Mushroom compost: What is it? Do you need it?

Laetitia Sands
Posted 5/7/15

The stuff mushrooms thrive on gets recycled to make top-notch garden soil compost. The recipes for mushroom compost vary from coast to coast and grower to grower. Mushroom compost is one of those …

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Mushroom compost: What is it? Do you need it?


MD-gardening column 2x-050415 The stuff mushrooms thrive on gets recycled to make top-notch garden soil compost. The recipes for mushroom compost vary from coast to coast and grower to grower.

Mushroom compost is one of those useful tools that offer the gardener a natural alternative to commercial fertilizers, which often contain chemicals. Listen carefully, all you friends of  birds, bees and frogs threatened by the chemical-ridden environments that surround us, including many gardens.

Most gardening books say nothing about this excellent material, also called mushroom mulch or mushroom soil. But here in Dorchester, a man who sells it and uses it in his own garden says people who’ve tried it swear it produces better tomatoes than anything else they’ve added to their soil.

After leafing through about a dozen of my gardening books, old and new, I found a brief entry in one of them describing this by-product of mushroom growing as “perfect for making your slightly acidic soil more alkaline.” What we don’t know about acidic soil in Dorchester County....Most of us would probably welcome some help making it less acidic.

A Pennsylvania State University website says mushroom compost also improves the structure and drainage of clay soil (we’ve plenty of that, too), reducing compaction, promoting drainage and feeding the microscopic creatures in soil that help keep it healthy and productive.

The gardening book notes that if you made friends with the owner of a nearby mushroom farm, he might give you the “spent compost” left over from growing mushrooms. Meaning for free. Unfortunately, the nearest place one can find mushroom growers is probably Pennsylvania. That’s why Mike Mowbray of the Neck District, outside Cambridge, charges $25 a cubic yard for the stuff. He said the price will rise to $30 next week, due to trucking costs and a smaller than usual amount of the compost available this year. (No, he didn’t offer me a free yard in exchange for this article.)

Landscapers and garden centers here and in nearby counties, as well as stores like Lowe’s and Home Depot, may have it, too.  Numerous ads online offer bagged mushroom compost at various prices, with amounts ranging from 40 pounds to 20 quarts – difficult to decipher and compare to the bulk price, if you’re not a mathematician or engineer.

During research I did recently for a column on permaculture, I discovered that several gardening gurus particularly recommended this compost for growing edibles. It can also serve as a mulch around perennials, shrubs and trees.

But the first I ever heard of the stuff dates back several years. A friend who knows I’m an enthusiastic wild mushroom hunter (no heads on the walls with this kind of hunting) called to say there were lots of interesting shrooms coming up in the ground around the house he rented. About 10 minutes later, I was there with a sharp knife harvesting delicious and nutritious “bolet” mushrooms to incorporate into every dish I could think of for the next few weeks – beef stew, omelets, sauteed chicken breasts, soup, etc.

The kind friend offered the opinion that the property owners had probably improved (“amended,” as horticulturalists say) their Dorchester soil by adding mushroom compost, which may have contained  bolet spores. Mushrooms propagate by spores rather than seeds. The idea that this soil amendment might, as a by-product, produce a crop of edible mushrooms has fascinated me ever since.

In their natural state, mushrooms come up in damp, shady places where decayed leaves — “leaf mold” — cover the ground. Commercial growers use a mixture of composted straw and manure, then add garden soil which contains no stones, weed seed, artificial fertilizers or chemicals.

After their season is done, growers replace the soil to prevent any leftover bits of rotting shrooms from spreading disease to the next crop. But evidently, rotten leftovers pose no threat to garden veggies. On the contrary, the mixture makes such good compost that the growers sell it to garden and landscape suppliers.

Recipes And An Equation

An Oregon State University website says the recipe for mushroom compost varies from grower to grower and may also incorporate used horse-bedding straw, peat moss, cottonseed or canola meal, grape crushing from wineries, soybean meal, potash, gypsum, urea, ammonium, nitrate and lime.

Mr. Mowbray said the mushroom compost he gets from Pennsylvania contains “various types of manure, straw or hay and ground-up corn cobs, mixed into a slurry.” This means “they add water, then let it dry up. There’s not a lot of weed seed in it.”

He went on to explain: “Typically, it’s at least two years old. It builds the soil up with organic matter so it retains water. Good compost is good compost!” Speaking of his customers in the Neck District, where he works out of a building beside the John Lewis General Store, Mr. Mowbray said: “People from around here say they’ve never grown such good tomatoes” as they do with the mushroom compost. (He can be reached at 443-521-5888.)

It’s important not to use the compost “fresh,” the Oregon State University site warns.

Like any compost, mushroom soil needs to be “cured” by letting it sit for a year or so. When fresh, the medium contains soluble salts and other nutrients that can kill germinating seeds and salt-sensitive plants like azaleas and blueberries.

If you have doubts about whether your shroom compost has aged enough, mix it with garden soil and let it cure, uncovered, over the winter before using it, the site says.

To improve veggie gardens and flowerbeds, till about 3 inches of shroom compost into the top 6 inches of garden soil when the ground is fairly dry. For containerized plants, use one-quarter mushroom compost and three-quarters regular soil.

A very important equation given by the site (which I plan to use when spreading mushroom compost on my own garden): one cubic yard will cover approximately 100 square feet of ground to a depth of about 2 inches.

In the meantime, happy gardening!

Editor’s Note: Laetitia Sands is a master gardener in Dorchester County.

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