Odor and Nutrient Management Newsletter

Spring 2000

Composting saves time, money, and nutrients

by Tom Glanville, Department of Agricultural and Biosystems Engineering

photoFor poultry and livestock producers, the daily task of animal carcass disposal poses constant problems. In the summer heat, prompt disposal is essential to avoid odors and biosecurity hazards. During the winter, time becomes less of a concern, but frozen ground makes on-farm burial difficult at best. Meanwhile, the valuable service provided by the rendering industry continues to become more scarce and expensive. In some regions, livestock operations no longer have access to rendering services. Those that do pay as much as $50 per week for services that used to be nearly free.

For some poultry and livestock producers, composting may provide the solution to animal disposal. Composting is a year-round on-farm carcass management option that greatly reduces weather-related concerns and dependence on availability of rendering services.

If you raise poultry or livestock, you already understand the basics of composting because bacteria and fungi that break down organic matter have the same basic needs as mammals and birds. If kept at a comfortable temperature and given suitable amounts of water, food, and oxygen, these microbes reproduce at a remarkable rate, decomposing small-animal carcasses in as little as 4–6 weeks. The primary end products are water, carbon dioxide, and a heat-treated humus-like product that adds organic matter and nutrients to the soil.

Composting can be done successfully on nearly any scale if you pay attention to the basic needs of the microbes: moisture, food, oxygen, and temperature.

photoMoisture. Moisture is the most critical factor in composting. Bacteria need water to help dissolve organic matter and transport vital nutrients through their cell walls. At moisture content below 35 percent, bacteria starve because their food source has literally dried up. In contrast, too much water can prevent the compost pile from "breathing." Moisture content above 60–70 percent causes pore spaces within compost to become filled with water. These spaces impede the movement of oxygen into the pile and the release of ammonia and carbon dioxide that are potentially toxic to bacteria. The good news is that you do not need a laboratory analysis to tell you when moisture levels are okay. Compost operators soon learn to judge moisture content by look and feel. Compost needs to be slightly damp, but if you can squeeze water out of a handful of compost, it is too wet and should be mixed with drier materials to keep bacteria healthy and productive.

Food. Like many plants and animals, bacteria need a balanced diet containing 20 to 30 times as much carbon as nitrogen. Carbon-to-nitrogen ratios (C:N) greater than 30:1 lack sufficient nitrogen to support rapid decay. C:N ratios below 20:1, however, contain more nitrogen than needed, which can lead to increased ammonia odor when the compost pile is turned. But as with moisture, compost operators soon learn to recognize these symptoms and correct C:N imbalances. Addition of high-carbon materials, such as sawdust, can raise the C:N ratio if needed. If decomposition is extremely slow and moisture levels seem adequate, low nitrogen is a likely cause. In this situation, addition of nitrogen-bearing organic material can help lower the C:N ratio and boost decomposition rates.

Oxygen.. Given sufficient oxygen, the bulk of carcass decomposition is carried out by aerobic bacteria. aAerobic bacteria produce more heat and fewer odor-causing by-products than their anaerobic counterparts. Sophisticated high-rate industrial composting operations are kept fully aerobic by blowing air through the pile or by frequently turning the compost. On-farm composting operations typically do not go to these extremes, however, because mechanical aeration equipment is costly and slower decomposition rates are not of great concern. Low-maintenance, passively aerated composting operations supply oxygen to the outer layers of the compost pile through natural diffusion of air into the pile. Covering carcasses with coarse sawdust or wood chips provides numerous large pore spaces for gas movement. Combined with proper sizing of the pile and occasional turning, this strategy maintains sufficiently aerobic conditions although portions of passively aerated piles may become anaerobic from time to time.

Temperature. Heat-loving microbes called thermophiles do most of the composting work. They produce heat as they degrade organic matter, causing temperature within the compost pile to reach 120–150F. In addition to keeping thermophiles working at top speed, these temperatures improve the safety and value of composted organics by killing disease-causing bacteria and weed seeds. Keeping the composting environment warm during cold weather is mainly a matter of constructing compost piles large enough to retain heat. Research conducted by Iowa State University has demonstrated that, even during extended periods of subfreezing weather, internal temperatures within unheated poultry composting bins (8 feet in width by 6 feet in depth by 5 feet in height) were in the 120–140F range. Occasionally, internal temperatures become too hot. At temperatures above 160F, even thermophilic bacteria start to die. Periodic monitoring with a long-stemmed compost thermometer and occasional turning if temperatures are too high can reduce internal temperatures when needed.

For more information on how to plan and operate an animal mortality composting operation, contact your ISU county extension office for a copy of Leopold Center for Sustainable Agriculture publication SA-8, Composting dead livestock: A new solution to an old problem. The National Pork Producers Council also offers an excellent videotape and guidelines for swine carcass composting. The On-Farm Composting Handbook, published by the Natural Resource, Agriculture, and Engineering Service (Cooperative Extension, 152 Riley-Robb Hall, Ithaca, New York 14853–5701, phone (607) 255-7654, E-mail: NRAES@cornell.edu) is an excellent resource on composting fundamentals.

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Iowa Manure Matters: Odor and Nutrient Management is published by Iowa State University Extension, with funding support from the USDA Natural Resource Conservation Service through Cooperative Agreement No. 74-6114-8-22. To subscribe or change the address of a current subscription, write to Angela Rieck-Hinz, 2104 Agronomy Hall, Iowa State University, Ames, Iowa, 50011-1010 or call 515-294-9590, fax 515-294-9985 or email: amrieck@iastate.edu. Please indicate you are inquiring about the Odor and Nutrient Management Newsletter. The newsletter's coordinators are Angela Rieck-Hinz, extension program specialist, Department of Agronomy, Wendy Powers, environmental extension specialist, Department of Animal Science, and Robert Burns, Department of Agricultural and Biosystems Engineering; the editor is Jean McGuire, the subscription manager is Rachel Klein, the production designer is Beth Kroeschell, and the web page designer is Liisa Jarvinen.

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