Growing Mushrooms on Compost
By Marc-Andrew Donsky
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
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,
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:
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" 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 160°F (71°C). Temperatures of up to 160°F are
due to thermophilic organisms. Temperatures over 170°F
are due to chemical bonds being broken as well as other
chemical reactions. Temperatures over 160°F 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
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 130°F 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 80°F 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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