What is Mushroom Compost?


Brown speckled mushrooms thrive on mossy forest floor, capturing intricate details in soft focus.

In gardening, good-quality soil is key to thriving vegetables and plants. Mushroom compost is an excellent addition to your soil, acting as a mulch or a conditioner and enhancing the overall health of your gardening environment.

When it comes to mushroom compost, there are two types to consider. The first is a specialized compost formular used by mushroom farmers for the purpose of growing mushrooms. This specialized formular can also be replicated by gardeners and is a highly effective compost, full of vital nutrients that are excellent for the growth of an array of plants. The second is spent mushroom compost, which is the substrate compost that mushroom farmers initially used to grow mushrooms and then sold to garden centers or hardware stores. It is a beneficial compost that is useful as a fertilizer or used to improve and aerate the structure of soil.

In this blog, we’ll look at mushroom compost in more detail, uncovering its origins and the benefits of adding it to your garden.

Garden compost bin full of scraps, leaves, and decomposing plants - utilizing mushroom compost for eco-friendly recycling.

What is Mushroom Compost?

Before we further explore the benefits of using this kind of compost in your garden, we need to understand what it is.

Put simply, it is not a compost made from mushrooms. It is a compost of organic matter that is specially formulated for the purpose of growing mushrooms. It comprises of straw or hay combined with manure, chalk, lime, sphagnum moss, gypsum and other agriculture by-products such as corn cobs, soybean meal and water. The whole process is carried out in two phases in a controlled environment. 

Phase one is when the ingredients are built up in long rectangular piles and regularly turned in a commercial compost turner to mix the ingredients. Water is added to help break down the matter used.

Phase two is when the mixture is transferred to a pasteurization room and heated for eight-hour periods over seven days. This process kills off bacteria and any plant seeds, thereby sterilizing the composted mixture. 

Once sterilized, the compost is ready to be used for growing mushrooms, whereby it is inoculated with mushroom spores.

This specialized compost is highly rich in essential nutrients and, as a rule, is made up of the ingredients below.

Mushroom Compost Material

Understanding what goes into commercial mushroom compost can help you better understand the benefits of using it in your garden. The combination of ingredients in mushroom compost material creates a unique mixture that has the ability to transform your garden.


Straw or hay is an essential component in this kind of compost because it provides structure for the compost pile. Furthermore, it provides effective aeration. During the composting process, straw breaks down and releases nutrients into the mix, acting as a vital element of the compost.

Agricultural Materials

Agricultural by-products like corn cobs, soybean meal, and hulls may be added to the compost mix. These materials provide essential nutrients to the compost and offer a well-balanced mix to ensure the blend is balanced for optimum results.


Chicken manure is an integral part of mushroom compost because it’s full of nitrogen, a vital nutrient for plant growth. During the composting process, manure is broken down to release nutrients, which are absorbed into the soil.


Gypsum can help regulate soil structure. This mineral helps prevent soil compaction, promoting better water movement and root penetration. Its ability to provide calcium to the soil is also important, as this is an essential nutrient that helps with the development of plants.


Though not strictly necessary, lime is generally added to mushroom compost as it helps to balance the pH level. The lime ensures that the final product is near neutral, resulting in a compost suitable for a wide range of plants.


As we all know, water is an essential element for all living life, as well as for the composting process. Water ensures that the beneficial microorganisms in the compost thrive and encourages the breakdown of organic materials. As I have mentioned, mushroom compost often has a high water content; if you don’t add water, it won’t achieve these same results.

Scattered golden straw displays natural texture and color, resembling mushroom compost.

Making Mushroom Compost

You can make your own mushroom compost by using the following ingredients. Note: Corn cobs and soya meals can be replaced with plant-based kitchen waste.

  • 40%  wheat straw
  • 20%  fresh horse manure
  • 40% garden soil
  • 2%  gypsum
  • A compost thermometer
  • Kitchen waste is an option

Using these materials, this is how to start the process of making your own mushroom compost.

  1. First, soak the straw or wheat in water for several minutes, then leave for 24-48 hours in order for the straw to fully absorb the water. Mix in some of the kitchen waste if using.
  2. Spread a layer of straw approximately 5ft by 5ft (1.5m x 1.5m) in diameter and around 9 inches (15cm)  high. Mix the gypsum and manure together and spread over the top, along with more kitchen waste, around 9 inches (15cm) high. Repeat these layers until you have a mound approximately 3 feet (1m) high. 
  3. Water the mixture again and cover using tarpaulin or black plastic sheets to prevent it from drying out.
  4. Leave the mixture for four weeks. Then using the thermometer check when the temperature has reached 140F-160F or 60C to 71C. This heating process will sterilize the mixture by killing off any seeds or harmful agents.
  5. When it has reached the ideal temperature, turn the pile again every couple of days, working the outer mixture into the centre and adding more water each time. Repeat this procedure for another two weeks.
  6. When the mixture is dark brown in color, uncover the pile and leave it to cool down completely before using it on your plants.
Green shovel near yellow sand-filled bucket on soil, capturing the essence of playful gardening or sand play with a touch of mushroom compost.

What Is Spent Mushroom Compost? 

Spent mushroom compost is a by-product of the mushroom farming industry. It consists of the substrate soil that’s left over after the growing and harvesting of the mushrooms.

Essentially, farmers create a specialized compost for the growth of mushrooms using a compost  similar to the one above. The main differences are that for commercial farming, agriculture by-products such as corn cobs and soya waste are also added. This mixture is then heated in a controlled environment whereby the compost is thoroughly sterilized before adding the mushroom spores. After the mushroom growing season has finished and the mushrooms have been harvested, what is left is a sterilized substrate soil.

Mushroom farmers bag up this by-product soil and sell it to garden centers or homeware stores, whereby it is purchased by gardeners. Although often referred to as mushroom compost, it is technically Spent Mushroom Soil.

Spent mushroom compost is a slow-release, light fertilizer with a lower percentage of nitrogen that has been depleted by the growing of the mushrooms. However it has high water retention qualities and acts as a fantastic organic amendment to your garden soil. It is ideal for breaking up dense clay soils or spread as a mulch over your lawn or around your garden plants.

Mushroom Compost Near Me

If you’re searching for “mushroom compost near me,” you’ll be pleased to know that you can purchase this mix in bags instead of making it yourself. Local suppliers or garden centers sell bags of mushroom compost, which is a compost that is rich in essential nutrients. In addition, you will also be able to purchase bags of spent mushroom compost, either labeled “SMC” (spent mushroom compost) or “SMS” (spent mushroom substrate). Though it may seem a little confusing, there is technically a difference between the two composts, and both have beneficial qualities. If unsure, either check the product label or ask for assistance.

You can often buy from large garden centers, but opting for local suppliers can help you to support local businesses. Plus, you cut down on transport costs and make your compost even more eco-friendly. Opting for local suppliers where possible is recommended.

The Benefits of Mushroom Compost

Both spent mushroom compost and mushroom compost have the power to transform your garden into a flourishing and blooming oasis. By utilizing the force of different ingredients, you can use it to your advantage and enhance the health and look of your garden.


Spent mushroom compost and mushroom compost are jam-packed with essential nutrients. From potassium to phosphorus and more, these types of composts are ideal for providing your soil with the essential nutrients needed for your plants, flowers, vegetables, and herbs to grow. Please note, though, that spent mushroom compost will generally have lower levels of nitrogen than mushroom compost.

Enhanced Soil Structure

Thanks to the organic matter in both composts, you can enhance soil structure and create well-aerated soil. This is especially true for spent mushroom compost, which has up to 30% more chalk than other compost varieties. As a result, it increases the soil’s pH level and can breathe life into heavy, dense soils.

Better Water Retention

Both mushroom compost and spent mushroom compost also increase the water-holding capacity of the soil, meaning you won’t need to spend as much time watering your plants. These types of compost have sponge-like properties, meaning it aids water retention and keeps your plants thriving despite being watered less.

Improved pH

I’ve already touched on this, but the use of lime and the high chalk content, especially in spent mushroom compost, means that it has a near-neutral pH. As a result, it’s a versatile compost type that can generally be used for a wide range of plants. This enables you to create a flourishing garden with a diverse array of plants. However, it is always wise to carry out a pH test on your soil before planting.

Sustainable Approach

Opting for compost of any type is a sustainable approach to gardening as you’re repurposing materials. In the case of spent mushroom compost, the materials are recycled not once but twice. Using compost for gardening reduces the need for chemical fertilizers, providing slow-release nutrients that won’t chemically burn your plants.

Note that spent mushroom compost is not suitable for use alone as a soil replacement due to its high salt content, which can harm some plants. Its water retention abilities can also lead to waterlogged soil if combined with heavy clay soil, resulting in poor drainage, root rot, and fungal infections. Mixing it with other compost soil or incorporating it into your garden soil is recommended.

Ripe red cherry tomatoes cluster on vine against backdrop of rich soil and vibrant green foliage, nourished by mushroom compost.

What Plants Benefit From Mushroom Compost

As we have touched on the benefits of both spent mushroom compost and mushroom compost, it’s worth covering what plants you can use it for.

Perhaps unsurprisingly, mushroom compost is great for growing mushrooms. After all, it contains all the essential nutrients needed for mushrooms to flourish.

However, you can also use it for a range of other plants, and it works particularly well for tomatoes, which are susceptible to blossom-end rot when they don’t get enough calcium. Since mushroom compost is high in calcium, it can help your tomato plants thrive. Veggies that will benefit are potatoes, sweet corn, beets and squash. It is also a fantastic fertilizer for growing roses, and other plants that are heavy feeders.

Its high water content also makes it great for tropical plants or any that require a lot of watering. Try using it for hibiscus plants and other similar flowers.

Spent mushroom compost is ideal to use as a light fertilizer for your lawn, especially if you live in an area with clay soil. You can incorporate a layer of spent mushroom compost before laying new turf or place it over your lawn to help the nutrients permeate through the soil. By improving soil quality, it can greatly help your drainage and lawn health. However, spent mushroom compost can have a higher pH level and is not always suitable for use on ericaceous plants such as heathers, rhododendrons, blueberries, and raspberries. Again, carrying out a pH test on your soil is always a good idea.

Get More Gardening Tips

I have had great results using both types of mushroom compost in my garden. From first-hand experience, I can assure you that this eco-friendly, nutrient-rich compost is a fantastic way to help your plants thrive and improve the quality of your soil. Whether you make your own mushroom compost or buy from local businesses,  mushroom compost can play a key role in creating a garden oasis.

Meanwhile, if you’re looking for more gardening tips, check out the rest of our blogs!

Frequently Asked Questions

Here are some frequently asked questions about using mushroom compost in your garden.

What is mushroom compost made of?

The key components of mushroom compost include organic matter, like hay, straw, and agricultural residues. It can also contain chicken or horse manure, gypsum, lime, peat moss, and corn cobs. This mixture and blend of organic materials contribute to a balanced and nutritious growing environment. 

How is spent mushroom compost made?

Mushroom compost is made by a blend of composted straw and horse or chicken manure. Mushrooms are cultivated on top of this mix, and after two or three harvests, it is sterilized at high temperatures to remove any unwanted pests, diseases, or weeds. What’s left is then sold in bulk as an effective soil conditioner. 

What are the benefits of using spent mushroom compost? 

Using mushroom compost in your garden offers many benefits, like improved soil structure, better water retention, more nutrients for plants, and a contribution to sustainable and eco-friendly gardening practices.

What does spent mushroom compost look like? 

Mushroom compost has a crumbly texture and a dark brown color with a pleasant, earthy aroma that indicates it’s undergone proper composting. The compost might also contain remnants of organic materials or straw, but this is to be expected. 

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