Carrington Research Extension Center
North Dakota State University
Many producers ask about composting as a method of treating manure, particularly manure scraped from beef feedlots. Proper composting requires a basic understanding of the process and some management, but it has several advantages when utilizing manure nutrients for fertilizer.
Composting is the aerobic (requires oxygen) decomposition of organic matter by certain microorganisms. These microbes consume oxygen and use nutrients including carbon, nitrogen, phosphorus, and potassium as they feed on the organic matter. They produce heat, carbon dioxide, and water vapor. The resulting composted manure is a humus-like organic material, fine-textured, low-moisture, and with a non-offensive earthy odor. If high enough temperatures have been reached during the composting process, pathogens and weed seeds have been killed. Because of the carbon dioxide and water vapor that escape during the process, the resulting compost can be approximately half of the volume and weight of the original material, a great advantage in hauling and spreading for land application.
Management of the composting process involves maintaining proper temperatures and oxygen levels in the composting material. This is best accomplished with feedlot manure by placing the manure in windrows. The windrows can be turned and mixed periodically to maintain the oxygen levels and temperature for proper composting. The windrows may be 3 to 6 feet high and 10 to 20 feet wide, depending mostly on the type of machinery to be used in turning them. The width of the windrow must also allow for air movement into the material to introduce oxygen. Bucket loaders can be used to turn the windrows. If the size of operation justifies, compost windrow turners are available from several manufacturers. They are usually powered by a farm tractor.
Composting microorganisms operate best at temperatures of 110 to 150 degrees F, 130 to 140 degrees being optimum. These temperatures are soon reached in the middle of the compost windrow. When the temperature begins to fall, it indicates the microbes are running out of oxygen and the windrow should be turned to introduce more oxygen. If the temperature climbs much above 140 degrees, the microbes will become dormant or die, so the windrow should be turned and cooled. The composting process is done when the material does not reheat after turning. Then a curing process begins during which the composting processes are still occurring but at a much slower pace. Curing can be considered complete when the temperature approaches near ambient conditions. This will take about one month. Composting manure may take from 2 to 8 months, depending on temperature, moisture, and how closely it is managed.
One drawback of composting manure to be used for fertilizer is the potential for loss of nitrogen during composting. Since manure already has a nitrogen:phosphorus ratio lower than most crops require, this loss of nitrogen aggravates the situation. However, this problem can be alleviated by attention to the carbon:nitrogen (C:N) ratio.
All organisms use 25 times as much carbon as nitrogen by weight. Most manures have a lower C:N ratio of 10:1 to 20:1. Thus the composting microorganisms use all the carbon before using all the nitrogen. The remaining nitrogen can be changed to the ammonia or nitrous oxide form and be lost to the atmosphere. Fortunately, small grain straw has a C:N ratio averaging about 80:1. Mixing a certain percentage of straw with manure, as would happen when straw is used for bedding, raises the C:N ratio of the composting mixture to the desired ratio of 25:1 to 30:1. Most of the nitrogen is then conserved.
Composted manure has many advantages over conventional manure handling. Compost is a finer-textured more uniform material which is easier to apply uniformly. It is easier to apply in no till situations. It is virtually odorless and may be free of pathogens and viable weed seeds. The weight and volume is much reduced so it is more economical to haul and spread. The nutrient value is largely preserved if the C:N ratio is near 25:1. However, the nitrogen will be much more slowly available compared to regular manure. As the technology of composting becomes more widely known, it will likely see increased use.
(Some material for this paper taken from NRAES-54, “On-Farm Composting Handbook”)
Dean and Director for Agricultural Affairs
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