Mushroom Composter Plans

How to generate superior compost for mushroom growing on a small scale.

This method uses a styrofoam picnic cooler with a steam humidifier and a common light dimmer (or variable transformer) to control the steam and compost temperature. The final product is about 3 gallons of pure pasteurized compost in just 6 to 7 days. Total cost for the bits and pieces to construct this composter should be no more than $30.

Styrofoam cooler
Steam humidifier or vaporizer (VICKS Warm Mist or similar)
Bottle, one gallon or larger
Hose (2 feet) that will fit mouth/cap of the bottle
Wooden sticks (5), each being 2 feet in length
Single pole, solid-state dimmer switch
Metal mesh screen (4 sqaure feet with a 1/4" or 1/8" mesh size)
Long stem thermometer, 120ƒ C, scientific or kitchen (meat therm.)

The construction of the composter utilizes a styrofoam cooler, preferably one close to 14 inches by 22 inches by 15 inches deep in size. Six 3/4" holes, evenly spaced across the surface of the lid, are melted with a soldering iron or cut with a knife. Six more holes of the same size are melted or cut into the body of the cooler, three along each side, two inches up from the bottom edge. On the short sides of the cooler body, one hole is cut or melted on each side at the bottom edge. One of these holes is sized 1/2", and will serve for drainage. The other end has a hole cut just the right size to accommodate the steam output head or nozzle of the steam humidifier.

The bottom of the cooler is lined with aluminum foil, being careful to not obstruct the drain or steam inlet. A screen support is made from the 1/4" or 1/8"-mesh screen (sometimes called hardware cloth). The screen is cut 8 inches larger than the dimensions of the bottom of the cooler. 4 inches of screen are folded 90ƒ at each edge to form supports, which hold the surface of the screen 4 inches above the bottom. The compost sits on the top surface of the screen so that the steam can flow evenly underneath the compost. This screen support should fit the bottom of the cooler without large gaps around the edges.

A single-pole solid-state dimmer is used to control the humidifier power. Good wiring and insulating procedures need to be followed. If you aren't knowledgeable about wiring, find someone who can help. It is important for safety that the wiring be correctly installed. The best method is to wire the dimmer switch (in a plastic electrical switch box) to both the power-cord of the humidifier (cut off the plug end[male]). Then take an extension cord (cut off the plug end[female]) and wire it to the dimmer switch.

Things to check for:

1. The maximum capacity of the dimmer switch (in Watts) should be greater than the load imposed by the humidifier. (almost all dimmers have the necessary rating)

2. The wiring methods should be well insulated and suitable for the damp environment it will be used in.

Finally, a water reservoir is constructed. Arrangements are made to secure a one-gallon or larger water bottle above the humidifier. A 1 and 1/2-inch hole is cut in the top surface of the humidifier body to accommodate the hose. The humidifier is filled with water to the "fill" line, and the hose is inserted so that the open end of the hose hangs 1/2-inch below the fill line. In this way a constant level of water is maintained in the humidifier, which makes the steam output constant for a given dimmer setting. It is important to maintain accurate steam / temperature settings during a composting run. During a run, the humidifier electrodes must be cleaned at least every other day, or output will be seriously effected. Using distilled water is a slightly expensive option to regular cleaning.

Before the first time a composter is used, it is best to set it up where in the final location, tape all the holes closed except the drain hole and the humidifier nozzle hole on the bottom, fill up the humidifier and reservoir jug with water and turn on the humidifier at full power dimmer setting allowing the cooler to steam for 4 hours or so to eliminate any plastic or chemical odors and to test your composter's operation.

To begin a compost run:

Mix in a bucket, approximately 2 and 1/2-gallons of shredded straw with 1 gallon shredded dry fresh manure (cow or horse). Mix in 3 to 4 ounces cottonseed meal or 1 to 2 ounces blood meal (nitrogen sources, available at most plant nurseries). Add 3 and 1/2-quarts of hot water, preferably with a sprayer, and mix thoroughly. This warm mixture is loaded into the cooler without delay, loosely filling the cooler to within 1 to 2 inches of the top. Five sticks are placed in the cooler before adding the compost mixture, so that after the compost is loaded, 2-inch square ventilation shafts can be created by wiggling each of the sticks around. The sticks are withdrawn, and the lid put in place, all the vent holes are taped shut except the one on the lid furthest from the steam nozzle, and the dimmer set on maximum. The thermometer is placed through the untaped hole so that it is measuring the middle of the compost.

During the first phase of the run pasteurization occurs, killing all insect life. Note that this is opposite of how it is done at a commercial mushroom farm or in outdoor nature composting, where pasteurization is done at the end of a composting run. After about 2 hours of full power steaming, the temperature should rise to approximately 65ƒ C (60ƒ to 70ƒ). Quoted temperatures are only a rough guide, try to maintain the temperatures within the given ranges. For this first phase it is necessary to reach a minimum of 60ƒ C. Once the temperature reaches 60ƒ C check the time and let the compost remain above 60ƒ C. for 3 to 4 hours.

At the end of this time remove all the tape from the ventilation holes and leave the dimmer setting on full. Within a day ammonia will be noticeable in the exhaust steam and the temperature should be 55 to 65ƒ C. Hold this temperature for 2 to 3 days. After the first day the ammonia odor should disappear. After this happens, white fire fang, or actinomycete fungi, will be apparent in the compost. It is best to allow the compost run to proceed undisturbed, but if you wish to check it is okay to briefly lift the lid and move the compost surface. Don't allow the compost to cool during any brief investigations. At the end of the 3 days actinomycete should be prominent and there should be no ammonia smell. If not, extend the high temperature phase for up to an additional day.

The dimmer setting is now lowered slowly so that the compost temperature drops 2ƒ to 3ƒ every 2 to 3 hours, until a temperature around 50ƒC is reached (45ƒ to 55ƒ). The compost is then held at this lower temperature for 3 to 4 days. At the end of this time the power is turned off and the compost is allowed to cool slowly inside the closed cooler. It is ready to lay out and spawn when the temperature cools to 24ƒ to 28ƒC. It is best to spawn the compost immediately after it has cooled, although it can be stored for a few weeks if necessary.

The compost made in this way is usually excellent, comparable to the best commercial mushroom farm compost. An added benefit is that new compost recipes can be experimented with in small quantities and in short time frames. One extra is that the drain water makes an exceptional organic fertilizer for houseplants.


1. The formula for compost here is very basic. With trial and error many materials can be formulated into usable compost. In particular, it is important to mix for the correct carbon / nitrogen ratio (saccharides / protein). Any properly balanced mixture suitable for large scale composting is suitable for use in this composter. Very useful tables of compost ingredients are included in The Mushroom Cultivator and Growing Gourmet and Medicinal Mushrooms, both by Paul Stamets.

2. Some species of mushrooms culture well on ordinary pasteurized straw with no additions. This composter is ideal to pasteurize straw. It is much simpler and more accurate than immersion in hot water or other methods. Just load with shredded dried straw and pasteurize at greater than 60ƒ for 4 to 6 hours. Allow to cool slowly covered, and spawn when the temperature is less than 28ƒC. Typically the straw will be at proper moisture content when unloaded.

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