC) or spent mushroom substate (SPS).

They now have the word spent in front of their labels because the can’t be used for growing mushrooms anymore. Instead, their nutrients have mostly been spent or used up.

However, SPS or SPC is often used as soil amendment or conditioner in gardening or lawn care because it improves water holding capacity, water infiltration, permeability and aeration of soil.

But, you also need to be careful when using mushroom compost because of its salt content. Thus, using too much will harm seeds and seedlings as well as hinder plant growth.




What is Mushroom Compost Used For?

Mushroom compost needs to be used with soil. While it helps improve soil, it does not and cannot replace soil.

In fact, you want to use more soil than mushroom compost. A ratio of 75% soil to 25% spent mushroom compost is a good starting point for container plants.

And, while the nutrients are mostly depleted, it still works as a slow release fertilizer. As such, you can think of it as something like a 2-1-1 formulation.

But, like soil, it is not meant to replace fertilizer because the amount of nutrients left is on the low side.

Unlike fertilizer it has the ability to improve soil including improve water retention and make soil less dense.

Keep in mind that it is high is soluble salts. As such, don’t use it for seeds, seedlings as well as salt-sensitive plants including camellias, azaleas and rhododendrons.

Spent mushroom compost also has a pH of around 6.6 in most cases. Although, it is a good idea to always check the pH since some are on the alkaline side.

In addition to being a soil amendment, I can be used for topdressing and as mulch as well.


Mushroom Compost Benefits

Like all things mushroom compost has its benefits and drawbacks. But, in this case, the pros outweigh the cons. Nevertheless, you do want to know how to use mushroom compost properly before applying it to your garden regardless of the fact that it is an inexpensive soil amendment.

  • Inexpensive – yes, mushroom compost is cheap. It costs less than other composts and fertilizer which makes it a good option if you’re on a
  • Improves water retention – one of its biggest advantages is its ability to improve the water holding capacity of soil. This makes it helpful if you want to save on your water bill or reduce the frequency with which you need to water your garden.
  • Eco-Friendly – mushroom compost is an organic soil amendment. It is a byproduct of the soil and is being reused to improve the quality of your soil.
  • It improves soil structure – adding mushroom compost to soil improves the soil’s drainage. It also helps to break down dense soil. And, it will help reduce waterlogging in the long term.
  • Low nitrogen content – since most of the nitrogen has been used to grow the harvested mushrooms, the mushroom compost you get has mild nitrogen levels. While this may sound bad from a fertilizer point of view, it actually helps in the right conditions. One example is if you don’t want too much vegetative or leaf growth and instead prefer flowering and fruits. Lower levels of nitrogen also keep weeds from quickly growing.
  • Acts as a slow release fertilizer – in addition to low nitrogen levels, the other nutrients have likewise been depleted. As such, mushroom compost works more like a slow release fertilizer instead of your fast-acting liquid formulations.
  • High calcium content – mushroom compost contains high levels of calcium which makes it useful for some plants, fruits and vegetables. Calcium is a secondary nutrient that’s essential to healthy plant growth. And, for some plant’s like tomatoes, lack of calcium be harmful. In this case, you get blossom end rot.
  • Attracts earthworms – earthworms are beneficial to soil as they improve its overall structure and drainage. In doing so, they help roots absorb more nutrients from soil.
  • Works well as mulch – in addition to being used as soil amendment, you can likewise use mushroom compost as mulch. Adding a layer over the soil will help with insulation and keep the roots of the plants moist and also cooler.


Disadvantages of Using Mushroom Compost

Most of the drawbacks of mushroom compost are a result of improper use or not knowing when or how to use it. As such, it is important to know when mushroom compost is helpful or not in your garden.

  • Increases the risk of waterlogging – for gardens or plants that have low water requirements, using mushroom compost will keep soil too moist. If you follow your regular watering schedule this can result in waterlogging. As such, make sure you know what plants you’re growing when using mushroom compost. Succulents, cacti and other plants that prefer dry conditions will experience problems due to mushroom compost’s water retention ability.
  • Increases the risk of root rot and fungal infections – too much moisture retention if not fixed soon enough will eventually encourage fungal growth as well as root rot.
  • High salt content – mushroom compost is naturally high in salt. As such, when using it, you want to dilute it and also only apply to plants that can tolerate the salt levels. Seeds, seedlings and unestablished plants are all no-no’s. Similarly, plants that are sensitive to high salt content won’t be able to tolerate it.
  • No beneficial microorganisms – to make sure that there are no pests, pathogens and weed seeds, mushroom compost is sterilized before it is packaged. While this ensures no diseases and other harmful contaminants, the process also eliminates all beneficial microorganisms.
  • Some are alkaline – before you use mushroom compost, check to see what its pH level in. In general, it should be neutral. But, that’s not always the case. Some have high pH levels, which if used can hinder or prevent some plants from being able to absorb nutrients. This will result in slow, stunted or no growth. As such, use a pH test to check before applying.



Now that you know what mushroom compost is, how it is made, when and how to use it, you can pick some up the next time you see a package in your local nursery.

The most important thing about this soil amendment is knowing the right situations where to use it. This way, you not only save money but also let your plants grow their best.

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