How to Produce Composted Substrate for Mushroom Cultivation

Substrate production is a complicated procedure of multiple aerobic fermentation stages (composting) and consists of two main phases: phase I (fermentation) and phase II (pasteurization – ripening). It occurs in special chambers equipped with modern equipment and under scientific supervision by agronomists. Creating a suitable substrate requires years of experience and proper equipment and space. There are many ways to produce substrate, some requiring special knowledge. However, we will analyze one commonly used method for substrate production for the common Agaricus mushroom.

It is crucial to select a protected substrate-preparation area of large size to fit the pile and allow the movement of raw materials. Since the pile will produce a very strong and unpleasant odor during fermentation, the facility should be far from residential areas, or measures should be taken to limit the spread of the emissions.

If poultry manure (or other nitrogen-rich substances) is used, the farmer should take all precautions to avoid any environmental leakage. Producers start the substrate production by collecting the straw or other bulk ingredients. They mainly prefer grain straw, corn cobs, or cotton straw combined in many cases with chicken or/and horse manure, alfalfa, or soy flour to increase nitrogen content. Producers may add gypsum (2.5-5% Ca, 40 lb per ton of dry ingredients) as a conditioning-flocculating agent if it is required. In most cases, the addition of nitrogen to the substrate is necessary. A good substrate usually has 1-2% of nitrogen and a pH 5-6.5 and a moisture content of 50-70% (depending on the mushroom variety and strain, the limits may deviate).

The next step is to chop and wet the straw. Nutrient-enriched water can encourage microbial activity, increase pile temperature, and soften fibers. On the other hand, excessive water and improper drainage must be avoided since anaerobic conditions may be created. During the wetting process, the mix of bulk ingredients should have humidity levels close to 68-72%. Producers create heaps of straw where the first zymosis begins. This procedure can last 2 to 14 days (more days are required if manure has been used). It is critical to remember that air, moisture (watering every 2-3 days), nitrogen, and carbohydrates should be available for the process to be completed successfully and for the compost to be produced. The compost ingredients in large-scale mushroom production facilities are mixed, wetted, and placed through a compost turner. The farmer must provide good aeration (use of forced aeration) and sufficient moisture (water). Usually, during fermentation, the mix is placed on a floor with openings (spigots, nozzles, etc.) that provide air (Forced-Aeration Composting or FAC). To define the successful compilation of Phase I, the substrate can withhold water, has a dark brown coloring and a strong ammonia smell, and is flexible.

Then it is time for the sterilization (initiation of phase II). This process is essential to eliminate other pathogenic microbes and give the mushrooms a clean head start to grow. Producers move the straw into pasteurization rooms and fill the pasteurizing tunnels, beginning the procedure. It is essential to sterilize or pasteurize before seeding the fungi spores. If the substrate is not sterilized, other unwanted fungi are almost guaranteed to grow inside it.

Moreover, this process is necessary to reduce the ammonia levels that, after fermentation, are in a concentration above 0.07%, making mushroom growth impossible. This can be executed by the beneficial microbial that can convert ammonia in solution and substances into proteins that the mushrooms can exploit. At the end of phase II, the farmer can test if the ammonia in the compost is within acceptable levels (ammonia testing kits are available in the market). Producers increase the temperature to 50-70 °C (122-158°F) for 8-12 hours until all unwanted pathogens are eliminated. Then, they let the straw cool down and mature for 2-7 days.

*It is also essential to sterilize the entire growing room, the beds, the bags, and all the media that will participate in the procedure.

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