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Making and Using Compost for Organic Farming

Last Updated: June 30, 2010

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eOrganic author:

Emily Marriott, University of Illinois

Introduction

Composting transforms raw organic wastes 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 the 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.

Compost provides many benefits as a soil amendment and source of organic matter by improving soil biological, chemical, and physical characteristics:

  • Increases microbial activity
  • Enhances plant disease suppression
  • Increases soil fertility
  • Increases cation exchange capacity
  • Improves soil structure in clayey soils
  • Improves water retention in sandy soils
  • Reduces bioavailability of heavy metals

Overview of the Composting Process

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. It is especially important to maintain adequate aeration during this phase of intense microbial activity because aerobic decomposition is most efficient and produces finished compost in the shortest amount of time. As decomposition slows, temperatures decrease to around 100°F and the "curing phase" begins. The compost can be stockpiled during this time.

Common methods of on-farm composting are 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 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.

Static compost piles with passive aeration tubes
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.

Profiles of compost windrows at a dairy in eastern Washington
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.

this farm-scale rotating drum is used at a Texas site
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.

How to Compost

A variety of 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, 2002), 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.

Compost and the National Organic Program

There are no restrictions on when compost can be applied in organic production, provided that the compost has been produced in accordance with the National Organic Program (NOP) final rule (United States Department of Agriculture [USDA], 2000). NOP Regulations state that compost must be made from plant and animal materials. Compost cannot contain:

Allowed materials include:

  • Plant and animal materials (and their ash)
  • "Natural" non-agricultural materials
  • Mined substances of low and high solubility
  • Synthetic materials accepted by the Organic Materials Review Institute (OMRI)

Composted materials must be produced through a process that (from section §205.203 of the NOP):

  • Establishes an initial C:N ratio of between 25:1 and 40:1
  • When using an in-vessel or static aerated pile system, maintains a temperature of between 131°F and 170°F for three days
  • When using a windrow composting system, maintains a temperature of between 131°F and 170°F for 15 days, during which period the materials must be turned a minimum of five times

Composts that don't meet the above production criteria can 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 (from section §205.203 of the NOP) 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

There are no restrictions on the application of uncomposted plant materials (see section §205.203 of the NOP).

Since the NOP composting rule is fairly prescriptive and received significant criticism upon release, the National Organic Standards Board (NOSB) formed a special task force to review the rule and develop some modified guidelines in the form of an addendum (NOSB, 2002). The guidance document allows for more flexibility in how compost piles are constructed and monitored. Assuming acceptable feedstocks are used, if the compost achieves 131°F for three days and the pile is mixed or managed to ensure that all of the mixture achieves this temperature, the compost is considered acceptable. Vermicompost (compost produced by the action of earthworms) is considered acceptable in the addendum as well, as long as:

  • It is produced from acceptable feedstocks.
  • Aerobic conditions are maintained.
  • Moisture content is maintained at 70-90%.
  • Composting occurs for at least 12 months outdoors or four months indoors (60 days for continuous flow systems).

The addendum does not address compost teas made from either thermophilic compost or vermicompost.

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. 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.

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.

Compost feedstocks may contain other contaminants that are not degraded in the composting process. 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.

Compost Quality

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).

Table 1. Qualities of compost for on-farm use and how to test. (after Cooperband, 2002a)
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 and Disease Suppression

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).

Compost and Soil Fertility

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 Application Rates

Compost should be considered a slow-release source of nitrogen. Most nitrogen is bound into organic forms during the composting process and thus is not immediately available 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.

This vegetable producer in Washington State built his own compost spreader from existing equipment.
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.

References and Citations

Additional Resources

 

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.

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