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Organic Gardening

All About Mushroom Compost for the Vegetable Garden

mushroom soil

If you’re a homesteader or gardener looking for ways to maximize your soil health and grow abundant gardens, then mushroom compost just might be the answer. It’s a versatile organic material used most often by organic farmers and gardeners because of its nutrient content that helps feed plants what they need throughout their growth cycle. In this blog post, we’ll look at how mushroom compost is made, where to buy it, and why sustainable gardeners prefer mushroom compost over traditional fertilizer or manure for nourishing their vegetable gardens.

This post is all about mushroom compost for the vegetable garden.

What is Mushroom Compost

Mushroom compost, or mushroom soil, is a great addition as a soil builder. It can’t replace your soil though. Contrary to what you may think, mushroom compost isn’t made from mushrooms.

Mushroom compost is what’s known as the ‘substrate’. The ‘substrate’ is the medium in which mushrooms are grown. The most common blends for the substrate have: wheat straw, gypsum, and horse or chicken manure.

What you buy at the store to use in your garden is actually ‘spent mushroom substrate’. This means it’s already been used to grow mushrooms. The substrate is given a fungal inoculation when it’s going to be used to grow mushrooms. This fungal inoculation causes the compost to break down and it then becomes more soil-like.

An important thing to keep in mind: since it has already been used to grow a crop, it is low in nutrients.

[RELATED POST: The 5 Best Composts for the Garden]

How is Mushroom Compost Made

Mushroom compost is the medium, or substrate, that mushrooms are grown in. Essentially it’s a by-product of mushroom farming. Commercial products commonly mix it with wheat straw, gypsum, and horse or chicken manure. However, there are many different ingredients that can be used. Manufacturers take large bales of straw and soak them for a few minutes in water. After a few mins the bales are then saturated and then run through a chipper to chop it up to help break it down. The mixture of other ingredients are then added.

Once all of the ingredients are thoroughly combined, it’s then put into a compost pile like any other compost. The pile sits for a number of days to heat up, which is an important part of the process. Bacteria forms in the pile, causing the rise in temperature. By allowing the pile to heat up, it helps to not only break down the materials, but kills off any weeds or pathogens. Compost piles can reach up to about 160 degrees Fahrenheit!

After the composting stage, the pile is then put in a cool, dark place to cool down and pasteurize. The pasteurization process allows for further elimination of any organisms or pests that could still be present.

Inoculation of the Mushroom Soil

After a few weeks, the pile is now ready to be used for growing mushrooms. The compost is inoculated with mushroom spawn, or mycelium. The mycelium form roots and after 5 weeks, mushrooms are ready to be harvested. The harvest lasts for another two to three weeks, after which, the substrate is considered “spent”.  On the bags, they’re labeled as SMC or SMS (spent mushroom compost or spent mushroom substrate).

However, this isn’t the end of the road for the substrate as it can be sold and added into the vegetable garden like any other compost.

What are Mushroom Compost Benefits

Although the substrate was already used to grow mushrooms and is lower in nutrients, there are still some benefits. It is still useful as a soil amendment since the straw continues to break down. The straw is beneficial to clay soil, and is great for water retention. Mushroom substrate also adds structure to soil. It’s rich in soluble salts, as well as the following mineral and nutrients:

  • Nitrogen

  • Potassium

  • Phosphorus

  • Magnesium

  • calcium

Here are the pros and cons of using mushroom compost:

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Pros of Mushroom Compost cons of mushroom compost
Eco-friendly High in soluble salts
increases water retention can absorb too much water resulting in water logging
High in calcium Because of risk of water logging risk, it can cause root rot
Attracts earthworms No beneficial microorganisms
Slow release of nutrients and fertilizer Reduces acidity of soil, can cause soil to be alkaline
Great to be used as a mulch Cell

What’s the Difference Between Mushroom Compost Versus Manure

While mushroom compost is a “spent” medium used to grow mushrooms, manure is animal dung. We’ve already examined how mushroom compost is made, but what about manure? How is that prepped?

Manure is either left in its original form, pasteurized, or mixed with straw and hot composted then matured.

The pH levels of mushroom compost and manure differ. Mushroom compost is alkaline, while manure can either be neutral or slightly acidic. What the animal ate affects the pH of the manure. Mushroom compost slowly releases its nutrients over a long period of time. However, it’s not suitable for all plants because of the high amounts of salts it contains and shouldn’t be used with acidic loving plants.

Where Can I Get Mushroom Compost Near Me

Mushroom compost is widely available in most  big box stores, and may be available at local nurseries and small businesses.

The most popular brands are:

  • Epsom Organic Mushroom Compost Blend
  • Organic Valley Mushroom Blend

If you’re not sure where it’s sold, type into a Google search, “mushroom compost near me”. Google will then provide results based on your location.


What vegetable plants do not like mushroom compost

Acidic loving plants don’t like mushroom compost so it’s recommended not to use them. It won’t work well with the following plants (examples, not complete list):

  • Tomatoes
  • Squash
  • Carrots
  • Cucumbers
  • Broccoli
  • Potatoes

You should also not use it with any acid loving trees, flowers, herbs, or bushes.  The reason why it shouldn’t be used with acid loving plants is because it lowers the pH levels of the soil.

Additionally, because of its high salt content, it shouldn’t be used to germinate seeds, with young plants, or any salt sensitive plants like blueberries.

Can I plant directly into mushroom compost

If you’re using any type of compost, including mushroom compost, it’s not recommended to plant directly into it. Mix the compost with garden soil and then plant your plantings.

Is mushroom compost good for tomatoes

Since mushroom compost reduces the pH levels in the soil, it’s not advised to use it with tomatoes. Tomatoes are acid loving plants.

When should I add mushroom compost to my garden

Add mushroom compost as you would any type of compost. I like to apply compost and mulch in both the fall and the spring.

Can you use too much mushroom compost

Yes, too much of a good thing can be a bad thing. Too much mushroom compost can result in soggy soil, cause burning of plants, and cause plants to wither and not thrive.

What We’ve Learned

Mushroom compost is a “spent” medium used to grow mushrooms, while manure is animal dung. Manure can be left in its original form, pasteurized or mixed with straw and hot composted then matured. Mushroom compost has an alkaline pH level while manure can have neutral or slightly acidic levels depending on the animal’s diet. Mushroom compost slowly releases nutrients over time but is not be suitable for all plants due to high salt content and because it reduces the acidity of the soil. Mushroom Compost is widely available at big box stores as well as local nurseries and small businesses; searching “mushroom compost near me” will provide results based on location.When using any type of compost, don’t plant directly into it. First mix the compost with soil before planting. Apply compost in both the fall and spring seasons. Beware that using too much can create soggy soil, burn plants, and/or cause plants to wither.

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