What is Mushroom Compost?

It’s a By-Product of Mushroom Growing!

Mushroom compost is actually the compost that mushroom growers make in which to grow mushrooms.  What you buy is the by-product of this growing process after the mushrooms are harvested.

mushroom compost

Mushrooms growing in mushroom compost

Mushroom growers prepare a compost made of different organic materials, such as wheat or rye straw, hay, ground corn cobs, peat moss, cotton seed hulls, gypsum, used horse bedding straw, cocoa shells, cottonseed hulls, canola meal, grape crushings from wineries, soybean meal, potash, gypsum, poultry litter, and other natural organic materials.    Most mushroom growers have their own special recipe for their compost.

For the first three to four weeks, the compost is closely watched to make sure the temperature reaches,  and exceeds, 160 degrees F for a few days.  This heat kills any weed seeds, pests, or pathogens.  The compost is also turned frequently to aid in aeration.

When the above stage is finished, the compost is moved  into the building where the mushrooms will be grown. Approximately one week before the mushroom spawn is added, the mushroom compost is steam pasteurized to about 140 degrees F. This kills any remaining surface disease-causing organisms and pests.

When the compost is finally ready, it is topped with sphagnum peat moss which has been mixed with some ground limestone and the mushroom spores are sprinkled on top.

Approximately five weeks later the mushrooms are ready for harvest and they will be harvested for about three to four weeks.

The “spent” compost, as well as everything else in the growing room, is then steam pasteurized.  The mushroom compost is ready.

Because the materials used to make mushroom compost do not contain many heavy metals,  the  compost itself  is low in heavy metals .  Also, the pesticide level is low as mushroom farmers do not, as a rule, use pesticides on their mushroom crops.

This is the mushroom compost you can buy as Spent Mushroom Compost (SMC), Mushroom Soil, or Spent Mushroom Substrate (SMS).

How to Use Mushroom Compost

Mushroom compost, even if it is labeled “mushroom soil”, is not a replacement for regular soil, and should not be used as such.  Especially in container gardening, you should not use more than 25% mushroom compost mixed with the soil in the containers.

Because mushroom compost has a high level of soluble salts, which can be harmful to your plants, it must be mixed 50/50 with soil, and then it can be used as a good slow release organic fertilizer (2-1-1,pH 6.8).  Be especially careful with plants  from the heath family, such as rhododendrons, azaleas, and blueberries.  Another way to make mushroom compost “safe” for young plants, is to let it sit, uncovered, over the winter months so it can “cure.”

A lot of people are concerned that  mushroom compost, after being pasteurized, is not “alive.”  They believe that the pasteurization process kills off  the good micro-organisims that normal compost contains.  If you feel tht this can be a problem, add some regular organic compost – or compost tea – to your mushroom compost and letting it cure for a while.  It doesn’t take long for the mushroom compost to be teeming with micro-organisms again.

compost tumblers

An Earthworm for worm composting

Putting mushroom compost into your wormery and letting the worms work on the mushroom compost over the winter months is also good for mushroom compost. The worm castings have many beneficial organisisms, the soluble salts can leach out of the wormery, and any synthetic fertilizers which may have been used on the mushrooms will be broken down by the worms.

Mushroom Compost Research and Statistics

Research from the Pennsylvania State University has shown that mushroom compost contains about 25% organic matter and 58% moisture.  This makes the mushroom compost perfect for handling and both making surface applications or incorporating it into the soil.  Due to an average of 1.12 % nigrogen, in mostly organic form, the nitrogen is slowly available to your plants.  It also contains an average of 0.67% phosphorous (phosphate), 1.24% potassium (potash), 2.29% calcium, 0.35% magnesium, and 1.07% iron.  The ideal pH range for most plants is 6.0 to 7.0, and mushroom compost averages 6.6.  Perfect compost contains a ratio of 30:1 or LOWER of carbon relative to nitrogen, and mushroom compost has a ratio of 13:1.

A good plan of action is to alternate the mushroom compost as a mulch one year and as a soil amendment the next year.

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