Mushroom Compost | What Is It And How Best To Use It

Spent mushroom compost is an excellent soil conditioner for gardens suffering from dense or dry soil. But, contrary to its name, mushroom soil doesn’t contain any actual mushrooms.

If you want to learn more about the benefits of mushroom compost and the ingredients it contains, I’ll break down everything you need to know in the article below.

Furthermore, if you’re into DIY projects, I’ll walk you through how to make organic mushroom substrate at home.

What Is Mushroom Substrate Used For?

Spent mushroom substrate offers gardeners the opportunity to aerate and condition their soil. People often use it to:

  • Break up dense soil, especially soil containing high clay levels
  • Increase their lawn’s soil quality
  • Aid with water retention in mixed or raised garden beds
  • Stop birds from eating seeds on newly seeded lawns

By breaking up dense soil, this organic material gives a plant’s roots more room to grow. Since worms help create air pockets beneath the ground, spent mushroom substrate is an excellent option for people who live in areas where worms aren’t abundant.

It’s natural to assume that mushroom substrate will boost the nutrient content of the soil you mix it with. However, this material’s purpose is to condition your soil so that your plants and grass have the optimal growing conditions.

Therefore, you’ll need to use it in conjunction with fertilizer if you feel your soil needs a nutritional boost.

Designed For Growing Mushrooms

Mushroom soil begins as a substrate that farmers use to grow abundant, healthy mushrooms.

The substrate typically contains a mixture of ingredients, with straw being the foundation of most formulas.

What Is Mushroom Substrate Used For

Manufacturers soak the straw in water before using a chipper to turn the bales into small pieces. They then mix in the rest of the ingredients and let the mixture sit in a hot environment for a couple of weeks.

After pasteurizing the substrate to avoid any unwanted plant sprouts, bacteria, or mold and waiting more time for the composting process to continue, the substrate lands in a mushroom farmer’s hands. The farmer then spreads mushroom spores on the substrate. The same substrate can be used for multiple mushroom growing cycles.

The benefits of using this material when growing mushrooms include:

  • A source of abundant nutrients
  • Holds in moisture well, promoting an ideal growing condition
  • Ability to pasteurize or sterilize it to prevent bacteria and mold from growing

In contrast to traditional gardeners, mushroom farmers don’t water their fungi even though mushrooms contain over 90% water.

Substrate offers farmers better control over the amount of water in their planting medium before it’s inoculated with mushroom spores. Mushrooms won’t grow on a substrate that’s too dry. Meanwhile, bad bacteria will grow on a substrate that’s too wet and hasn’t undergone pasteurization or sterilization.

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Mushroom Compost pH

According to The Royal Horticultural Society, spent mushroom substrate has a pH level of approximately 7.1 for weathered compost, with some soils ranging from 5.8 to 7.7, depending on how fresh it is.

A soil’s pH can range from 3.5 to 9.0, with 3.5 categorized as ‘extremely acid’ and 9.0 categorized as ‘strongly alkaline.’ Most spent mushroom soil is slightly alkaline.

The pH of mushroom soil is ideal for some but not all plants. Examples of plants that you can supplement with mushroom substrate include:

  • Broccoli
  • Cauliflower
  • Brussels sprouts
  • Kale
  • Cabbage

You should avoid using this material on plants that prefer acidic soils. Examples of such plants include:

Should you wish to grow plants that prefer a more alkaline pH level but your soil is too acidic, you can add lime to the earth. If you have the opposite problem, you can mix sulfur into your soil to make it more acidic.

What Is Mushroom Compost Made Of?

Mushroom compost is a misnomer since it doesn’t contain mushrooms. Instead, it’s a 100% organic mixture that mushroom farmers no longer need because the mushrooms that grew there exhausted most of the substrate’s nutrients.

Although this material is best for enhancing the structure of your soil, it contains small amounts of minerals that offer some nutrition for your garden.

Examples of the nutrients that mushroom soil contains (in the percentage of dry weight) include:

  • Potassium: 1.93 – 2.58%
  • Magnesium: 0.45 – 0.82%
  • Calcium: 3.63 – 5.15%
  • Iron: 0.18 – 0.34%
  • Phosphorus: 0.45 – 0.69%
  • Ammonia: 0.06 – 0.24%
  • Aluminium: 0.17 – 0.28%
  • Sodium: 0.21 – 0.33%

The numbers above assume that the spent mushroom substrate is in its fresh stage. If the substrate weathers for 16 months before you use it, you can expect the potassium, ammonia, and sodium levels to fall. In contrast, the magnesium, calcium, iron, phosphorus, and aluminum levels will rise.

I should note that mushroom soil has a high sodium content. Certain plants, such as azaleas, won’t grow well in it. However, the salt content varies significantly depending on how much time the compost had to weather (0.06% dry weight for 16-month-old compost compared to 0.21 – 0.33% dry weight for fresh compost).

It’s critical to understand the impact that weathering has on the chemical makeup of mushroom soil so that you can choose accordingly.

Mushroom Compost Ingredients

The ingredients in mushroom soil vary, as it depends on what mushroom farmers used in their substrate. For garden use, however, it should not contain mushrooms nor traces of mushroom spores that could grow once you mix the compost with your soil. I’ll talk about the science behind that next.

That said, straw is present in most mushroom soils. As it decays, microbes consume the available nitrogen and help break down carbon, so that the final total nitrogen content becomes a healthy 1.42 to 2.05% dry weight.

Other ingredients in mushroom substrate include but aren’t limited to:

  • Peat moss       
  • Cottonseed or canola meal      
  • Grape crushings from wineries       
  • Potash     
  • Soybean meal      
  • Gypsum      
  • Chicken manure      
  • Urea       
  • Lime
  • Ammonium

Because of its organic ingredients, mushroom soil often has a strong odor. Although you can use this compost in its smelly state, some manufacturers allow the mixture to continue composting for 6 to 12 months or longer. By then, the compost becomes odorless and more pleasant for use in your backyard.

Pasteurization and Sterilization of Mushroom Soil

The purpose of pasteurizing or sterilizing mushroom soil is so your garden benefits from the soil’s properties without you having to worry about leftover mushroom spores taking over your plants. Furthermore, since mushroom soil is an organic product, there’s a high chance that mold or bacteria may grow on it.

For these reasons, most spent mushroom soils you find at garden stores already come pasteurized or sterilized. However, the circumstances in which mushroom soil needs pasteurization or sterilization varies.


Pasteurization is an effective method for non-hardwood substrate materials. For example, spent mushroom substrates containing straw, manure, and cottonseed are excellent candidates for pasteurization.

To pasteurize mushroom soil, manufacturers simply put the substrate in a pot of boiling water. They then turn off the heat and let the water cool down with the substrate inside. Within 1.5 hours, the substrate has complete pasteurization protection.


Sterilization is the most effective technique for spent mushroom substrate that contains hardwood sawdust, although you can sterilize any substrate using this method.

To sterilize, manufacturers put the substrate under conditions of over 250°F in a high-pressure cooker set at 15PSI (pounds per square inch).

This is an effective technique for killing any living or dormant bacteria, mold, and mushroom spores. In the end, you have a clean slate to work with in the garden.

Organic Mushroom Compost for Garden Soil

Environmentalists and savvy gardeners alike love using mushroom substrate as a soil conditioner because of its recyclable and organic properties. This compost is a relatively inexpensive way to improve soil, and vegetables, in particular, grow well in it.

Mushroom Compost Benefits

By incorporating this material into your soil, you can expect to receive the following benefits:

  • Less watering of your plants since the compost retains moisture
  • Plants with a stronger foundation because the roots have aerated soil to grow in
  • A pasteurized or sterilized product that won’t introduce mold, bacteria, or mushrooms into your soil
  • A small source of nutrients for your plants

Remember, to receive these benefits of spent mushroom substrate, you need to grow plants that thrive under the substrate’s slightly alkaline pH properties.

Another benefit of mushroom substrate is that it’s easy and inexpensive to make on your own. I’ll teach you how to make this compost at home shortly!

Mushroom Compost vs. Cow Manure

Unlike spent mushroom substrate, cow manure contains a rich source of nutrients for plants. It also encourages good bacteria to grow, which helps crops absorb more nutrients.

Mushroom soil and cow manure share similar properties in that they help keep the soil moist, meaning you don’t have to water your plants as frequently.

Nevertheless, cow manure must undergo a lengthy composting process. If you don’t do it right, you set yourself up for the possibility of getting E. coli from the vegetables you eat.

However, if you learn how to compost cow manure or purchase it already composted, it can be an excellent organic fertilizer to supplement mushroom substrate’s soil-conditioning properties.

Mushroom Compost for Vegetable Gardens

Mushroom soil is an excellent choice for vegetable gardens, as most vegetables tolerate its high salt content. Furthermore, this compost has a slightly alkaline pH level, which helps plants like Brussels sprouts, cauliflower, broccoli, and cabbage thrive.

Mushroom Substrate as a Soil Conditioner

Spent mushroom compost is an excellent soil conditioner as it helps aerate the soil, especially if you live in an area with a lot of clay. It also retains water.

You don’t have to worry as much about the sun drying out your plants on a hot day or spending excessive money on your water bill. It can even help balance your soil’s pH level if your soil is too acidic or alkaline.

You might be wondering—what’s the difference between soil conditioner and fertilizer?

Soil conditioners help change the soil’s physical structure. Fertilizers inject nutrients into the soil. There’s often some overlap between the two types of material.

Since mushrooms already extracted most of the nutrients from the substrate, it’s almost solely a soil conditioner.

How do you know when your garden needs a soil conditioner? Below are some situations where I find soil conditioners may come in handy:

  • Rocky soil
  • Soil with a lot of clay
  • pH level that’s too high or low
  • Soil that drains too quickly

Planting in Mushroom Compost

You can use two methods when planting your garden with spent mushroom soil: Mix it in with your soil before you plant or put the compost on top (called top-dressing).

If you choose the top-dressing option, pile the compost up to 6 inches thick around established plants only, and keep it away from the plants’ stems.

Moderation is key when using spent mushroom soil. Because of its large soluble salt concentration, seeds and newly germinated plants may die if you condition their soil with mushroom substrate.

Best Mushroom Compost-To-Soil Ratio

A ratio of 25% mushroom soil to 75% regular soil is ideal under most circumstances. Since mushroom substrate doesn’t have many nutrients, your plants will need to extract them from the regular soil it’s mixed with.

How To Make Mushroom Compost at Home

If you’re interested in making your own mushroom substrate, here’s an easy-to-follow step-by-step guide.

By creating your own substrate, you’ll have a huge advantage over the product sold at garden stores — your mushroom soil will contain the rich nutrients that mushrooms consume before manufacturers turn the substrate into a spent commercial product.

So, homemade mushroom soil serves as both a soil conditioner and organic fertilizer (although depending on your soil’s quality, you still might need to use an additional fertilizer).

Step 1: Gather Your Ingredients

You can use a combination of any of the ingredients I covered in the ‘mushroom compost ingredients’ section of this article. However, make sure to include straw and manure, as these are essential ingredients in mushroom soil.

Step 2: Make a Hot Compost

Mix the ingredients together. Then leave the compost for 30 to 60 days. During this time, bacteria will eat the straw, manure, and any other ingredients you use in the compost. Most of these bacteria are aerobic. The bacteria will release heat as they eat (and thus, break down) the organic ingredients.

You’ll be able to physically feel the heat coming from your compost pile if you stick your finger in it. As your compost temperature increases, other compost-friendly microbes will come into play, too.

Step 3: Monitor the Temperature

One of the hallmarks of making effective mushroom soil is managing the temperature as your ingredients decompose. Your compost will start by having psychrophilic bacteria, as these microbes become active around the low 55°F mark.

Once your compost reaches a temperature range of 70 to 100°F, mesophilic bacteria take over to continue the decomposing process. Thermophilic bacteria continue from there, as they love temperatures up to 160°F, which is the temperature goal you should aim for with your compost.

However, if your compost reaches a temperature over 160°F, the excessive heat will start killing off the thermophilic bacteria, and the composting process will begin to slow down.

It is completely optional as to whether or not you have a thermometer to check your compost’s temperature in the 30 to 60 days that you let it sit. Irrespective of knowing the exact temperature, you should take a shovel and mix up your compost every two to three days. That’ll help it cool down enough to remain in a temperature range where essential bacteria thrive.

Step 4: Keep Your Compost Moist

When you turn over your compost, you should make it a habit to spray some water on the freshly turned pile. Moisture is a critical part of keeping bacteria happy and ensuring the ingredients decompose at a quick pace.

Another advantage of mixing your compost every few days and injecting it with some moisture is that you allow the lesser-composted ingredients on the outside of the pile to reach the inner area where there’s higher heat and moisture retention. It also helps aerate the compost, which is critical for aerobic bacteria.

Step 5: Cure Your Compost

A couple of weeks before you plan on using your compost, take a shovel and move the amount of compost you want to use to form a second pile. Then let the pile sit for about two weeks. During this time, you don’t have to turn over the compost.

You’ll notice that your compost becomes dark brown, and the ingredients break down into even smaller pieces.

Step 6: Pasteurize or Sterilize Your Compost

As a final step, you should pasteurize or sterilize (if you’re using hardwood sawdust) your compost. Without taking this measure, you could introduce unhealthy bacteria, mold, and fungus into your soil.

To pasteurize, place your compost in a cloth or mesh bag. Then set the bag in water that’s at a rolling boil, turn off the heat, and let it sit in the cooling water for up to 1.5 hours.

In the case of sterilization, you’ll need to use a pressure cooker that’s set at 15PSI with a temperature of more than 250°F. Let your compost sit in the pressure cooker for about 2.5 hours.

At that point, your compost is ready to use.

The Importance of Composting Mushroom Soil

Needless to say, you’ll need time and space to make mushroom compost. So, you might be wondering—can’t I just mix the ingredients together and use them in my garden right away?

No, you can’t! Placing fresh ingredients in your soil before they have time to compost can cause nitrogen burn. Nitrogen is crucial for a plant’s health, but when there’s too much of it in the soil, it’ll cause your plants to wilt and rot their leaves.

By composting mushroom substrate ingredients, the bacteria will eat excess nitrogen, bringing the nitrogen levels down to a point that’s healthy for plants.

Mushroom Soil FAQs