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32 APRIL 15, 2015 GOOD FRUIT GROWER www.goodfruit.com Think years, not weeks, when calculating nitrogen release from compost. by Melissa Hansen C ompost is revered as being one of the best things to add to your soil. But what does it do (or not do) and what is the best way to use it? Making compost is decomposition en masse, says Dr. Lynne Carpenter-Boggs, soil scientist and researcher in Washington State University's Department of Crop and Soil Sciences. It's a biological process that transforms raw materials called feedstocks—the organic materials that decompose—into a stabilized, soil-like material called compost. With the right blend of feedstocks, oxygen, water, and microbes, activity ramps up quickly. Heat and carbon dioxide are generated by the activities and respiration of the microorganisms. Legally, compost must reach 131°F and stay at that temperature for three to 15 days, according to Carpenter-Boggs. Once it reaches 130°F, the compost environment changes significantly and there's a change in the microorganisms that are active, she explained. "The 131°F is the magic number at which you know the compost has made the switch to the heat loving or thermophilic organisms. Many of the patho- genic organisms and weed seeds that we don't want die at 130°F." During the active phase of unturned composting sys- tems, temperatures climb quickly to around 130°F and then decline over time. During the fi nal or curing phase, another switch is made when the heat-loving organisms die off and benefi cial organisms recolonize the material. "Although we lose about half of the carbon that was in the initial material—which is why compost is so dark in color—most of the other nutrients are retained," she said during a Washington State Grape Society meeting. Because the fi nished compost reduces the mineraliza- tion rate of nitrogen, it is unlikely to burn crops or leach nitrogen below the root zone. Compost retains—and tends to concentrate—any heavy metals that were in the original feedstocks. "So if it came in on the feedstocks, it doesn't go anywhere in the composting process," she said. Carpenter-Boggs described fi nished compost as hav- ing complex humic acids, fi ne texture like good topsoil, no unpleasant odor or undecomposed material, and sta- bility in long-term storage. "Humus is valuable because it encourages microbial activity and holds soil particles together, reduces erosion, and improves soil structure," she said. An optimum composting environment has the following attributes: —Carbon-to-nitrogen ratio of 25-30:1 —Moisture content of 50 to 60 percent —Oxygen greater than 5 percent —pH 6.5 to 8 —Varied particle sizes (generally from 1/8 to 1/2 inch) Compost approved for certifi ed organic agriculture must be moderate to high quality and follow special National Organic Program regulations that include the following requirements: —Feedstock mix has a carbon-to-nitrogen ratio of 15-60:1 —Temperature must reach 131°F at least three days —All parts of pile must reach 131°F —Compost must be tested for salmonella, Escherichia coli, and heavy metals —No biosolids or prohibited materials used If any of the requirements aren't met, the material cannot be used as compost in organic agriculture. However, as long as no prohibited material is in the feedstock, it can be used as raw manure. Current rules require a 90-day interval between manure application and harvest of food crops that don't touch the soil (such as tree fruits), or a 120-day interval for food crops that do touch the soil. These rules are being revised due to the new Food Safety and Modernization Act. What's in your compost? Lab analysis is the only way to know what's in your compost. There are no legal requirements that compost be tested for nutrients, only certain human pathogens and heavy metals, so you may have to test the material yourself. No two composts are the same. The feedstock source and treatment can greatly impact the plant availability of nutrients, particularly nitrogen, says Kyle Bair of Soil Test Farm Consultants, Inc., in Moses Lake, Washington. For example, composted yard wastes will be vastly dif- ferent from composted fi sh by-products or composted manures. Soils and Weed Control Getting the most from COMPOST ONLINE How much nitrogen is in your compost? See application rates at soilplantlab. missouri.edu/soil/compost.aspx Finished compost has a fine texture, like good topsoil, and stability in long-term storage. MELISSA HANSEN/GOOD FRUIT GROWER