Composting the Mushroom Substrate.
Composting is the most critical aspect of mushroom cultivation. Ironically, there is some evidence showing that this is the stage of the process most prone to farmer error.
The mushroom growing process starts with the formulation of a species specific “substrate,” a mixture comprised of composted organic and mineral ingredients which is intended support the growth of the given type of mushroom.
The set of potential compost ingredients is extremely broad, and varies widely according to the location of the farm, the nature of surrounding industries, and the types of mushrooms grown.
Given the fundamental requirements and the spectrum of alternative ingredients, much of the material can be obtained from waste streams of other processes.
Composted substrates include: wheat and rice straw, corn cobs, hay, water hyacinth, decaying manure, and various other agricultural by-products including coffee husks and banana leaves.
Managing the Pile
Once the prepared ingredients are mixed, they start to decompose — a process through which the embedded nutrients are converted into forms that are useful to mushrooms.
The goal of composting is to produce a food source suited to the growth of a specific mushroom, to the exclusion of competing fungi and bacteria.
The proper proportions and amounts of water, oxygen, nitrogen, and carbohydrates must be present throughout the process to achieve optimal growing medium.
The preparation of mushroom substrate occurs in two steps. Phase I preparation usually occurs outdoors, although an enclosed building or a roofed structure may also be used.
The product is managed in a compost turning yard, which consists of a flat slab of concrete, asphalt, or a low-permeability earthen material. Compost-turning machines are used to mix and water the ingredients, while bucket loaders move the ingredients on the turning yard.
Phase I of the process begins on many mushroom farms with a “pre-wet” step, in which large heaps of a hay/straw mixture are showered with water.
The wetting step accelerates the growth and reproduction of microorganisms naturally present in the mixture, which leads to the production of heat. This serves to soften the hay and straw, making it more water absorbent. These heaps may be mixed together to produce a uniform starting compost. The pre-wet stage lasts from between 3-4 days to 12-15 days, depending on a range of operating conditions.
Following the pre-wet stage, the materials are arranged in a long pile over which nitrogen supplements and gypsum are spread. The pile, often referred to as a “rick” by farmers, is thoroughly mixed with a turning machine. Aerobic composting continues after the pile is wetted and formed.
The pile must be carefully erected and managed. Most compost piles are roughly five to seven feet wide, five to ten feet high, and as long as necessary or practical.
The rick must hold its shape, while remaining loose enough to allow for aerobic conditions throughout. Turning and watering are done at approximately two-day intervals.
Turning provides the opportunity to water and mix the ingredients, as well as to relocate the substrate from the cooler exterior to the warmer interior, and vice versa.
The aeration accomplished by turning is short-lived, so pile construction, structure, and contents are critical in promoting aerobic degradation.
The number of turnings and the time between turnings depends on the condition of the starting material and the time necessary for the compost to heat up.
Water addition is critical. Too much water will exclude oxygen by occupying pore spaces, and may lead to an unnecessary loss of nutrients due to leaching, while too little water can limit the growth of bacteria and fungi. As a general rule, most of the water is added when the pile is formed and at the time of first turning.
Thereafter, water is added only to adjust the moisture content. On the last turning of Phase I composting, water may be applied generously to carry sufficient water into Phase II.
Water, nutritive assets, microbial activity, and temperature are like links in the composting chain. When one factor is limiting, the efficacy of the process may be diminished.
One of the management issues that farms often face is the creation of odor during the composting. These odors, which constitute a significant negative externality, are generated if mixtures are improperly formulated, or if piles are poorly managed.
One way that farms can address this problem is through the use of aerated silos that force air into the mix.
These innovative silos employ air jets embedded in the floor to introduce oxygen to the substrate, and feature solid walls to ensure the even distribution of air throughout the structure.
Once a proper state of decomposition has been reached, the pile is transferred to a separate room, where it sits for 48 hours at 132oF.
Raising the air and temperature to 140oF initiates the pasteurization process, which lasts two hours. The pile is then gradually cooled over the next five days, or until it reaches a temperature of 85oF. Pasteurization uses far more energy than any other process during substrate preparation.
Pasteurization is conducted to kill any insects, nematodes, competing fungi, or other pests that may be present in the compost. The heating process also reduces ammonia levels by favoring the growth of thermophilic (heat loving) organisms that consume carbohydrates and nitrogen.
Caution:High ammonia levels can be lethal to mushroom spawn.1