What Is Mushroom Compost and Should I Use It?

When walking the aisles at your local garden center looking for some compost to put on your garden, you may find yourself overwhelmed with the number of options available. With so many types of compost on the shelves, how do you choose which is right for your plants? One of the most misunderstood–and underutilized–types of compost is mushroom compost.

Mushroom Compost

Mushroom compost is a substrate that has previously been used to grow mushrooms commercially. Once mushrooms can no longer grow in it, it’s sterilized and then sold to consumers for use in gardens and lawns.

Keep reading to find out how mushroom compost is made, the benefits and drawbacks of adding it to your soil, and the best ways to use spent mushroom compost in your garden.

What Is Mushroom Compost Made Of?

Though the name seems to imply it, mushroom compost doesn’t actually have any mushrooms in it. The ingredients in mushroom compost vary slightly depending on the company producing it. However, the basic recipe typically includes:

  • Corn cobs
  • Straw
  • Horse or chicken manure
  • Peat moss
  • Potash

Other ingredients that may be added in certain mushroom composts include gypsum, soybean meal, canola meal, and lime. These ingredients create an incredibly fertile soil perfect for growing mushrooms.

How Is Mushroom Compost Used Before It’s Sold At Garden Centers?

The ingredients in mushroom compost aren’t left to decompose for months like normal compost. According to Oregon State University, mushroom compost sits for about 30 days before being placed in beds.

While the compost sits, it reaches temperatures of 160 degrees Fahrenheit or greater. This kills any bacteria from animal manure or weed seeds. At this point, it’s ready to be used to grow mushrooms.

Before mushroom spores are added to the soil, the compost mixture is pasteurized to kill any remaining bacteria or pathogens. The mushrooms are ready to be picked just over a month after being placed in the compost beds, though they will continue producing mushrooms for several more weeks.

Once mushrooms can no longer grow in the compost, it’s sold commercially for gardeners to use. Spent mushroom compost is typically sold in bags, but can also be purchased by the truck load.

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Is Mushroom Compost Sterilized After Growing Mushrooms?

Yes, mushroom compost goes through a sterilization process before hitting store shelves. This kills any mushroom or mold spores in the soil, making it safe for use in home gardens. To sterilize the mushroom compost, it’s placed in a pressure cooker and heated to over 250 degrees Fahrenheit.

What Are The Benefits Of Using Mushroom Compost In A Home Garden?

Mushroom compost helps with water retention in soil. If you treat your garden soil with mushroom compost, you’ll have to water your plants less often.

You can add mushroom compost to your garden without fear of introducing bacteria or illness to the soil since the sterilization process kills any diseases in the compost.

Mushroom compost has high levels of calcium. This can be beneficial for plants like tomatoes, which can suffer from blossom-end rot when they are calcium deficient.

Another benefit of mushroom compost is that it helps to aerate the soil. This is especially beneficial in places with heavy clay soils. Improved aeration helps plants grow stronger, deeper root systems.

Compost in wheelbarrow

Because mushroom compost helps to aerate soil, it’s a great soil conditioner. According to Penn State University, commercial potting mixes sold in stores are primarily made up of mushroom compost for this reason. Mushroom compost helps to create a light, airy soil structure.

What Are The Drawbacks Of Using Mushroom Compost?

Though mushroom compost helps with soil aeration and is weed-free, it’s not without imperfection.

Mushroom compost is available to consumers only after it can no longer support the growth of mushrooms; this means that many of the nutrients have already dissipated. While some nutrients remain, mushroom compost is best used to condition soil rather than to fertilize it.

Some spent mushroom compost may contain traces of pesticides. The sterilization process effectively decomposes much of the pesticide content, but a small amount may still remain. Mushroom farmers typically use the same pesticides as gardeners do, but this may be a concern if you are following organic gardening protocols.

Another issue with mushroom compost is the high sodium content. Many plants have a sensitivity to salt, so using too much can stress your plants or even kill them. Germinating seeds and young plants are among the most vulnerable, along with azaleas and rhododendrons.

What Is The Best Way To Use Mushroom Compost In My Garden?

To get the most out of using mushroom compost, here are a few things to keep in mind:

  • Let it sit before you use it. Allowing your mushroom compost to sit outside over the winter months will help reduce salt and pesticide levels.
  • Use fertilizer to boost nutrient levels. Since mushroom compost contains fewer nutrients than other types of compost, adding fertilizer to soil amended with mushroom compost will help make sure your plants are receiving the nutrition they need to thrive.
  • Use it in moderation. Mushroom compost should only make up a small portion of your soil. Combining it with natural soil or other types of compost allows you to benefit from mushroom compost’s increased aeration without having to worry about high salt levels in your soil.
  • Use it on your lawn. Though many vegetable seeds dislike the high sodium content of spent mushroom compost, grass seed actually thrives when planted in it. The compost acts as a ground cover to keep birds from eating the seeds, and the mild nutrient contents will slowly feed the seeds as they grow. The increased water retention also keeps the seeds from drying out and blowing away.

Final Thoughts

Though there are a few drawbacks to using mushroom compost, the benefits far outweigh them. The increased water retention and increased aeration in the soil make mushroom compost a good choice for dry, compacted soils. It’s also often cheaper than other types of compost. The next time you’re at the store looking for compost to add to your garden, consider giving mushroom compost a try.

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