Composting manure transforms raw animal waste into a valuable soil amendment that is both stable and safe for gardening and agriculture. This biological process relies on microorganisms to break down complex organic materials, resulting in a dark, crumbly product called humus. The primary goals are to stabilize nitrogen and other nutrients, reduce the physical volume of the waste, and destroy potentially harmful pathogens and weed seeds. Manure must undergo this controlled decomposition because applying it raw can burn plants, introduce disease, or foster weed growth.
Selecting Appropriate Manure Sources
Manures from herbivores like cows, horses, chickens, and rabbits are generally suitable for composting because they typically contain fewer pathogens transferable to humans. These animal wastes are rich in nitrogen, providing the necessary material for microbial activity. However, even these sources carry a potential risk of contamination from persistent herbicides.
Herbicides such as aminopyralid, picloram, and clopyralid are designed to control broadleaf weeds in pastures. These chemicals can pass through the animal’s digestive tract and remain active in the manure, even after composting. If this contaminated compost is applied, it can damage or kill susceptible broadleaf plants like tomatoes, beans, and potatoes. It is essential to confirm the source of the manure and the herbicide history of the feed or bedding used by the livestock.
Manures from carnivores and omnivores, including dogs, cats, pigs, and human waste, should be avoided for home composting, especially for food gardens. These wastes pose a higher risk of carrying zoonotic parasites and bacteria. Cat feces may contain Toxoplasma gondii oocysts, which are highly resistant. Dog manure frequently contains roundworms and ascarids, which also present a human health hazard.
Balancing the Carbon-to-Nitrogen Ratio
Achieving the correct blend of materials is governed by the carbon-to-nitrogen (C:N) ratio. Microorganisms require carbon for energy and nitrogen for growth, and the ideal starting ratio for efficient composting is typically between 25:1 and 30:1. If the ratio is too high, decomposition slows significantly. If the ratio is too low, excess nitrogen is released as ammonia gas, resulting in unpleasant odors and a loss of valuable nutrients.
Raw manure is often high in nitrogen, with cow manure averaging around 20:1. Nitrogen-rich manure must be mixed with high-carbon “bulking agents” to raise the overall ratio toward the 25:1–30:1 range. Common carbon sources include wood chips (C:N ratio around 500:1), straw (C:N ratio between 40:1 and 100:1), and dry leaves (C:N ratio between 40:1 and 80:1).
A practical approach involves combining two parts of carbon-rich bedding material, such as straw, for every one part of raw manure. This mixing creates a porous structure, allowing air to circulate within the pile. Adequate porosity ensures that the composting microbes receive the oxygen necessary for aerobic decomposition, preventing the anaerobic conditions that cause foul odors.
Managing the Active Heating Phase for Safety
The active heating phase, known as the thermophilic stage, is when the compost pile reaches the high temperatures necessary to sanitize the material. Microbial activity generates heat, which must be sustained to destroy pathogens and weed seeds. To effectively reach and maintain these temperatures, the compost pile should be built to a minimum size, typically at least three feet cubed, to provide enough mass for insulation.
To ensure the destruction of most common pathogens, the internal temperature of the pile must reach a minimum of 131°F (55°C). This temperature should be maintained for at least three consecutive days in a static system, or for 15 days in a windrow system. Temperatures should not exceed 160°F (71°C) for prolonged periods, as excessive heat can kill the beneficial microbes responsible for decomposition.
The moisture content of the pile is ideally maintained between 40% and 65%, which should feel like a wrung-out sponge. If the pile is too dry, microbial activity stops; if it is too wet, oxygen is excluded, leading to anaerobic conditions.
The most important management practice is turning the pile, which reintroduces oxygen and ensures that all material is exposed to the sanitizing heat. A common turning schedule involves mixing the material whenever the temperature drops below 131°F or rises above 160°F. Monitoring the temperature with a long-stemmed thermometer inserted deep into the pile is the only reliable way to confirm that the sanitization standards are being met.
Curing and Applying Finished Manure Compost
The active heating phase transitions into the curing or maturation phase once the internal temperature stabilizes and no longer reheats after turning. This indicates that readily degradable organic matter has been consumed and the pile has cooled to near-ambient temperatures. The compost should appear dark, uniform, and crumbly, possessing an earthy smell rather than ammonia or sourness.
The curing phase is a period of stabilization where mesophilic microbes continue to break down resistant organic compounds. This stage converts remaining unstable organic acids into stable humus, which improves soil structure and nutrient retention. A curing period of two to four months after the active heating phase is recommended before the finished compost is used.
Once properly cured, manure compost can be safely applied as a top dressing, a soil amendment, or a mulch around plants. Because composting stabilizes the nitrogen, the finished product can be incorporated into vegetable gardens without the risk of burning plant roots. Finished compost is a soil conditioner that slowly releases nutrients over time, enhancing the overall fertility and health of the soil structure.