Substrate is the growing medium that provides the nutrients needed for mushrooms to grow. Mushroom composting is designed to change the nutrients e.g. ammonia bearing compounds found in the initial ingredients such as cotton seed meal into compounds that are available to and selective for mushrooms. Composting produces a food source most suited for the growth of the mushroom to the exclusion of competing fungi and bacteria. Since the mushroom mycelium does not have a chlorophyll system it must have a growing media that supplies the sugars and carbohydrates it needs as well as the correct moisture, oxygen, and nitrogen throughout the process or the nutrients will be inadequate to support the future mushroom crop. Mushroom farmers use various recipes when formulating mushroom compost based on the availability and cost of ingredients in their area and the individual characteristics of their farms. Generally speaking, however, mushroom farmers in the United States use straw based stable bedding containing horse manure. A variety of other ingredients are added in order to develop the nutrition and structure of the developing compost. The preparation of mushroom growing media recycles vast quantities of agricultural nutrients, especially stable bedding from horse farms and race tracks and/or poultry manure. By utilizing these nutrient-rich materials productively, the mushroom farming community performs a service to the community in terms of minimizing waste disposal. At Mountain Meadow we only utilize stable straw, cotton seed hulls, cotton seed meal, along with canola fines and gypsum to make our growing substrate.
The preparation of mushroom compost occurs in two steps referred as Phase I and Phase II composting. Phase I compost preparation occurs outdoors on a cement slab, referred to as a wharf. A specially designed compost turning machine is used to mix and water the ingredients, and a front-end loader is necessary to move the ingredients on the wharf. Mountain Meadow has two such loaders costing about $200,000+ each.
Phase I composting begins on all mushroom farms with a preliminary or “pre-wet” step. Piles of the straw are wetted down by pumping reclaimed wharf water on them. This wetting step initiates the heating process, a result of the growth and reproduction of microorganisms naturally present in the bedding straw. This serves to soften the hay or straw and makes it more water absorbent. These piles are mixed several times to produce uniform starting compost. The pre-wet stage may last from 5 to 15 days depending on the season and condition of the straw. At this point the manure in the straw has been consumed by many strains of bacteria that were naturally present in the bedding straw.
Following the pre-wet stage, the materials are arranged in a long pile over which are spread nitrogen supplements and gypsum, and the pile or rick is wetted again and thoroughly mixed with a turning machine. Aerobic composting continues after the pile is wetted and formed.
The compost rick must be carefully created and managed. Most compost ricks are roughly 5 to 6 feet wide, 5 to 6 feet high, and as long as necessary. The rick must hold its shape, but be loose enough to allow for aerobic conditions throughout. Turning and watering are done at approximately 2-day intervals based on internal temperature profiles. We strive to reach temperatures of 170o to 180o F in the ricks. Turning provides the opportunity to water and mix the ingredients, as well as to relocate the compost 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 depend on the condition of the starting material, the season and the time necessary for the compost to heat up.
Mountain Meadow is a technological leader in aeration of compost ricks. This experimental system was installed approximately 5 years ago and has eliminated all anaerobic odors in the rick. We are currently designing a similar system for our pre-wet piles.
Water addition is critical. Too much water will exclude oxygen by occupying the pore space and may lead to unnecessary loss of nutrients due to leaching. 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, and 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 value, microbial activity and temperature are like links in a chain. When one condition is limiting, the whole chain is often affected.
Phase I composting lasts from 16 to 28 days, depending on the nature of the material at the start, its characteristics at each turn, and the season. At the end of Phase I, the compost should have: a) a brown color; b) short, pliable straws; c) a moisture content from 68 to 74 percent; and d) a slight odor of ammonia. When these conditions have been reached, Phase I composting is completed.