Some people rave about mushroom compost – and wax lyrical about its impact on their soil and plants.
It’s true that mushroom compost can be good for your soil – but there are caveats.
Ultimately, the impact of this compost depends on different factors. These include soil, the plants you are growing, and the quantity you add to your soil.
So before you rush out and start using mushroom compost on your soil, let’s take a detailed look at it.
What is mushroom compost?
Mushroom compost can either be compost specifically made for mushrooms or mushroom compost which has already been used (called spent mushroom compost).
Most gardeners purchase spent mushroom compost.
The exact properties of each mushroom compost will vary, as there is no standard way to make it.
Compost for mushrooms was traditionally made from straw, manure (often from chickens) and gypsum, but nowadays it may well be made without manure.
Mushroom compost is also likely to contain the remains of fungi. However, as mushroom compost is pasteurized after being removed from the mushroom bed this, along with any weed seeds, will be dead.
Spent mushroom compost often contains chalk, which makes it alkaline in nature, and this needs to be borne in mind when choosing what plants and soil to use it with.
It is also possible to obtain unused mushroom compost which does not have chalk in it.
Mushroom compost is used to improve the soils in your gardens, flower beds, or anywhere else where you grow plants.
However, as we’ll see, it’s important to know that, at least when fresh, it’s not good for all soils and plants.
Why would you consider using mushroom compost?
Nutrients
Some guides state that spent mushroom compost can be high in nutrients and contains a variety of minerals and trace elements that plants need to grow strong and healthy.
However, other sources state that mushroom compost, while having certain benefits, is low in nutrients.
Mushroom compost can be made in different ways, which might explain the discrepancy.
The graph below shows levels of three key nutrients from two different studies.
| Spent Mushroom Compost (Fresh): Nutrient Analyses | ||
| Uzen et al | Penn State University | |
| Nitrogen | 1-2% | 1.42 – 2.05%< |
| Phospherous | 0.2% | 0.45 – 0.69% |
| Potassium | 1.3% | 1.93 – 2.58% |
| Sources: Uzen et al, Use of spent mushroom compost in sustainable fruit production; Penn University: Spent Mushroom Substrate | ||
Mushroom compost is able to hold onto nutrients much longer than other soil amendments because of its porous structure.
Indeed, when Courtney and Mullen tested different types of compost and fertilizers on agricultural land, they found that mushroom compost had the strongest correlation between the nutrients in the soil and the final yield of the plant.
Benefits to soil structure
That porous structure helps soil in other ways too. It can retain moisture, which can help your plants through dry seasons.
Its ability to absorb water can also help in times of heavy rain – and help prevent water pooling which can lead to root rot or other issues with plant growth over time.
Also see: How Compost Helps Soil Structure
Impact on soil
Spent mushroom compost can also benefit certain types of soil.
Spent mushroom compost often contains chalk, and this has a liming effect. This can reduce the PH of acid soils and have a fertilizing effect.
(This does not usually apply to unused mushroom compost.)
| Mushroom Compost Slows Germination, Increases Yield |
|---|
| Scientists Wang, Lohr and Coffey wanted to find out how mushroom compost would affect common greenhouse plants. They tested different mixing spent mushroom compost with garden soil, testing different quantities. The results? Germination was slower – but yields increased by 30-50%. The best results happened when mushroom compost was used at a ratio of 20/30% – when a 50% concentration was used, lower yields were reported. The researchers attributed this to high salt content. Source: Growth response of selected vegetable crops to spent mushroom compost application in a controlled environment |
What are the drawbacks of using mushroom compost?
The study by Uzun et al (linked above) also found that spent mushroom compost contains high levels of salt and unstable organic material. Uzun advised leaving mushroom compost to mature before use because of this.
The salt in mushroom compost, unless used in large quantities, won’t be a problem for most garden soils.
However, it is probably advisable to avoid adding fresh mushroom compost to soils with high levels of salts, or with soils that have alkaline soil.
Plants that do and don’t like mushroom compost
You’ll find some long lists of plants that do and don’t like mushroom compost, but these aren’t always backed up by evidence – and sometimes contradict each other!
To try and clarify the situation, I’ve searched for research that indicates mushroom compost is positive or negative for plants. (Unfortunately for flower growers, these results are mostly limited to vegetables.)
Do note that the reaction of the plant can depend on the quantity of mushroom compost added.
For example, tomato yield increased in one experiment when mushroom compost was added but then decreased when the amount of mushroom compost was doubled.
In the table below, I’ve included plants where some mushroom compost can help.
| Plant | Result | Source |
| Lettuce Seedlings | Spent mushroom compost led to quality improvement | Marques et al, 2014 |
| Tomato seedlings and plants | No difference in yield/increased yield depending on the study | Castelo-Gutiérrez, 2015 |
| Snap bean | Increase in yield | Wang et al, 1984 |
| Cucumber | Increase in yield | Wang et al, 1984 Gonani et al, 2009 |
| Radish | Increase in yield | Wang et al,1984 |
| Spinach | Increase in yield | Wang et al, 1984 |
| Cabbage | Increase in yield | Wang et al, 1984 |
| Onion | Decrease in yield as more spent mushroom compost added | Wang et al, 1984 |
It makes sense that alkaline-loving plants like brassicas will thrive in soil amended with mushroom compost.
Using the same logic, we also know that it’s best to avoid using mushroom compost with ericaceous (acid-loving) plants. As the chalk increases the pH of the soil, it makes the soil more alkaline – and less palatable to these plants.
These plants include
- Azaleas
- Rhododendrons
- Camellias
- Gardenias
- Hydrangeas
The RHS also recommends avoiding using mushroom compost for fruit-bearing plants and lawns. Some studies have found that it can be used as a compost for fruit, but in general, these studies aged the compost first.
Penn State University also advises that mushroom compost can be used on turf – but accompany this with a list of analyses that should be done first!
| Where to buy mushroom compost |
|---|
| For large amounts of mushroom compost, it’s best to look around for a mushroom farm in your area. You can often get large amounts at a good price, but you may need to pick it up yourself. You can also order bags of compost online, and these can vary from small compact bags to 1000-liter bulk bags. You may want to look for aged mushroom compost, as this has lower salt levels. |
In a nutshell
We’ve seen that mushroom compost can be a great amendment to acid soils, and can improve soil structure too.
It does have a different impact on different plants. However, most of the studies have looked at adding significant amounts of spent mushroom compost.
The main thing to remember is to avoid using mushroom compost with acid-loving plants. If you’re unsure about a specific plant, the best thing to do is either research that plant, or do your own trial side-by-side!