Substrate Preparation for White Button Mushrooms

I. Guidelines for calculating nitrogen (N) content

1. Calculate the starting N content of pile to be 1.5 to 1.7% before composting. The starting N for a synthetic compost formulas may be slightly higher than the wheat straw horse manure formulas.
2. The percent N will increase throughout Phase I composting and Phase II, and at spawning time, the N content of the compost should be 2.1-2.3 %.
3. Knowing the N and % moisture of the bulk ingredients and supplements will increase the accuracy of the calculated and finished nitrogen content.
4. If supplements are added by volume, occasionally weigh volume added to confirm calculated formula.
5. At the end of Phase I and again at the end of Phase II, compost may be analyzed for N, ammonia, ash and moisture. It is important to take representative samples, several small handfuls thoroughly mixed. When taking a sample, do not shake the compost.

II. Examples of Mushroom Compost Formulas

III. Suggested watering procedures during composting:

1. Add as much water as possible without runoff during pre-wet conditioning or during the first two turns.
2. Avoid adding too much water early during Phase I, always be able to control moisture.
3. Add only enough during next turn or turns to wet dry spots.
4. Bring up compost moisture to desired water content by adequate watering just before filling.
5. During pre-wet, it is advisable to flip or turn the compost every day. After the rick or pile is built, the compost should be turned every other day unless pile temperatures have not peaked.

IV. Changes in organic matter, carbohydrates, and nitrogen during mushroom composting.

1. Soluble carbohydrates are simply absorbed by the microorganisms, and they are converted into new living matter or provide energy for the cells.
2. As these microorganisms grow, energy in the form of heat is released.
3. As the pile heats to temperature above 150°F, the activities occurring within the pile change from biological to chemical reactions.
4. It is at these higher temperatures that carmelization takes place. Carmelization is the process in which water is eliminated from the carbohydrates and carbon is concentrated. This process can be compared to boiling sap down to make maple sugar.

V. Phase I is considered complete when as soon as the raw ingredients become pliable and are capable of holding water, the odor of ammonia is sharp and the dark brown color indicates carmelization and browning reactions have occurred.

1. Moisture content at filling should be 70-73%. Water should drip from compost squeezed in the hand. But a good rule of thumb to follow is: the longer, greener, or more coarse the compost, the more moisture it can take. The shorter, more mature or dense the compost, the less water it should have.
2. The shorter or wetter the compost, the more loosely it should be filled into the beds or trays. The longer or greener the compost, the more it can be firmed into the beds.
3. Attempt to fill uniformly in both depth and compaction.
4. Edges or sideboards should be packed slightly tighter, whereas the center should remain looser.

VI. Phase II composting has two objectives:

1. Pasteurization - elimination of undesirable insect pest, microbes, and pathogens.
2. Conditioning - Creation of specific food for the mushroom and creating a selective and suppressive compost to favor the growth of the mushroom.

VII. Ensure adequate ventilation during Phase II. When in doubt, ventilate. A flame should be burned at all times.

1. The higher the nitrogen content of compost, the greener the compost or the more dry weight at filling time, the greater the ventilation required.
2. When outside temperature is high, as in summer or early fall, more ventilation is required than when Phase II occurs during the cold winter weather. This is especially important when the grower does not have a forced air ventilation system.

VIII. During Phase II, keep compost in the temperature range where microorganisms grow best (115-140°F).

1. Microbes convert ammonia and ammonia-containing salts into protein and other nitrogen compounds that the mushroom uses for food.
2. The growth of these microbes depends on having the available food, adequate moisture, sufficient oxygen and suitable temperature. A shortage of one of these requirements will limit growth and often result in incomplete conditioning.

IX. Heat up (pasteurization) for insect kill early in Phase II (perhaps 1-4 days after filling) so as to avoid a second heating cycle of the compost.

1. A good indication that the compost is ready to be pasteurized is the subsiding of microbial activity, which is indicated by a decrease in compost temperature at the same air temperature.

X. After pasteurization, slowly lower compost through the temperature ranges of the microorganisms. A general rule is to lower compost temperature no more than 4–5°F per day.

1. Provided that enough food, water and oxygen the microbes will continue to grow.
2. Different microbes use different compounds and grow at different temperatures. Therefore, it is important to make sure all areas of the beds and room gradually drop through all temperature ranges.
3. Thermophilic fungi grow at lower temperatures and are important because they are able to grow into denser areas of compost.

XI. Composting is considered complete when no trace of ammonia odor can be detected and the compost has a uniform flecking of white colonies of actinomycetes, called fire-fang.

1. Compost should have less than 0.1 ppm of ammonia and the N content on a dry weight basis should be in the range of 2.0 to 2.5.

Prepared by David M. Beyer, Penn State