Using Mushroom Compost: Complete Guide for Home Gardeners

You’ve probably heard lots of things about mushroom compost – some good, some bad – leaving you unsure of what to make of it.

When used correctly, this organic-rich soil improver has many health benefits for your garden soil or container plants.

What is mushroom compost best used for? Mushroom compost is best used as a soil amendment in poor garden soil, such as free-draining or compacted mediums. Its fertile composition can be used to improve low-nutrient substrate and neutralize acid soil. It is also good for improving the structure of soil, allowing it to retain more moisture.

Cool, but you’re probably still wondering: how will it affect certain vegetables; where can I find it, and how does it differ from regular compost?

Don’t sweat it – I’m going to answer all these questions and more. Here’s your ultimate guide to mushroom compost.

All About Mushroom Compost

Also known as Spent Mushroom Compost (SMC), mushroom compost is inexpensive and consists of many components that can be beneficial to organic gardening.

SMC has notable differences from normal compost, and recipes can vary depending on from where you source it.

What Is Mushroom Compost?

Mushroom compost is part soil conditioner and part slow-release fertilizer, but – despite the name – it usually does not contain mushrooms.

The majority of commercial mushroom compost is harvested from the leftover substrate in which edible fungi previously grew, hence the common term “spent” mushroom compost.

This mix of organic materials typically consists of straw, hay, corn cobs, horse manure, and poultry manure, though some mushroom growers will also add peat moss, lime (pulverized limestone or chalk), and soybean meal.

Some suppliers, on the other hand, sell pure mushroom compost which consists of a similar recipe but with mushroom spores injected into the mix.

How Is Mushroom Compost Made?

Once a commercial crop of edible mushrooms has been harvested, farmers will put the leftover substrate through a steam pasteurization process for several hours at around 140°F (60°C).

This is to kill any residual insects, weeds, and harmful bacteria.

It is then aged for around 2 years to leach out decomposing matter and land-contaminating solvents before being sold on as organic fertilizer.

The majority of mushroom compost from garden stores will have already been aged, but you can create your own.

Mushroom Compost Nutrients

The poultry and cattle manure within mushroom compost provides the majority of nutrients, but additional nutrients are present in the form of macro and micro salts, which respectively improve soil fertility and encourage plant growth.

Is Mushroom Compost High in Nutrients?

Mushroom compost is dense with essential nutrients because the substrate needed to grow mushrooms successfully has to make up for a lack of carbohydrate production.

Unlike plants, fungi don’t contain chlorophyll that helps produce energy for themselves during photosynthesis, so instead, mushrooms need a large supply of organic materials to feed on.

What Minerals Are in Mushroom Compost?

As well as mainly containing nitrogen, phosphate, and potassium, mushroom compost is also comprised of further macro and micronutrients including calcium, magnesium, and sulfur as well as manganese, iron, zinc, copper, and sodium.

Mushroom Compost pH

Mushroom compost generally has a pH reading of 6.7 (or 6.6 for fresh/younger compost).

This fairly balanced range makes it an appropriate compost for growing many crop varieties but will be too neutral for acidic-loving (ericaceous) plants.

Is Mushroom Compost Good for Plants?

Assuming it is used on the right plants, mushroom compost can enrich the soil in your garden and container-grown fruits, vegetables, herbs, and flowering plants by supplying various nutrients to encourage healthier growth.

Its richly organic components and water retention can also improve soil fertility and structure in some mediums.

For example, mushroom compost will correct the quality and structure of compact, poor-draining soils such as clay and also the overly fast draining, nutrient-light soil of loose, sandy substrates.

Mushroom Compost vs. Manure

Like mushroom compost, manure is highly nutritious due to the animal’s diet of pasture grass and grains.

Furthermore, both forms of fertilizer improve soil aeration and are respectively eco-friendly (cow manure specifically has a low greenhouse gas production, while mushroom compost’s excellent water retention conserves water in its environment).

However, manure contains high traces of ammonia that could damage certain plants and can be more tedious to prepare for composting.

Mushroom Compost vs. Regular Compost

Generally speaking, both compost types have a similar nutrient content but this will vary across suppliers.

Though they offer much the same benefits to garden soil, regular compost tends to be more versatile and cost-effective.

While mushroom compost is made with a mix of animal waste and various other organic matter, regular compost can be made with readily available home materials, such as leaf cuttings and food waste.

Normal compost is also well-suited to seeding medium whereas the salt levels within mushroom compost can be damaging if not fatal for seedling germination when used on its own.

Is Mushroom Compost Acidic or Alkaline?

Most commercially available mushroom composts lean subtly towards alkaline due to the inclusion of chalk (lime).

Without this, it is more or less neutral in composition, which makes it ideal for amending acidic soils.

Mushroom Compost Pros & Cons

Is Mushroom Compost Good Fertilizer?

Yes, mushroom compost is good fertilizer. Thanks to its mild nitrogen content, mushroom compost delivers a slow-release of essential nutrients into the soil to provide long-term fertility and soil structure benefits.

It is also a highly sustainable fertilizer choice since it reduces landfill waste.

Where To Buy Mushroom Compost

Some local mushroom farmers may be happy to offload their leftover compost for free.

But if you have no luck down that route, mushroom compost is widely available in gardening stores and online – this compost in particular is a good choice, especially if you like a great price and speedy shipping.

Using Mushroom Compost

Mushroom compost needs to be applied in specific amounts to avoid health issues in the soil. It’s also important to know that the application of SMC can greatly benefit certain plants but will be deadly to others.

How To Use Mushroom Compost in the Garden

It is best applied to garden soil in early spring or long into summer for vegetable gardening. When amending garden soil, spread the compost evenly and till it 1-3 inches into the top 6 inches of soil.

For container plants, it’s best to mix in ¼ mushroom compost with ¾ of your potting soil mix.

Can You Use Too Much Mushroom Compost?

As mushroom compost has fantastic water retention, use it conservatively with drought-loving plants.

While moist soil is beneficial for plant growth, too much SMC may cause poorly circulated, waterlogged conditions that risk the development of root rot.

What Plants Benefit From Mushroom Compost?

The high calcium levels can benefit tomatoes as they are prone to blossom end rot due to a calcium deficiency, while many tropical plants in need of long periods of hydration such as the iris, marsh marigold, and hibiscus will appreciate this compost’s moisture-retaining quality.

What Is Mushroom Compost Not Good For?

“Keep mushroom compost away from salt-sensitive plants, young plants, and germinating seeds,” warns John Hart, a soil scientist at Oregon State University.

The rich combination of soluble salts and high-nutrient content can overwhelm plants in the heather family of flowering plants (Ericaceae) and will kill off germination seeds when used on its own.

The following fruits, veggies, and flowering plants can be killed by the salts in mushroom compost:

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

Can You Grow Vegetables in Mushroom Compost?

Spent mushroom compost makes a perfectly suitable soil amendment for growing vegetables and other crops.

However, crops should never be planted solely in pure mushroom compost as this has not been allowed to age/cure and will have too high a concentration of soluble salts.

The following vegetables can benefit from mushroom compost:

  • Tomatoes
  • Carrots
  • Lettuce
  • Cabbage
  • Broccoli
  • Potatoes
  • Spinach
  • Peas
  • Squash
  • Bell peppers
  • Kale
  • Beets
  • Sweetcorn

Is Mushroom Compost Good for Roses?

Any organic compost, including mushroom compost, is beneficial for rose beds and gardens, though some growers recommend tilling mushroom compost into the bottom of the soil bed before planting rather than adding it to the topsoil.

This is due to a concern over the slow release of nutrients spreading fungi to rose foliage.

Can You Use Mushroom Compost in Raised Beds?

Yes, mushroom compost can be a great addition to raised beds, but it’s important that the rest of the garden soil is a well-draining medium with a loose, fluffy structure.

This ensures plant roots can spread far and absorb nutrients sufficiently due to well-ventilated soil.

Can You Use Mushroom Compost as a Mulch?

Mulching your plants with an inch-layer of mushroom compost on the surface of your garden soil will help to conserve much-needed moisture for developing plants and will reduce your need for frequent watering.

The high salt content will also help to suppress competing weeds in the soil bed.

Conclusion

In essence, mushroom compost or “spent” mushroom compost as it is commonly sold can deliver a burst of beneficial nutrients to your flower bed or fruit and vegetable patch when mixed in with your native garden soil.

However, the high salt levels aren’t suitable for all plant types, and when used in its purest uncured form the concentration of nutrients can be too much for crops to handle, so it’s wise to consider your plant/soil type before using it.