Gardening: Examining Mushroom Compost

Last week I wrote about the flower gardening seminar led by Shannon Ezer that I attended at The Jersey Barnyard. I ran out of space and couldn’t get around to the part I learned the most about – bed preparation.

For me, this was the most interesting part of her class. For the past few years, I’ve been experimenting with all kinds of ideas for bed preparation. In my beds, I’ve messed around with different combinations of compost, leaf mould, whole ground cornmeal, rock minerals, molasses, lava sand, greensand, charcoal – you name it, I’ve probably tried it. And I’ve had good results. Adding any of these products to your native soil will improve it.

Ezer takes a much simpler approach. When she started the cut flower garden at The Jersey Barnyard a few years ago, Ezer said she ordered a dump truck load of mushroom compost and mixed it in to the native soil. Every year since then, she said, they add at least three inches of mushroom compost to the top. They don’t mix it in. They plant straight into the compost. The farm does this year after year, she said, and the garden keeps improving.

I haven’t used a lot of mushroom compost because you typically need to hire a gravel company to deliver it to you. There is a mushroom farm in Gonzales that produces a lot of compost certain times of the year. Some garden centers sell mushroom compost in bags, but the quality can be hit or miss. Ezer said the compost delivered from Gonzales is very high quality. She convinced me to give it a try.

Some organic gardeners warn against mushroom compost due to concerns about salinity. I have heeded this warning in the past. But the most I research, I am beginning to wonder if this is a myth.

Commercial mushroom operations grow mushrooms in fresh compost, which they call “substrate.” The compost recipes vary from producer to producer, but they commonly contain wheat straw bedding containing horse manure, hay, corn cobs, cottonseed hulls, poultry manure, brewer’s grain, cottonseed meal, cocoa bean hulls and gypsum. These products contain a lot of nitrogen, so the compost heats up quickly. The finished compost is pasteurized to kill any diseases or pests. Then the mushroom grower inoculates the compost with mushroom spawn. After the mushrooms are harvested, the growers get rid of the “spent” compost because it doesn’t have enough nutrients to grow another crop of mushrooms. But this spent compost still has lots of nutrients for plants. So the growers often sell it to nurseries or gardeners.

According to an article by Dr. David Meigs Beyer, Professor of Mushrooms at Penn State, fresh spent compost can contain somewhat high levels of salt, as much as 0.21 to 0.33 percent by dry weight. However, that number drops to 0.06 percent if the spent mushroom compost has been weathered or aged for 16 months. Beyer said the salt content drops considerably even after just six month of weathering. Therefore, he recommends allowing it to age for a few months before adding it to the garden.

I would assume that bagged mushroom compost has already been aged at least six month, so I would not be concerned about using it in the garden. If you buy mushroom compost from a gravel hauler or straight from the mushroom farm in Gonzales, I would stockpile it for a few months before using.

“Obtaining spent substrate in the fall and winter, allowing it to weather, will make it ready to use in a garden the following spring,” Beyer said. “Spring and summer are the best time to use weathered material as a mulch.

“As a soil amendment, spent substrate adds organic matter and structure to the soil,” he adds. “Spent substrate primarily improves soil structure and it does provide a few nutrients. Spent substrate is the choice ingredient by those companies making the potting mixtures sold in supermarkets or garden centers. These companies use spent substrate when they need a material to enhance the structure of a soil.”