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.

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.

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.

Parameters	Average values (1,2)	What it means
pH	6.7	Good pH range for growing most crops, but not suitable for acidic-loving plants.
N-P-K ratio	1.8 – 0.6 – 2.2	Generally good for all-around plant growth
Organic Materials	25.86% (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.
Moisture	57.33%	Excellent water absorption keeping the soil moist.
Soluble salts	0.38 SAR	Good 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 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).   

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.