Managing for Better Compost
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Manure Management Technology Development Team East National Technology Support Center Managing for Better Compost June 2007 Manure Management Information Sheet Number 2 INTRODUCTION phosphorus (P), and potassium (K). The relative amounts of available carbon and nitrogen greatly Making consistently good compost from manure effect the composting process, so the takes skill and attention to detail. Experience carbon/nitrogen ratio [written as C:N] is used as will show you what works; understanding the a primary indicator of nutrient content. environmental factors that control the composting process will help you interpret what Carbon is a source of energy for microbe you see. growth. In aerobic decomposition, part of the carbon is released as CO2 while the rest is Keep in mind the definition of composting: combined with nitrogen in the bodies of “The controlled aerobic biological microorganisms. As a result, the carbon content decomposition of organic matter into a stable, of a compost pile is continuously decreasing. humus-like product.” Nitrogen is used for the synthesis of cellular Note: Composting mortality is a viable and safe way material, amino acids, and proteins and is to dispose of deadstock and slaughter waste, but is continuously recycled through the microbe’s beyond the scope of this document. Refer to NRCS bodies. Any nitrogen that is incorporated into and university websites for more information on the cells becomes available again when the large and small carcass composting. microorganism dies. This document discusses the factors that most An initial C:N ratio of 20:1 to 40:1 is influence the success of a composting operation: recommended for rapid composting. (See C:N ratio, aeration, moisture, particle Figure 1.) However, C:N ratios as low as 14:1 characteristics, and temperature, along with have worked for composting animal mortalities. some information on environmental concerns If carbon is present in excessive amounts connected to composting. It is intended to relative to nitrogen so that the C:N ratio is above supplement the information in Conservation the optimal range, the composting process Practice Standard 317, “Compost Facility.” slows. In this case, nitrogen availability is the limiting factor. With only limited nitrogen resources to use, microorganisms take longer to use the excess carbon. Several life cycles of organisms are required to reduce the C:N ratio to a more suitable level. If the C:N ratio is too low because the raw material is rich in nitrogen, the limiting nutrient will instead be carbon. If enough carbon is not available to provide energy for the microbes to incorporate nitrogen compounds into their cells, unstable ammonia will form. The calculated C:N ratio of a compost mix does Figure 1. From 'Compost Fundamentals', not always accurately reflect the amount of Washington State University nutrients available to the microorganisms. Microbial systems, especially bacteria, respond C:N RATIO C:N RATIO primarily to readily available nutrients. The mix Microorganisms require certain nutrients in large of raw materials not only needs the proper C:N amounts, including carbon (C), nitrogen (N), ratio, but the nutrients need to be in forms that Managing for Better Compost can be consumed. Material containing simple Waterlogged pores block uniform convective air sugars, such as fruit waste, decomposes rapidly movement. One of the reasons to mix and turn while woody material bound by decay-resistant compost is to redistribute moisture and maintain lignins is more difficult. Most nitrogen sources porosity. decay easily except for keratin (horns, hair, Diffusion through smaller pores and into the wool, and feathers). aqueous film surrounding compost particles is Maximize the availability of the carbon and essential to maintaining aerobic conditions for nitrogen in your compost feedstock by mixing active microbes. Water saturation dramatically thoroughly and grinding material into smaller reduces oxygen diffusion, which is 6000 to pieces if necessary. 10,000 times greater in air than in water. OXYGEN AND CARBON DIOXIDE: WATER is essential for the survival of compost Oxygen is obviously necessary to the survival of microbes. They require an aqueous environment aerobic organisms. If there is not enough in which to move, reproduce, and transport oxygen to sustain aerobes, anaerobes will nutrients, and water is the medium for most dominate the pile, slowing the composting chemical reactions. Aqueous does NOT mean process and producing unpleasant odors. A saturated. The ideal moisture content for minimum oxygen concentration of 5% is composting is a compromise between achieving required to maintain aerobic conditions. adequate moisture for the microbes and adequate Oxygen (O2) can be supplied to the pile using either forced or passive aeration. Regardless of the method, the amount of air being supplied does not necessarily reflect the amount of O2 actually reaching microorganisms. Bacteria live within a thin liquid film on the surface of compost particles. [See Figure 2] Since the diffusion of O2 through water is significantly slower than through air, the pile must be porous enough, and particle sizes small enough, to allow air to reach all the wetted surfaces. Figure 2. Properly wetted compost. [Cornell Waste Oxygen levels can be used as an indicator of Management Institute] active composting. As aerobic activity oxygen flow. Forty to 65% moisture is increases, O2 levels should drop. Measuring O2 levels is not quite as accurate as measuring recommended, making the compost the temperature, but adds useful information. It is a consistency of a wrung-out sponge (the “squeeze good way to show that stability has been test”). reached. As the pile reaches maturity and Another function of moisture is evaporative microbial activity slows, O2 levels rise. cooling. If the pile becomes too dry during the thermophilic stage, it may overheat and even Because carbon dioxide (CO2) is a product of aerobic respiration, it can also be used as an burn because of insufficient evaporative cooling. indicator of microbial activity. CO2 levels In hot, arid climates, moisture is especially should increase as microbial activity develops difficult to maintain because of excessive and decrease as the composting process evaporation losses. If the pile gets too dry, approaches maturity. composting can be halted prematurely. In wet or Oxygen transport -- Oxygen moves from the humid climates, piles may require additional open air into a pile by convection and diffusion. turning, adding more dry amendments, or Convection can be "forced" (driven by roofing to prevent them from getting too wet. mechanical means) or "natural" (caused by the The simplest way to correct a low moisture buoyancy of hot air). In a passive system, heat problem is to turn the pile right after rainfall, or can often be seen rising out of the pile, while to spray water on it during turning. Runoff and natural convection pulls cool oxygen rich air in leachate from the composting area, collected in a to replace it. 2 Managing for Better Compost pond or sump, should be returned to the rate of decomposition, and the ability of a pile to compost. maintain aerobic conditions. Porosity is a measure of air space and indicates the capacity for airflow. If pores are filled with water because of high moisture content, then resistance to airflow increases. Improve porosity by uniformly mixing materials, maintaining proper moisture, and including some larger particles or a bulking agent to increase pore size. Texture is the relative proportion of various particle sizes of a material and dictates the amount of surface area available to the Figure 3. Adding water while turning [Midwest microorganisms. The finer the texture, within Biosystems Inc.] reasonable limits (see porosity), the greater the surface area exposed to microbial activity. The inherent pH LEVELS of raw material do not necessarily influence the composting process Structure refers to the ability of a particle to because different microorganisms thrive at resist compaction and settling. It is a key factor different pH levels. The first several days of in establishing and maintaining porosity during active composting may show a drop in pH to as the composting process. A mix that has all of low as 4.0, due to formation of organic acids. the necessary components, but poor structure, The ideal range for microbial activity is between may not be able to sustain rapid composting. If 6.5 and 8.0. the pile settles and air spaces close up as the material decomposes, composting slows down. Composting continues at extremes, but the process slows. Acidic conditions are detrimental TEMPERATURE -- Temperature is a critical to aerobic microorganisms, particularly bacteria, indicator of microbial activity. Daily and slow the composting process. It will not temperature records will help detect deviation stop; a population of fungi eventually develops from normal patterns. Temperature should rise that can use the acidic compounds. As they are steadily as the microbial population develops. If consumed, the pH goes up. By the end of the it does not begin to rise within the first several composting process, pH generally stabilizes days, adjustments must be made in the compost between 7.5 and 8.0, regardless of the beginning mix. pH. Temperature is the one factor that most In most cases, pH does not need to be adjusted, operators will measure while composting, even because of the natural buffering capacity of if they pay little attention to anything else. organic matter. However, if the mix is too high • Lack of heat indicates that aerobic in nitrogen, a basic pH (> 8.5) promotes decomposition is not happening. Check for conversion of excess nitrogenous compounds to lack of aeration, inadequate carbon or ammonia. Ammonia formation further increases nitrogen content, low moisture, or low pH. It alkalinity and not only slows the rate of may be necessary to add a bulking agent to composting, but also promotes loss of nitrogen improve porosity, or dry amendments with through ammonia volatilization.