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Weekly UpdatesMany scientists and growers have described their thoughts on mushroom composting through the years. We will describe our understanding of what goes into quality compost and how one may go about assessing the important details during Phase I and Phase II composting. We will also touch on what the mushroom uses for food and end with some cropping research conducted at Penn State.
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The objectives for Phase I and Phase II composting are to: 1) soften rigid plant materials; 2) increase the water holding capacity of the raw materials; 3) form complex carbohydrates to preserve carbon for the mushroom; 4) generate and subsequently convert ammonia to microbial protein; 5) create substrate with sufficient nutrients to grow a good crop of mushrooms while providing little or no nutrition for other fungi and competitor organisms. By accomplishing the above goals you will develop a substrate with adequate nutrients to insure rapid, healthy mushroom growth and yields, and at the same time provide little to no nutrients for other potential competitor organisms.
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Growers need to assure that these goals are being accomplished during and at the completion of the composting process. Describing and measuring compost characteristics is one way to maintain a successful composting process. This process involves sensory perception as well as chemical and physical assessment throughout the composting procedures. Sensory assessment includes touching, seeing and smelling the compost. The chemical assessment includes a laboratory analysis for pH, percent nitrogen, moisture, ash and ammonia. Presently, physical analysis of mushroom compost is confined to the research laboratories. However, with the development of a new technology for Municipal Waste Composting systems, commercial application of these analyses is becoming available to the grower. Thermal and spectroscopic analysis of compost may someday soon be as common as the nitrogen, ammonia and ash analysis you use now.
Is composting different with an aerated Phase I system? Dr. James Sinden once wrote "With complete aeration, composting will proceed more rapidly" and he continued with "the practical composting process should be limited to the minimum time necessary to produce a favorable substrate. Composting beyond that minimum merely destroys organic material usable by the mushroom without serving a purpose. Growers should make small tests to see whether they are composting longer than is necessary. By maintaining the minimum composting time, a grower should recover a maximum return per unit of ingredients purchased." That was written over 50 years ago, and accurately describes Forced Aerated Composting, a concept that was not brought to commercial fruition in the United States until the 1990s. His other comments remain accurate and provide good advice in these days when the cost of compost ingredients continues to rise.
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So what sensory assessments can growers use to assess mushroom compost? The most common is visual. Growers are often looking at the color of the compost to determine the maturity level at certain times during the Phase I process and prior to filling. The degree of compost maturity and breakdown may differ from farm to farm depending on the system used for Phase II. It is often more desirable to have more mature and denser compost in a bed system in order to achieve high dry weights needed for higher yields. However, less mature compost has a greater water holding capacity at spawning time and later, which is one advantage of a bulk system where longer, less mature compost seems more desirable. The yellow-green color of the compost also may indicate whether the compost was anaerobic at some time. In anaerobic compost, organic acids are formed that are detrimental to mushroom yield or microbial activity, and will encourage the growth of Trichoderma Green Mold disease.
Another visual observation includes accessing the steam coming out of the pre-wet piles, indicating microbial activity has started. Since the compost ingredients harbor many naturally occurring microbes, wetting and mixing ingredients provides the necessary microbial growth and reproduction requirements. Steam is the indicator that microbes are active and that they have sufficient food, adequate moisture, suitable temperatures and enough oxygen. If any of these growth requirements are limiting, so is the microbial activity. In bunker composting, the oxygen is controlled so managing the amount of air into the pile is critical. In general, more air is required during the early part of the process when there is more microbial activity, but as temperatures reach and exceed 150[degrees]F (70[degrees]C), chemical reactions take over and less air or oxygen is required. This spike and following decline in oxygen requirements is repeated immediately after turning with the addition of supplements as well.
Growers should visually assess microbial activity (fire-fang) in traditional Phase I ricks. The pausing and backing up of a turner to view the cross-section of a pile and to assess this layer of fire-fang should be a common practice on a compost wharf. Growers need to note how close it is to the edge or if it is excessive. Both observations are good indicators that the compost pile has tight sides and optimum temperatures. With bunker compost, growers should access the amount of microbial activity present prior to filling a room, tray or tunnel. Because so much of the compost is at high temperatures there's a need for microbes to be re-inoculated throughout the compost before Phase II is started. This microbial activity helps to kick start the Phase II process, which is why some growers rick or pile up the compost for a short time prior to filling and Phase II.
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Additional visual details include looking and checking to see how well supplements and water are uniformly distributed with the bulk materials, a very critical detail for bunker composting where there is less mechanical turning and mixing of the compost. For ricks, how high and wide are piles and is the width reduced for the later turns so when the compost becomes more dense it is easier for air to reach the center of the pile. The uniformity of filling a bunker regardless of the systems or equipment used should be carefully accessed.
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Another important sensory assessment for making good compost is touch. Feeling and squeezing the compost throughout the process is so critical to determine proper moisture levels, uniformity of the moisture and if the moisture is "inbound" into the straw. Tearing of the compost may indicate its maturity or can sometimes provide an indication of pile/bunker temperatures. During Phase I at higher temperatures (chemical reactions), the straw tends to maintain the structure compared to lower temperatures (microbial decomposition) where the straw would be softer and more friable. Just look at the difference in straw strength during Phase I and after Phase II.
The feeling of greasiness in compost is caused by the organic colloids found in compost. A colloid is a material that is a liquid but has a solid firmly suspended in it such as butter or gelatin. Organic colloids left alone would accumulate during the composting process on the surface of the straw and because they are impervious to air would choke off oxygen to the microorganisms. Therefore gypsum, a salt, coagulates or flocculates these colloids and transforms them into stable protein matter. It has also been suggested that gypsum lowers the pH of the compost, which plays a role in the formation of ammonia or ammonium compounds.
These chemicals bring us to the last important sensory assessment tool, our nose. Our sense of smell is obviously important during the Phase I and Phase II composting process. The smell of ammonia is one of the first clues that the composting process has started and is properly proceeding. As the indigenous microbes in the bulk ingredients and supplements begin to grow and reproduce, the pile temperatures begin to rise. First the mesophilic microbes grow, and they feed on the simple carbohydrates in the compost forming organic acids, lowering the pH. But as the temperatures start to reach 110-115[degrees] F then the thermophiles (heat-loving) organisms takeover and give off more heat. At this time, much of the nitrogen is ammonified, and ammonia is given off by many of the urea loving bacteria, which starts to raise the pH.
As mentioned earlier, once the temperature in the compost rises above 150[degrees]F (65[degrees]C) the biological activity changes over to chemical activity. At high temperatures, reactions like carmelization and Mailliard browning reactions occur. Carmelization is the elimination of water from carbohydrates and changes the long chain carbohydrates into shorter chain carbohydrates that the mushroom prefers and other microbes do not. Like making candy, the higher temperature mixed with ammonia speeds the process of carmelization and the darkening of the compost. Some growers can even detect the smell of burnt sugar that indicates this reaction is occurring.
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In addition to carmelization, the reaction of carbonation is occurring. This reaction is started by oxidation, a way in which both lignin and cellulose are partially or wholly changed back into water and carbon dioxide. Oxidation releases heat that could start combustion, but if combustion is limited by lack of oxygen and moisture so that it doesn't reach a combustion level, there is still a separation of the hydrogen and oxygen from carbon as water and the straw gets darker and darker. One of the few organisms adapted to digesting the carbon from these intermediate dark compounds is our mushroom. If carbonation is left to continue, charcoal is formed, whereas in plant materials a different type of decomposition continues in which the end product is coal. Coal is formed when peat is altered physically and chemically. This process is called "coalification." During coalification, peat undergoes several changes as a result of bacterial decay, compaction, heat and time. Did you ever see mushrooms grown on just peat moss?
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