Mushrooms growing in Mushroom Compost

Mushroom compost is not the same as the compost you make in your compost bins, compost tumblers, or compost piles.  It is actually a byproduct of growing mushrooms.

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

Each compost growing company can have their own precise recipe for their compost making.

It takes three to four weeks for the compost to be ready and it is closely watched to make sure that the temperature reaches and exceeds 160° F for a few days to kill any weed seeds, pests, or pathogens, and is turned several times.

The compost is then moved into the buildings where the mushrooms will be grown.  Approximately one week before the mushroom spores are added, the mushroom compost is steam pasteurized to about 140° F. This kills any remaining surface disease-causing organisms and pests.

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

In about five weeks, the mushrooms are ready for harvest and the harvesting period can continue for three to four weeks.

At that time, the “spent” compost, as well as everything else in the growing room, is steam pasteurized again and the mushroom compost is ready to be disposed of.  The resulting mushroom compost is low in heavy metals as the materials used to make the compost do not have many 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).

Even if it is labeled “mushroom soil”, it is not a replacement for soil, and should not be used as such.  Especially in container gardening, no more than 25% mushroom compost should be placed with the soil in the containers.

Because mushroom compost has a high level of soluble salts, which can be harmful to the plants in your garden, especially those from the heath family, such as rhododendrons, azaleas, and blueberries,  it must be mixed 50/50 with soil.   Then it is a good, slow release, organic fertilizer (2-1-1,pH 6.8).

If you want to use the mushroom compost on young plants, an acceptable procedure  is to let it sit, uncovered, over the winter months to allow it to “cure.”

A lot of people are concerned that the mushroom compost, after being pasteurized, is not “alive.”  They maintain that the pasteurization process kills off all of the good micro-organisims that normal compost contains.  You can solve this problem by adding 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.

Vermiculture

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

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 when 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.  Mushroom compost 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 a soil amendment the next year.

In conclusion, mushroom compost should not replace your standard compost, but can be used with the compost you make from your compost bins, compost tumblers, or compost piles.

See the list of articles in the column on the right for more information about Compost Making, Compost Tumblers, and the History of Compost Making.

(c) 2010 Renee Benzaim