Mushroom Compost: What It Is, Benefits, pH and How to Use It

Mushroom compost is the leftover growing material from commercial mushroom farms. Once the mushrooms are harvested, that mix of straw, manure and other ingredients is sold as a rich, crumbly soil amendment.

It’s a good, cheap way to add organic matter to most gardens. It does have two quirks, a slightly alkaline pH and extra salts, so here’s how I’d use it and where I’d skip it.

What Is Mushroom Compost Made Of?

Mushroom farms grow their crops in a pasteurized mix, usually straw or hay, horse or chicken manure, gypsum and a top layer of peat moss and lime.

After a few harvests, that mix is “spent” and gets sold to gardeners. Despite the name, there are no whole mushrooms in it. It looks like dark, slightly stringy compost.

If you want to try some, bagged versions are sold for home gardens.

Check price on Amazon.

Benefits: What It Does for Your Soil

• Adds organic matter that loosens heavy soil and helps sandy soil hold water.
• Feeds soil life, like earthworms and beneficial microbes.
• Supplies calcium from the gypsum and lime, which many vegetables appreciate.
• Recycles a farm by-product, often cheaper than other composts by the truckload.

Is Mushroom Compost Acidic?

No. Because growers add lime and gypsum, it usually tests neutral to slightly alkaline. Penn State Extension reports aged material testing around pH 7.

That’s fine for most vegetables and flowers, but it’s the wrong choice for acid lovers like blueberries and azaleas. Not sure where your soil stands? Test it first.

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Is It High in Nitrogen?

Not really. It usually carries only about 1 to 2% nitrogen, released slowly as it breaks down.

Think of it as a soil conditioner, not a fertilizer. Hungry crops like corn and tomatoes will still want regular feeding.

Will Mushroom Compost Burn Plants?

It can, if it’s fresh and used heavily around sensitive plants. Fresh mushroom compost is higher in soluble salts than most composts.

The fix is simple. Buy aged material or leave it in a pile outdoors for about 6 months, and mix it into the soil instead of planting straight into it. Rain washes most of the salt away.

According to the American Mushroom Institute, weathered mushroom compost is safe for most garden plants when used this way.

How to Use It

• New beds: dig 2 to 3 inches into the top 6 to 8 inches of soil.
• Existing beds: spread 1 inch on top each spring and let worms work it in.
• Containers: no more than about a quarter of the mix.
• Lawns: a thin topdressing of about a quarter inch, raked in.

A sturdy garden fork makes mixing it into beds much easier than a shovel. Check price on Amazon.

For crop-by-crop advice, see how to use mushroom compost in a vegetable garden.

Mushroom Compost for Clay Soil

This is one of its best uses. The coarse organic matter opens up heavy clay, improving drainage and making it easier to dig.

Work 2 to 3 inches into the top 8 inches, and repeat each year. The gypsum in it can also help clay particles clump into a crumblier texture.

Will It Kill Grass?

Not when you use it thinly. A quarter-inch topdressing, raked into the lawn, feeds the soil without smothering the grass.

A thick layer of an inch or more can block light and suffocate the turf, so go light and water it in.

Mushroom Compost vs Regular Compost

Homemade or municipal compost is usually closer to neutral and lower in salts, so it’s more versatile. Mushroom compost is often cheaper by the bulk load and richer in calcium.

Leaf compost is gentler still and slightly acidic, a nicer choice around shrubs. For head-to-head comparisons, see peat moss vs mushroom compost and worm castings vs mushroom compost.

Can You Use It as Mulch?

You can, but it’s not ideal. It breaks down quickly and can form a crust. I’d use it as a soil amendment and mulch with shredded bark or straw instead.

Can You Make Your Own?

Not true mushroom compost, since it comes from mushroom farming. You can make a similar blend by composting straw, horse manure and a little gypsum together for several months.

Or simply age store-bought mushroom compost in a pile for 6 months to mellow its salts.

Plants That Like It, and Ones That Don’t

Most vegetables, roses, lawns and many perennials do well with it. Acid lovers and seedlings do not.

The full list is in plants that like (and dislike) mushroom compost.

Mushroom Compost Questions

Sources

• Penn State Extension: Spent Mushroom Substrate.
• American Mushroom Institute: Mushroom Compost FAQ.