Emily Marriott, University of Illinois at Urbana-Champaign
Ed Zaborski, University of Illinois at Urbana-Champaign
Composting transforms raw organic residues into humus-like material through the activity of soil microorganisms. Mature compost stores well and is biologically stable, free of unpleasant odors, and easier to handle and less bulky than raw organic wastes. In agronomic and horticultural operations, compost can be used as a soil amendment, seed starter, mulch, container mix ingredient, or natural fertilizer, depending on its characteristics. Composting can also reduce or eliminate weed seeds and plant pathogens in organic residues.
Compost provides many benefits as a soil amendment and a source of organic matter by improving soil biological, chemical, and physical characteristics:
Microorganisms drive the composting process, so creating an optimal environment for microbial activity is crucial for successful and efficient composting. Assembling an appropriate mix of organic residues or feedstocks and maintaining adequate moisture and oxygen levels are all necessary.
As soon as feedstocks are compiled, the composting process begins. As microorganisms begin to decompose the organic materials, the compost pile heats up and the active phase of composting begins. During this phase of rapid decomposition, temperatures in the pile increase to 130–150°F and may remain elevated for several weeks. Maintaining adequate aeration during this phase of intense microbial activity is especially important because aerobic decomposition is most efficient and produces finished compost in the shortest amount of time. As readily available organic matter is consumed and decomposition slows, temperatures in the compost pile decrease to around 100°F and the curing phase begins. At this stage, the compost can be stockpiled.
Common methods of on-farm composting include static piles, windrows (elongated piles), and in-vessel (enclosed) composting. Static piles are compost piles that are not turned. To meet National Organic Program requirements, static pile systems must be aerated to sustain microbial activity and adequate temperatures. To that end, perforated pipe is installed at the base of the pile and in some cases fans or blowers are used to force air through the pile.
Figure 1. Static compost piles with passive aeration tubes. Photo credit: Robert Rynk, Compost Education and Resources for Western Agriculture project, Washington State University.
Windrows, or enlongated piles of compost feedstocks, are turned or mixed regularly to aerate the pile and to reestablish pore space.
Figure 2. Profiles of compost windrows at a dairy in eastern Washington. Photo credit: David Granatstein, Compost Education and Resources for Western Agriculture project, Washington State University.
Figure 3. An example of in-vessel composting. This farm-scale rotating drum is used at a Texas site. Photo credit: Robert Rynk, Compost Education and Resources for Western Agriculture project, Washington State University.
Several comprehensive resources providing detailed explanations of the composting process and specific information on how to make compost are available; examples include The Art and Science of Composting (Cooperband, 2002a), Composting on Organic Farms (Baldwin and Greenfield, 2009), and On-Farm Composting Handbook (Rynk, 1992).
Large-scale composting is regulated in most states. Check with your state government to ensure compliance with composting regulations.
The use of composted plant and animal materials to maintain or improve soil organic matter is supported by the National Organic Program (NOP) final rule (United States Department of Agriculture [USDA], 2000):
The producer must manage plant and animal materials to maintain or improve soil organic matter content in a manner that does not contribute to contamination of crops, soil, or water by plant nutrients, pathogenic organisms, heavy metals, or residues of prohibited substances.
~ 7 CFR 205.203(c)
The composition, production, and use of compost in organic production systems is regulated by the NOP final rule; the NOP provided clarification of these regulations in guidance on the allowance of green waste in organic production systems (NOP, 2010a) and in draft guidance on compost and vermicompost in organic crop production (NOP, 2010b). In addition, the NOP provided guidance on uncomposted, processed animal manures in organic crop production (NOP, 2010c).
According to NOP's guidance on the allowance of green waste in organic production systems (NOP, 2010a) and draft guidance on compost and vermicompost in organic crop production (NOP, 2010b), approved feedstocks for compost include:
NOP regulation states that compost that is produced with prohibited feedstocks (urea, recycled wallboard, or sewage sludge, for example) is prohibited, and it does not permit the use of compost that contains synthetic substances that are not on the National List of synthetic substances allowed for use in organic crop production (see Can I Use this Input on My Organic Farm?). However, recognizing that background levels of pesticides are present in the environment (referred to as unavoidable residual environmental contamination—UREC—in the regulations) and may be present in organic production systems, NOP regulation does not mandate zero tolerance for synthetic pesticide residues in inputs, such as compost. According to NOP guidance,
Green waste and green waste compost that is produced from approved feedstocks, such as, non-organic crop residues or lawn clippings may contain pesticide residues. Provided that the green waste and green waste compost (i) is not subject to any direct application or use of prohibited substances (i.e. synthetic pesticides) during the composting process, and (ii) that any residual pesticide levels do not contribute to the contamination of crops, soil or water, the compost is acceptable for use in organic production.
~ NOP, 2010a
What constitutes "contamination of crops, soil or water"? The NOP final rule states (USDA, 2000, 7 CFR 205.671) "When residue testing detects prohibited substances at levels that are greater than 5 percent of the Environmental Protection Agency's tolerance for the specific residue detected or unavoidable residual environmental contamination, the agricultural product must not be sold, labeled, or represented as organically produced." NOP is thus far silent on what constitutes contamination of soil or water.
Compost feedstocks may contain synthetic pesticide contaminants that are not degraded in the composting process, and can contribute to crop, soil, or water contamination. This was the case for the herbicide clopyralid, which was used on turfgrass as well as in agriculture. It passes through animals in the urine, and therefore if they eat forage with clopyralid residues, the herbicide ends up in the bedding and potentially in the compost. Similarly, clopyralid can contaminate compost made from clippings from treated lawns. The uses of this herbicide have been restricted to avoid this problem, but it is advisable to ask the compost vendor or the provider of raw feedstock materials about such potential contaminants. For more information, see the Washington State University Puyallup Research Center publications on clopyralid in compost.
The source of all compost feedstock materials should be known to ensure that they are allowed for use in organic production. Knowing the feeding practices used for manure sources and having the manure tested can also provide information about possible antibiotic and heavy metal contamination. The use of compost containing these contaminants is not permitted in organic crop production; however, the organic rule does not require that manures come from organic livestock farms to be used in organic compost production.
The use of broiler litter as a feedstock for compost production poses some additional concerns. Arsenic is a component of some feed medications or growth promoters used in commercial broiler operations. The majority of arsenic consumed by poultry is excreted and incorporated into the litter, leading to the potential for build-up in the soil and leaching from compost piles into lakes and streams. For more information, consult the ATTRA publication, Arsenic in Poultry Litter: Organic Regulations, by Bellows (2005).
Increasing use of copper in broiler and hog operations may result in manures with high concentrations of copper. Copper foot baths are also common in cattle production. While copper is a necessary plant nutrient, it can become toxic in very high concentrations. Sustained use of compost from these sources could contribute to copper build-up in the soil in the long-term, especially in operations that rely on copper as a pesticide.
The NOP regulations refer to production methods for compost in the context of managing plant and animal materials to maintain and improve soil organic matter content:
The producer must manage plant and animal materials to maintain or improve soil organic matter content in a manner that does not contribute to contamination of crops, soil, or water by plant nutrients, pathogenic organisms, heavy metals, or residues of prohibited substances. Animal and plant materials include:
(2) Composted plant and animal materials produced though a process that:
(i) Established an initial C:N ratio of between 25:1 and 40:1; and
(ii) Maintained a temperature of between 131°F and 170°F for 3 days using an in-vessel or static aerated pile system; or
(iii) Maintained a temperature of between 131°F and 170°F for 15 days using a windrow composting system, during which period, the materials must be turned a minimum of five times.
~ 7 CFR 205.203(c)(2), USDA, 2000
The NOP's draft guidance on compost and vermicompost in organic crop production (NOP, 2010b) identifies these processes as examples of methods for producing acceptable composts, and states that:
An example of another acceptable composting method is when:
a. Compost is made from allowed feedstock materials (either nonsynthetic substances not
prohibited at §205.602, or synthetics approved for use as plant or soil amendments), and
b. The compost pile is mixed or managed to ensure that all of the feedstock heats to the minimum of 131°F (55°C) for a minimum of three days. The monitoring of the above parameters must be documented in the Organic System Plan in accordance with §205.203(c) and submitted by the producer and verified during the site visit.
~ NOP, 2010b
NOP compost requirements can also be met by vermicompost (compost produced by the action of earthworms), so long as:
a. It is made from allowed feedstock materials (either nonsynthetic substances not prohibited at §205.602, or synthetics approved for use as plant or soil amendments);
b. Aerobicity is maintained by regular additions of thin layers of organic matter at 1–3 day intervals;
c. Moisture is maintained at 70–90%; and
d. The duration of vermicomposting is at 6–12 months for outdoor windrows, 2–4 months for indoor container systems, 2–4 months for angled wedge systems, or 30–60 days for continuous flow reactors.
~ NOP, 2010b
Compost production practices, including the type and source of all feedstock materials, temperature monitoring logs by date, and practices used to achieve uniform elevated temperatures, should be described in the organic system plan (OSP).
Compost made in accordance with the above production criteria may be applied in organic production systems without restriction on the time interval between application and crop harvest.
Composts that don't meet the above production criteria may still be used in organic farming. However, if they contain animal manure, they must be applied to agricultural land in accordance with NOP regulations for manure, which state that raw animal manure must be composted unless at least one of the following conditions is satisfied:
- Applied to land used for a crop not intended for human consumption
- Incorporated into the soil not less than 120 days prior to the harvest of a product whose edible portion has direct contact with the soil surface or soil particles
- Incorporated into the soil not less than 90 days prior to the harvest of a product whose edible portion does not have direct contact with the soil surface or soil particles
~ 7 CFR 205.203(c)(1), USDA, 2000
Compost quality varies depending on the raw organic materials (feedstocks), the composting process used, and the state of biological activity. Before using compost as a soil amendment, it is a good idea to evaluate its quality by determining moisture content, organic matter content, C:N ratio, and pH (Table 1).
|Quality||Optimum||How to test|
|Source of organic matter||Should have a good organic matter content (40-60%)||Have organic matter tested by a soil lab|
|Source of nitrogen||10–15:1 C:N ratio||Have C:N ratio tested by a soil lab|
|Neutral pH||6–8||Use soil pH kit at home or have pH tested by a soil lab|
|Low soluble salts||If compost will be spread in the fall, no test necessary||N/A|
|If compost will be spread before planting, levels should be below 10 dS||Have soluble salts tested by a soil lab|
|No phytotoxic compounds||Good seed germination (>85%)||Plant 10 seeds in a small pot|
|Weed-free||No or few weed seeds||Moisten compost and watch for weed seedling growth|
Compost can be effective at controlling some soil-borne diseases, particularly root-rot diseases. By providing a favorable environment and food source, compost encourages the growth of microorganisms that compete with, parasitize, or produce natural antibiotics against plant pathogens. Additionally, increased plant vigor due to compost application can increase resistance to plant pathogens. For more information see the chapter on Compost and Disease Suppression in the ATTRA publication Sustainable Management of Soil-Borne Plant Diseases by Sullivan (2004). See a related article to learn how composting can reduce or eliminate weed seeds and plant pathogens in crop residues and other organic feedstocks.
Generally, compost can be considered more as a soil conditioner than as a fertilizer substitute because it improves plant productivity primarily by improving physical and biological soil properties and increasing soil organic matter, rather than by directly supplying significant amounts of plant-available nutrients. By increasing soil organic matter content, which fuels microbial activity and nutrient cycling, compost applications will increase overall soil fertility. Over subsequent growing seasons, the nitrogen applied in compost will become plant-available.
Compost should be considered a slow-release source of nitrogen. Most nitrogen remaining after completion of the composting process is bound into organic forms and thus not available immediately for plant uptake. Compost routinely applied at rates high enough to meet immediate crop N requirements will almost always result in excess P and K application. Excess P can result in surface water pollution (and potentially threaten organic certification). In some cases, excess K can upset crop nutrition balance.
Compost application rates can be calculated using fertilizer recommendations from soil tests, compost nutrient analysis, and methods similar to those used to determine manure application rates. When using this method, nutrient availability in compost must also be taken into account. General guidelines suggest that 10 to 25% of compost N will be plant-available during the first year of application. Estimates for P and K availability in the first year are higher, 40% and 60% respectively. It is important to keep in mind that these are only estimates and actual availability will depend on the nature of the compost and—for N especially—conditions during the growing season that affect N mineralization. Composting on the Organic Farm, by Baldwin and Greenfield (2009), provides detailed instructions for calculating application rates.
Figure 4. This vegetable producer in Washington State built his own compost spreader from existing equipment. Photo credit: David Granatstein, Compost Education and Resources for Western Agriculture project, Washington State University.
This is an eOrganic article and was reviewed for compliance with National Organic Program regulations by members of the eOrganic community. Always check with your organic certification agency before adopting new practices or using new materials. For more information, refer to eOrganic's articles on organic certification.