What is in Mushroom Compost?

Mushroom cultivators produce mushroom compost to foster an environment suitable for the fungi necessary for mushroom development.

Mushroom Compost could refer to one of several forms of anaerobic compost specifically formulated for mushroom cultivation. Mushroom compost could also refer to compost previously used for mushroom growing, a residuary product still loaded with plant-beneficial microbes.

Aerobic Composting 101

Compost is one of the keys to creating healthy soil and, as such, is an essential skill every gardener should develop. I strongly advise you to get my book on the topic, not only because I want the sale, but because I sincerely believe it will benefit you.

Decomposition is the natural process of reducing organic waste into a stable form. Composting manages decomposition, orchestrating the interaction between diverse microorganisms, carbon, nitrogen, water, and air. The process generates heat which effectively destroys pathogens and weed seeds.

Composting’s product is humus (the Latin word for earth), a dark-brown substance essential to sustaining life and positively affecting the soil environment in myriad ways. The smell of humus (petrichor) is strongly associated with rain or healthy soil, an odor caused by actinomycetes, bacteria spores in humus. Their presence is an indicator of the soil’s health.

The Aerobic Composting Process

 The formula for a successful composting batch is simple:

  • A carbon-to-nitrogen ratio of 30:1.
  • Moisture content below 60%
  • Air content above 5%

Provide that, and the ubiquitous microorganisms will do their thing. The carbon (brown plant material) and nitrogen (green plant material) must have the maximum surface area exposed, so they must be shredded before being added to the pile. Still, because you need air inclusion, some parts of the mix must be bigger to avoid compaction.

The process is a thing of beauty. First, the initial microbes will consume the carbon as food, releasing CO2 and nitrogen to boost their cellular growth and replication. The water provides transport and functions as a reactant in some processes. Finally, we need aerobic microbes, so a constant air supply must be provided for them to flourish and for anaerobic microbes to be limited.

The activities of the microbes develop heat, triggering an additional cohort of microorganisms (thermophilic) to become active, pushing the temperatures even higher.

For higher temperatures to be reached, some insulation is needed, which is only possible if the batch is big enough (bigger than a cubic yard).

Regular fresh air inclusion and added water will keep the environment optimal for microbe activity, consuming the material and converting it to humus. Air and water management is achieved by turning the batch and incorporating the outlying insulation material into the center for thermophilic consumption.

The final product has carbon-rich plant material reduced to its essence. Most of the nitrogen bound in the plant material has now been reduced to plant-available nitrogen captured in the living and dead microorganisms. When added to the soil, the living microbes will migrate to the plant roots, making their bioavailable nitrogen accessible to the plant.

Spent Mushroom Compost

The soil-like substance left over after a crop of mushrooms is known as spent mushroom substrate and is what is commercially available to gardeners. The high amount of organic matter in the spent substrate makes it a suitable soil conditioner or additive.

Making Compost for Mushroom Growing

Making compost for mushroom growing (mushroom compost) is similar to the process above but with a twist. The process is the same, but the recipe is quite different. The final product is richer in nitrogen because of a recipe that includes the addition of synthetic ammonium nitrate.

Also, rather than being a single pile repeatedly processed until humus is formed, fresh mushroom compost goes through three processes before being used to grow mushrooms. Spent mushroom compost is the soil-like material that remains after the mushrooms have been harvested.

Phase I of Making Mushroom Compost

Until recently, mushroom compost was a mix of horse and poultry manure, brewer’s grain, and gypsum. The new synthetic blend includes hay, corn cobs, poultry manure, ammonium nitrate, potassium, and gypsum, with the hay and corn cobs constituting about 85% of the total mix, about 10% is chicken manure, and the remaining 5% made up of the synthetic additives.

The process depends on the presence, in the right proportions, of moisture, oxygen, nitrogen, and carbohydrates. The performance of mushroom compost regarding spawn run and mushroom output is influenced by the quality of the raw materials utilized, which can vary greatly.

The addition of gypsum boosts aeration, an essential factor in aerobic composting. Nitrogen supplements include maize distiller’s grain, seed meals made from soybeans, peanuts, or cotton, and chicken manure.

Ammonium nitrate or urea is added to synthetic compost at the beginning of the composting process to give the compost microbes a readily available nitrogen source for their development and reproduction.

At intervals of around 2-3 days, the pile is turned and watered when temperatures above 145°F (63⁰C) are reached. Turning allows hydration, aeration, and mixing of the components, including incorporating non-decomposed material into the center, where the action is.

Mushroom composting is known for its potent ammonia smell, typically accompanied by a sweet, moldy scent. When ammonia is present, and compost temperatures reach 155°F (68⁰C) or above, chemical reactions produce the nutrition mushrooms need.

During the second and third turns, when the desired degree of biological and chemical activity occurs, temperatures in the compost can reach 170° to 180°F (77⁰C – 82⁰C). Finished mushroom compost, at this first stage, has:

  • a chocolate brown color
  • soft, malleable straw
  • a moisture level of between 68 and 74%
  • a strong smell of ammonia.

Phase II of Making Mushroom Compost

Phase II composting serves two main objectives: Eliminating potential insects, nematodes, and pest fungi and reducing ammonia to below 0.07%.

Phase II composting can be considered a managed, temperature-dependent ecological process that uses air to keep the compost at a temperature where microorganisms can thrive and multiply.

To thrive, these thermophilic (heat-loving) organisms require both useable carbohydrates and nitrogen, some of which must be ammonia. The mushroom mycelium thrives on these bacteria as they produce or act as nutrients in the compost.

The first goal of this second phase is to get rid of extra ammonia. De-ammonifying organisms thrive at temperatures between 125° and 130°F (~52 – 54⁰C). The other goal is to eliminate pests by pasteurizing the batch.

Before planting (spawning) can start, the compost temperature needs to be decreased to 75 and 80⁰F (24 – 27⁰C), moisture levels should be between 68 and 72%, and nitrogen content should be 2.0 to 2.4 percent.

The Third Component of Spent Compost

The casing is a top dressing added to the compost where the mushroom spawns eventually form. Peat moss and ground limestone are the main ingredients and serve as a water storage area and a location for rhizomorph formation.

Sphagnum can vary in color from brown (young, less decomposed, loose textured, surface peat) to black (compact, more decomposed, deep dug).

Wet-dug peat is transported in a saturated state, whereas milled peat is partially dried before packaging and shipment. Certain farmers prefer wet-dug peat because it has more water-holding capacity than milled peat.

In Closing

Mushroom compost is a soil additive rich in organic matter and nitrogen residue. Because of its high salt content, it is unsuitable for some salt-sensitive plants and as s starter medium but is brilliant for plants in general.

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