Why compost on the farm?
Across ornamental nurseries, vegetable fields/greenhouses, and livestock operations large and small, use and production of compost is considered a best practice. Composting keeps materials out of the landfill, prevents water quality impacts resulting from unmanaged debris/manure piles, and provides a beneficial re-use of waste products. Compost can improve soil health by increasing organic matter, supporting nutrient cycling, increasing soil water holding capacity and aeration, and improving soil structure. The improvements in soil health can in turn benefit farm crops. In this article, I am sharing a summary of best practices to support farmers in producing good quality compost from their vegetative waste in piles and windrows.
How to compost
Compost is created through the work of microorganisms that break down organic materials. The microorganisms need oxygen, the right amount of moisture, the proper mix of carbon and nitrogen, particle sizes less than about 2 inches in diameter to feed on, and temperatures in the range that they can live (and work!). To create good quality compost, farmers must gather and mix the materials, monitor the conditions, and adjust as needed.
Mixing the compost materials
The ratio of Carbon (C) to Nitrogen (N) in the compost source materials will affect how well the composting process works. If the C:N ratio is outside of the recommended range, the feedstocks may not completely break down. If not thoroughly digested, then weed seeds, pathogens, and other contaminants may remain in the compost. To create good quality, well-digested compost, a C:N ratio between 25:1 and 40:1 is recommende
d. Certified organic producers creating compost from a mix of plant and animal feedstocks are required to stay within the 25:1 to 40:1 C:N ratio range. Particle sizes of source materials should be about 1/8-inch to 2 inches in diameter.
Getting the right balance of carbon to nitrogen
So, what tools can farmers use to get the C:N ratio right? Appendix A of the On-Farm Composting Handbook provides the percent N and the typical C:N ratio of selected feedstock materials. We can then calculate the % C by multiplying the % N by the C:N ratio. For example, vegetable produce has about 2.7% N and a C:N ratio of 19. That means the C would be at 51.3%. Non-legume hay has about 1.3% N and a C:N ratio of 32, so the C would be 41.6%. The table also includes the typical percent moisture for most of the materials listed.
Say you were using a mix of discarded vegetable produce and hay to make your compost. You can look up their individual C:N ratios, but how much of each should you add to your compost pile to get the right ratio overall? Cornell University has a C:N ratio calculator for mixes of up to three materials. Input your materials, the approximate wet weight that you plan to add of each, as well as their % C, % N, and % moisture from the data in Appendix A of the Handbook. For example, I entered this recipe:
- 20 lb of hay with 1.3% N, 41.6% C, and 9% moisture content; and
- 100 lb of vegetable produce with 2.7% N, 51.3% C, and 87% moisture.
The online calculator tells me this would give a mix with a C:N ratio of 24. If I increase my hay to 100 lbs, the ratio would change to 29. The revised mix is well within the good range for C:N.
Establishing the compost pile or windrow
Next, the compost feedstocks need to be mixed together into a pile or windrow where the microbes can digest the materials. The microbes will need a continuous supply of oxygen. Before setting up your pile, think about how you will keep the compost aerated. Farmers aerate their piles either by turning them periodically (windrow method) or by sending air through the pile (aerated static pile method).
Windrow method
Composting materials are usually placed in a pile or a windrow (i.e., a long, narrow pile). The size of the pile will be determined by the type of equipment available to turn the pile and the density of the pile materials. Dense materials will not allow as much air to penetrate the pile, so the pile will be smaller and narrower than they would be for porous materials. The size of the windrow could be 3 feet high by 10 feet wide for dense material, or up to 12 feet tall by 20 feet wide for porous material. The farmer turns and mixes the pile regularly, so that the pile stays oxygenated from inside to outside. Turning also helps to regulate temperatures within the pile and to reestablish pore spaces between compost particles.
Aerated static pile method
The aerated static pile method uses perforated pipes, instead of turning, to keep the pile supplied with oxygen. To establish the aerated pile, start with a layer of wood chips, chopped straw, or another bulky porous material. Run a perforated pipe through this base layer. Next, pile the compost materials on top, up to a height of 5 to 8 feet. Again here, the height of the pile will be determined by the equipment and feedstocks. The base layer should only run about 1/4 to 1/3 of the total pile width. The distance from the edge of the base layer to the edge of the pile should be about equal to the height of the pile. To provide an even distribution of air, piles should typically be no longer than 70 to 90 feet. Having the perforated pipe and base layer fully enclosed within the pile ensures that air drawn through the pipe is going through the pile. The pipe is used to either blow or suction air through the pile. The aerated static pile method usually produces finished compost more quickly than the windrow method.
Monitoring temperature and moisture
To keep the compost process going, the temperature and moisture will need to remain within optimal ranges.
Temperature
During the composting process, temperatures should pass through two stages: mesophilic and thermophilic. The mesophilic stage should initiate early and last 1-2 days. At this stage, temperatures will be around 90 to 110 degrees F. Microorganisms are starting the work of decomposition, raising the temperature as they consume starches, fats, sugars, and proteins. The microorganisms generate heat, and the pile becomes “active”. Temperatures rise into the 120-150 degree F range next, which is the thermophilic stage. Now the heat-loving or thermophilic bacteria get into action, breaking down the compost further. Above 131 degrees F, pathogens in the pile are destroyed. Next, when temperatures climb to 145 degrees, fly larvae and most weed seeds are destroyed. As long as there are materials in the pile to decompose and enough oxygen to support the microbes, the thermophilic stage will continue.
When temperatures decrease
If your compost pile begins to cool down, it likely means that either the oxygen or moisture levels are too low to support microbial activity. To introduce more oxygen, turn the pile (windrow method) or turn on the blowers/fans (aerated pile method). In turning the pile, materials on the outer layers of the pile are moved into the hotter thermophilic center of the pile, speeding up their decomposition. Turnings also help to destroy weed seeds and pathogens. A general guide is that a pile should be turned when its interior temperature drops below 120 degrees F. Use a thermometer with a 2- to 3-foot stem to measure temperatures at 50-foot intervals along the windrow.
When temperatures get too high
If temperatures get above 150 to 160 degrees F, even the heat-loving organisms begin to die. Composting then slows down or halts. If the pile is too dry and too hot, the pile may spontaneously combust. Monitoring, turning, and re-hydrating the pile as needed are key to preventing it from reaching critical temperature and moisture levels.
Moisture
Microbial organisms need adequate moisture to continue the composting process. Therefore, water lost through the heating of the pile may need to be replaced through re-wetting. The composting material should feel moist to the touch. However, if you can squeeze water out of a handful of the material, then it’s likely too wet. Water occupies pore spaces that would otherwise supply oxygen. If the water content is above 65-70%, then the oxygen-filled pore spaces are limited. Without oxygen, the pile can become dominated by anaerobic microbes. Anaerobes break down materials more slowly than aerobic microbes. Additionally, the anaerobes produce bad odors and compounds that can inhibit crop growth.
Determining when the compost is ready to use
Growers must determine when the compost is “mature” or ready to apply in their fields. Determining compost maturity is important because applying immature compost can hinder seed germination and plant growth, inhibit root respiration, and contribute to heavy metal phytotoxicity. A combination of compost recordkeeping and physical examination are the best on-farm methods. Sample recordkeeping forms are provided in Appendix D of the On-Farm Composting Handbook. First, check the recordbook to ensure the compost stayed in the thermophilic temperature range for long enough. A time of 15 days and 5 turnings is recommended. How does the compost smell? A bad odor may indicate a lack of oxygen. Have you tried turning the pile and adding moisture? If the pile has been turned and the moisture is in a good range, but the temperatures are still not rising, then active composting has likely completed. Keep monitoring the temperature. Once the temperature of the pile is about equal to the air temperature, the compost is ready for the curing step. For curing, let the compost rest undisturbed for about 30 days. This gives time for destruction of any remaining weed seeds or pathogens and allows beneficial microorganisms to colonize the compost. Once the compost is cured, it is ready for application.
Resources for more information
This article summarizes information presented in the Center for Environmental Farming Systems’ Composting on Organic Farms publication. The publication is part of an Organic Production series. The Center for Environmental Farming Systems is a cooperative effort between North Carolina State University, North Carolina A&T State University, and the North Carolina Department of Agriculture and Consumer Services.
Cooperative Extension’s On-Farm Composting Handbook, NRAES-54
Cornell University’s Co-Composter Planning Tool for calculating the space needed and cost analyses for different compost systems
Sarasota County’s “Learn More About Composting” information pages
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