Mushroom Compost: Advantages, Disadvantages, and How to Use Mushroom Compost

A lot of controversy surrounds mushroom compost. Your uncle swears by it, but it also killed your neighbor’s garden. You might even be thinking, “What is mushroom compost anyway?” 

We’re giving you the real scoop on what makes this compost special and how to make your garden thrive using it. So let’s grab some spent blocks and give this misunderstood compost a chance, shall we?

What Is Mushroom Compost?

Before we get into the details, let’s just make one thing clear: Mushroom compost is NOT made of mushrooms. It’s the medium where the edible fungi used to grow (aka spent mushroom compost). 

Ever tried starting one of the best mushroom growing kits in the market? Well, mushroom compost is the block of material left in the plastic bag after you’ve harvested the final flush. You can also find mushroom compost for sale in your local gardening supply store.

bags of spent mushroom subtrate (a.k.a. "mushroom compost") stacked together
Why was a mushroom invited to a party? Because he is a fungi.

Each block is different depending on the producers, but essentially mushroom growers use a mix of the following (1):

  • Wood chips or sawdust
  • Peat moss or sphagnum moss
  • Wheat straw or hay bedding
  • Chicken manure or horse manure
  • Hulls of grains and beans
  • Gypsum 
  • Corn cobs
  • Wheat seeds
  • Soybean meal

Check out what goes on behind commercial mushroom substrate production.

Again, the process varies depending on how big the mushroom substrate operation is. After it’s packed, growing mushrooms is the next step.

Remember, mushrooms are not plants. They can’t make their food. Instead, they break down the substrate materials and absorb the nutrients they need.

When they’re done, the mushroom substrate becomes gross and smelly. But that icky substrate is now nutrient-rich because of all the metabolic processes that happened. You can crumble up the block from the kit and immediately use it as fertilizer for your garden. 

Not all mushrooms grow on a mixture of materials. For example, growing shiitake mushrooms indoors involve solid oak logs inoculated with mushroom spores. After the final flush, the spent shiitake logs can be thrown into a wood chipper and used the same way as other spent substrates.

Here’s the chemical breakdown of mushroom compost and what it means for general gardening.

ParametersAverage values (1,2)What it means
pH 6.7Good pH range for growing most crops, but not suitable for acidic-loving plants.
N-P-K ratio1.8 – 0.6 – 2.2Generally good for all-around plant growth
Organic Materials25.86% (wet weight)Great for soil amendment and plant production.
Macronutrients(Ca, Mg, S)2.32%, 0.36%, 0.86%(wet weight)Keeps garden soil fertile.
Micronutrients(Fe, Mn, Cu, Zn, Na, and Al)<0.01% to 0.18% (wet weight)Moderate amount for garden plant growth.
Moisture57.33%Excellent water absorption keeping the soil moist.
Soluble salts0.38 SARGood as mulch for killing weeds, but it can also kill some crops.

Given the values above, mushroom compost is most effective for gardening as a soil additive. However, using it in its pure form as “mushroom soil” is unsuitable for all garden plants because of the concentrated salts and waterlogging properties.

Why Use Mushroom Compost For Your Garden Soil?

Now that we have answered the question, “what is mushroom soil?” Let’s get into why it is good for your garden. Organic mushroom compost benefits your soil in many ways, as you can see from the table in the previous section. 

Most mushroom compost has a good balance of organic compounds, making your soil fertile. Plus, it has a good mix of macro and micronutrients for overall plant growth. The relatively neutral pH makes most nutrients available to plants.

Mushroom compost also improves the quality of sandy and clay-like soil. Sand drains out moisture and nutrients quickly. On the other hand, clay soil is too compact for planting. Mixing mushroom compost with regular soil balances out the need for water retention and proper aeration for these soil types. 

What Plants Love Mushroom Compost

Mushroom compost is a popular fertilizer choice for landscape supply firms. Turfgrass, trees, shrubs, and most flowering plants love mushroom compost because of its nutrient value and water retention properties. You can also add mushroom compost to crops that don’t mind the higher salt levels (3). Some garden veggies that love mushroom compost are:

  • Beets
  • Bell peppers
  • Broccoli
  • Cabbage
  • Kale
  • Loquats
  • Spinach
  • Tomatoes
  • Carrots
  • Cauliflower
  • Lettuce
  • Peas
  • Potatoes
  • Squash
  • Sweet Corn

Mushroom Compost Is Sustainable

The mushroom farming industry produces a lot of spent mushroom substrate. Using mushroom compost lessens waste in landfills, making it a sustainable fertilizer option. It’s cheap and sometimes even free if you get them from local mushroom farmers. Trust us; they are usually happy to get rid of their leftover compost.

What Are The Disadvantages Of Mushroom Compost?

The often overlooked problem with using mushroom compost is not knowing the exact composition of the substrate. As we said, it varies depending on the producers. 

“Does it matter?”

Yes, because you might end up adding harmful substances like pesticides into your garden without you knowing it. Plus, if you get mushroom compost by bulk without knowing what’s in it, guess what? The crops you harvest may not be organic after all. Selling those “organic” crops presents another issue. You don’t want to be another phony selling questionable organic produce, do you?

“So, how do you prevent this?”

Some farmers prefer to let spent mushroom compost blocks dry out on the garden beds for months before using them. Alternatively, you can shorten this process by sterilizing the spent mushroom substrate with steam. Drying out or sterilizing the mushroom substrate degrades any pesticides or antibiotics that may have been in the mix. And it kills off any unwanted weed seeds or mold spores. 

You get a clean material that you can now customize. Introducing new beneficial microorganisms into the spent mushroom substrate is the first step. Then, again, make sure the manure is organic. After that, your fresh, clean, and 100% organic mushroom compost is ready to mix with other materials for your garden.

Tracking the ingredients of the spent mushroom compost is easier for those using commercial mushroom kits. You can just check the packaging and confirm things with the company before using the substrate as compost. Growing shiitake mushrooms on logs presents no issues with tracing since the only material used is an oak log, easy peasy.

Plants That Don’t Love Mushroom Compost

Before thinking about “how to make mushroom compost,” you should be aware that this compost could kill some of your plants. The most common issue with using mushroom compost is the salt content. And it is a concern because it makes absorbing essential nutrients harder for plants (3). Some of the plants that don’t like mushroom compost are the following:

  • Azaleas
  • Magnolias 
  • Rhododendrons
  • Blueberries 
  • Beans
  • Celery
  • Cucumbers
  • Radishes

But you can easily fix this problem by adjusting the amount of mushroom compost you use in your garden or compost heap. 

Another less common problem with mushroom compost is its ability to retain water. We know plants need moist soil for growth. But too much water is not good either. Waterlogged soil causes root rot. Bad bacteria can also start growing when air cannot flow through the soil. 

When And How To Use Mushroom Compost?

The best time to use organic mushroom compost for vegetable garden plots is from spring to summer. 

Mulch

Using fresh mushroom compost as mulch keeps moisture in the soil for germinating seeds. It also prevents weed seeds from growing because of the high salt levels (4).   

“When mulching, you should use between 1 and 3 inches of mushroom compost. Use 3 inches of compost for the best water retention and weed suppression.”

Mulching with mushroom compost saves you from watering your plants often. Plus, you don’t need to add fertilizer because all the nutrients seep down for the growing plants to use. 

Soil Improvement

Adding mushroom compost into your garden beds or pots enriches the soil. But a little goes a long way with mushroom compost. Remember, the salt content will be a problem for most plants. However, using 25% to 50% mushroom compost is good enough to keep the soil moist and healthy (4). 

Compared to manure and materials from your backyard compost pile, fresh mushroom compost has a more even texture. This is because the finer soil-like particles of the spent mushroom substrate make it easier to mix into soil. 

Plus, it doesn’t need more time to break down, unlike some clumps of hay found in cow manure. You also don’t need to water or fertilize crops as often because mushroom compost is a slow-release fertilizer that retains moisture. 

Animal compost like those made of horse manure and fresh mushroom substrate make good hot compost piles. The generated heat from good microbial activities kills off weed seeds and bad bacteria (5). 

This type of compost also develops faster than your average backyard compost pile. You can add your food waste from your kitchen into the hot compost to break things down faster, making the nutrients readily available to the plants. Ensure you don’t mix vermicompost into hot compost piles because the worms will die in the underground heat.

Growing Mushrooms

Another way to use spent mushroom compost is to use it for growing mushrooms! A lot of mushroom grow kit companies are all about sustainability. 

Even if the grow bag doesn’t give off flushes anymore, it still has many mushroom spores inside it. You can crumble it up in a shaded patch of your garden. Cover it with regular mulch and wait for a new flush to come out. 

If all things go well, you can have a patch that produces mushrooms all year long.

Our Take

Mushroom compost is a good middle ground between animal manure and backyard compost in terms of nutritional value for your plants. But it’s not a universal soil additive. So you need to consider your soil type and the kind of plants you will be growing before using mushroom compost. 

Most plants benefit from a small amount of mushroom compost in the soil because of the moisture it retains and the nutrients it provides. However, some plants can use a higher ratio of mushroom compost, while others can’t tolerate it because of the salt levels. 

In any case, using mushroom compost has more benefits for your garden and the planet compared to the downsides. Adding the right amount of mushroom compost to your soil creates a healthy environment for your plants, beneficial microorganisms, and even soil-aerating bugs to grow. In addition, this “living soil” pretty much takes care of itself, leaving you with less work to do.

FAQs

You can keep mushroom compost in your backyard for years as long as you don’t get it too wet. Excess moisture causes bad bacteria to grow, changing the chemical composition of the compost. For this reason, it’s better to dry out mushroom compost before storing it in bags.

Leaving a pile of mushroom compost in an area where the sun can dry it up but rain can’t get to it is a good way to dry it out. Once it’s dried out, you can bag the compost up and store it. Then, when you’re ready to use it, just rehydrate it, and you’re good to go.

Yes, you can add mushroom compost to raised beds. But you have to make sure the bed has proper drainage. You should also be careful not to compact the soil in raised beds because the plant roots won’t have space to spread out.

Mushroom compost is excellent at keeping the fluffy soil structure to let air circulate through the root systems. It also helps the roots absorb nutrients more efficiently.

Yes, mushroom compost can be toxic to dogs. Remember that it usually still has mushroom spores and other microorganisms inside it. Compost from farm supply yards even has a higher possibility of containing poisonous mushroom spores. 

The digging activity of dogs can cause these harmful substances to get into their system, causing mycotoxicosis (6). The dogs can develop digestive problems or even neurologic damage depending on the severity of the infection. Make sure you keep your compost out of digging range to prevent any incidents.

The best manure for mushroom compost is either chicken or horse manure because they provide the most nutrients. Between the two, many gardeners prefer chicken feces because its smaller particle size is easier to break down. 

The downside is that chicken poop can have traces of antibiotics. Meanwhile, horses can graze in places where they use pesticides. Both options have risks. But when sourced organically, it produces good organic material with high macronutrients for your soil.

The best compost for vegetable gardens is the aged and dark variety. Aged compost has no recognizable plant particles that will make you say, “oh, look, that’s a banana peel.” Instead, the fine texture of aged compost means that the plant matter has broken down to a point where the plant roots can easily access the nutrients from them. This lets you harvest bigger crops faster.

You can age mushroom compost by letting it sit out for a few months before mixing it into your garden soil. This process doesn’t take as long as waiting for your compost bin of kitchen scraps to break down.

  1. Spent Mushroom Substrate. Retrieved from: https://extension.psu.edu/spent-mushroom-substrate
  2. Analysis of Fresh Mushroom Compost. Retrieved from: https://journals.ashs.org/horttech/view/journals/horttech/20/2/article-p449.xml
  3. Salt Tolerant Vegetable Gardening. Retrieved from: http://blogs.ifas.ufl.edu/pinellasco/2014/03/28/salt-tolerant-vegetable-gardening/
  4. How To Use Mushroom Compost. Retrieved from: https://homeguides.sfgate.com/use-mushroom-compost-40833.html
  5. Guidelines for Using Animal Manures and Manure-Based Composts in the Garden [fact sheet]. Retrieved from: https://extension.unh.edu/resource/guidelines-using-animal-manures-and-manure-based-composts-garden-fact-sheet
  6. Mushroom Toxicity. Retrieved from: https://vcahospitals.com/know-your-pet/mushroom-toxicity

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