What is Mushroom Compost? Not What You Think

What is Mushroom Compost

You probably know that many gardeners use mushroom compost as a soil builder to keep their soil moist, break down clay, and provide key nutrients to their garden beds, shrubs, or lawn. But exactly what is mushroom compost? 

Let’s break it down because it’s not what you think.

What is mushroom compost?

Nope, mushroom compost does not refer to mushrooms broken down into compost. It’s actually the leftover soil from mushroom farming. Once mushrooms are harvested, leftover soil is gathered, pasteurized, and sold as a fertilizer. 

Mushroom Compost

It is a combination of organic materials such as straw, hay, poultry manure, horse manure, and hulls. Different variations also include peat moss, soybean meal, gypsum, and lime.

Dr. Fidanza and Dr. Beyer (Pennsylvania State University) advise using mushroom compost as a soil conditioner. When used as a soil conditioner it slowly releases organic matter and macronutrients into the soil.

Mushroom compost is high in primary macronutrients such as nitrogen, phosphorus, and potassium. It also contains key secondary macronutrients such as calcium, magnesium, and sulfur. Lastly, it is full of key micronutrients including iron, manganese, copper, and zinc.

Mushroom compost is a powerhouse of nutrition for your garden, unlike any other soil amendment out there!

Types of mushroom compost

The term “mushroom compost” can be applied to both mushroom substrates, the medium in which mushrooms are grown and spent mushroom compost.

Mushroom Substrate

Mushroom substrate is the bulk material used by mushroom farms to grow mushrooms. Substrate is typically a mixture of manure, gypsum, and wheat straw. Wheat straw is worked into manure and gypsum, composted, and sterilized.

Spent mushroom substrate

Mushroom Substrate

As the name implies, spent mushroom substrate is the soil-like material left over from mushroom farming. It is more economical for growers to dispose of old mushroom substrate and replace it.

Spent substrate can then be “pasteurized” with steam to kill any pathogens or pests present in the casing and substrate. This also kills off any organisms that may cause mushroom diseases, weed seeds, and insects.

After pasteurization happens, growers can use spent mushroom substrate in their gardens.

How to make mushroom compost at home

To make it at home, you’ll need: raw organic materials, an open and spacious composting site, and a compost turner or tractor-loader.

Phase 1: Mixing the Compost

  1. Mix the raw materials together. A mixture of horse and poultry manure, hay, gypsum, and ammonium nitrate is an example formula often used in commercial mushroom composts. There are others. 
  2. Begin the composting process by moistening the ingredients. Soak straw in water and then crush it with a crusher machine. Be sure that it is not too wet or too dry. PRO TIP: You want it to be as moist as you keep your traditional compost pile.
  3. Blend chicken manure, horse manure, and gypsum together. If you live in a rural area, reach out to a local farmer and offer to take any manure off their hands. It’ll be free for you and they’ll thank you for the service. Pro Tip: If you have leftover manure, make manure tea
  4. Move all the ingredients into long rectangular piles. Turn the piles every day or two. The piles should heat up during the composting phase to 140-160F. This phase of composting lasts anywhere from 7-14 days.

Phase 1 is complete when a) the hay becomes soft and pliable, b) the raw ingredients can hold water, and c) the mixture has a sweet smell and is dark brown.

Phase 2: Finishing the Compost

15-20 days after composting, the pasteurization process can begin. Pasteurization kills weed seeds, bacteria, and ammonia to create a clean product.

To pasteurize, boil a large pot of water then place the composted material in a cloth or mesh bag and submerge it into a pot.

Turn off the heat and let it sit for one to two hours. This will ensure everything is thoroughly pasteurized.

If you have hardwood sawdust in your mixture, it is import to fully sterilize your compost. The most effective method to sterilize mushroom substrate is pressure cooking it at 15PSI and over 250F for 2 to 3 hours.

Prior to pressure cooking, ensure that all the mushroom substrate is moist and inside its growing bag.

What is it used for?

Mushroom compost can be used as a soil conditioner or mulch. It helps add key nutrients to soil, retains moisture, and improves soil texture. Moreover, it is extremely beneficial for soils with low organic matter. The added chalk and lime doesn’t hinder plant growth like it would in alkaline or chalky soils.


Is mushroom compost good for tomatoes?

Yes! It provides calcium and increased alkaline matter to the soil which helps ensure that tomatoes don’t fall victim to blossom end rot. It also helps soils retain water to help tomato plants thrives.  

Are there plants that don’t like mushroom compost?

Dr. John Hart (Oregon State University) highlights that it might be “too much of a good thing” for germinating seeds, seedlings, and young plants. When using it on young plants, he recommends mixing it with garden soil.

Most plants typically grow well in a soil pH range of 6.0-7.0. The average pH of mushroom compost is 6.6, which makes this fertilizer an ideal fertilizer for most plants that thrive at a typical range. 

What are tips for planting in mushroom compost?

This compost contains quite a bit of chalk and is alkaline in nature. Prior to applying it, remove all visible pieces of chalk. It is also advised to mix it with well-rotted manure and/or traditional finished compost.

What are the benefits of mushroom compost for vegetable gardens?

This compost is a great fertilizer and soil builder for vegetable gardens. It has an array of macronutrients and micronutrients that make it a great soil conditioner.

It can work to improve the overall soil structure, provide key plant nutrients, increase plant root structure, and improve water soil status. Moreover, it has a relatively balanced pH so the available nutrients can easily be taken up by most plant roots.

It is especially beneficial in raised beds that are growing cabbage, broccoli, cauliflower, Brussels sprouts, and kale.

What are the benefits of mushroom compost tea?

Mushroom compost tea is a great liquid fertilizer to protect plants from diseases and bacteria.

Mushroom compost vs cow manure: which one is better for my soil?

Both fertilizers are great sources of nutrients for soils. However, un-aged cow manure that hasn’t been composted contains a risky amount of bacteria pathogens, ammonia, and weed seeds. You should definitely (at least partially) compost cow manure prior to using it on your garden.

On the other hand, mushroom compost (pasteurized substrate) doesn’t have these issues. In the end, it all comes down to what you have access to and if you plan to compost yourself.

Allowing cow manure to age is definitely an easier process than making mushroom compost at home. On the other hand, if you plan to purchase one option or the other, the commercially available products are very comparably priced at around $3-$5 for a 40 lb. bag.