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Growing Mushrooms on Compost

By Marc-Andrew Donsky
Cultivation III

By preparing compost, you are creating an ideal medium for mycelial growth. Basic mushroom compost is made up of wheat straw, horse manure and gypsum (calcium sulfate). There are a variety of optional ingredients that may be added. A brief outline of some materials used in making composts follows:

serves as a carbon source (carbohydrate) source wheat - considered the best - contains xylan oat, barley - break down more rapidly than wheat rye - breaks down slower than wheat also corn cobs, oak and beech leaves, etc.

Other Carbohydrate Sources:
Rice straw, molasses, brewer's grains, cottonseed meal (provides the fatty acid - linoleic acid -which is reported to stimulate yields.)

nitrogen source, provides organisms essential to composting horse - most commonly used, fresher the better poultry - higher in nitrogen and phosphorous than horse, not so rich in potash (provided in wheat straw), faster and hotter than horse, use dry pig and sheep - must be used before they become sticky - used partly dry

Other Nitrogen Sources:
Blood meal (dried blood), bone meal urea, ammonium sulfate ((NH4)2SO4) Gypsum: calcium sulfate (CaSO4) - essential to mushroom compost preparation - prevents the compost from becoming too "greasy" - by forming an equilibrium matrix with the water, also helps the colloids to flocculate producing a compost with a more granular structure with increased water holding capacity: provides Ca++ ions; a mineral essential to mushroom growth: helps to prevent the loss of nitrogen (from the breakdown of proteins during the act of composting) by chelating the ammonia

Optional Mineral Sources:
Superphosphate; is said to promote vigorous mycelial growth, but an excess may make the beds too acid too soon which depreciate the crop. 14 lbs./ton of compost should be added at the last turn. It should not be used if there are a lot of droppings 9 fresh) in the compost.

Sulfate of potash; used in synthetic composts. the ubiquitous calcium carbonate.

Activators; compost "activators" can be obtained from nursery and garden stores and assures the presence of the organism essential to composting.

The following recipes create about one half ton of compost. One half ton of compost will provide enough compost for about 60 square feet of beds (surface area). At least one quart of grain spawn per 15 square feet of bed surface should be used.

Sample Compost Recipes:
5 bales wheat straw, half a pickup (half ton) horse manure, third of a pickup of horse manure, 30 lbs. gypsum, 2 lbs. activator, 70 lbs. chicken manure, 4 lbs. Blood meal and 30 lbs. gypsum.

To prepare compost, the straw must be soaked for several days until it just about, but not quite, squeezes water out in your hands. The compost pile is then built by stacking alternating layers of straw, activator, manure and gypsum until all the materials are used up. The stack should be 4-6 feet high.

In about 48 hours the heap will begin to generate heat and will sink somewhat in height. By the fourth to sixth day the temperature in the interior of the pile should reach 160F (71C). Temperatures of up to 160F are due to thermophilic organisms. Temperatures over 170F are due to chemical bonds being broken as well as other chemical reactions. Temperatures over 160F are undesirable. After the pile reaches a peak temperature the temp will then begin to fall and the pile should be turned. The pile is turned by moving the middle half third to the bottom, the top and sides to the middle, and the bottom to the top. If any parts appear excessively dry, water should be sprinkled on those parts at this time. There should be no need to add any water after the first turn.

The heap will again heat up and be ready for a second turn after six more days. It should now be turning a rich brown color. With the second turn, no water should be given unless there are very dry patches - wet sparingly. One more turn should complete the mixing but if the temp (peak) is above 130F a fourth turn may be necessary, (some authors recommend even another turn). If on the final turn the compost is too wet or has a greasy appearance, more gypsum may be added.

When done, the pile should be brown to gold in color, open in texture, and have a rich humus smell. The straw should break readily when twisted, and the compost should be just moist enough to bind together when squeezed in the hand. Initially the compost will have an alkaline pH. When mature and ready for inoculation the pH should be between 7.0 and 8.0. The heating of the compost has pasteurized the compost by the action of the thermophilic organisms. These organisms will not grow at the lower temperature at which mycelium grows. With proper composting the resulting compost will be free from competing organisms. Insects in all their forms will be absent from the medium and the rapid growth of the thermophilic composters will have also eliminated bacterial and fungal competitors.

Inoculating Beds: The compost is then filled in boxes about 10-12 inches deep. The temperature should be 80F or less and there should be no ammonia fumes present when the boxes or beds are inoculated (spawned). The compost is inoculated with grain spawn either by mixing throughout the compost in the bed or box or by sprinkling a tamped down box with spawn and then covering thin layer of compost. In either cased the compost and spawn are then tamped down and covered with moist newspapers or a sheet of plastic to retain the humidity. The inoculated compost should be allowed to sit for 2-5 weeks (until the mycelium has taken over the compost). It may be necessary to moisten the newspapers occasionally during this time.

When the compost is permeated with mycelium it is then cased for fruit initiation. A drop in temperature and increase in ventilation induce fruiting. As the mushroom and mycelium grows there will be a drop of pH from the excreted metabolites until the pH reaches 5.0-5.5 at which time mushroom production will cease. At this time, the boxes/beds should be removed and the area thoroughly cleaned and sanitized.

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