Manure is a valuable natural resource, but its raw form presents challenges for use in gardens and agriculture. Composting transforms this raw material, which contains unstable compounds and potential hazards, into a stable, safe, and highly beneficial soil amendment. This transformation relies on controlled biological decomposition, converting a waste product into a humus-like substance that enhances soil health and plant growth.
Defining Composted Manure
Composted manure is animal waste that has undergone controlled aerobic decomposition, a process managed by microorganisms in the presence of oxygen. This biological breakdown results in a dark, fine-textured, and relatively odorless material, often described as humus-like. The stability of the final product is a defining characteristic, meaning the organic matter is largely broken down and will not rapidly decompose when applied to soil.
It is important to distinguish true composted manure from merely “aged” or “rotted” manure. Aged manure is raw manure that has sat in a pile, which may reduce volume and some strong odors, but it has not achieved the high temperatures needed for safety. Genuine composting requires careful management to ensure the material reaches a mature, stable state, making it a reliable and homogenous soil input.
The Composting Process
Creating quality composted manure depends on maintaining specific environmental conditions to support microbial activity. The process begins with balancing the carbon-to-nitrogen (C:N) ratio, ideally aiming for a range between 20:1 and 40:1. Since manure is high in nitrogen, carbon-rich materials like straw, wood chips, or dry leaves are added to achieve this balance.
The material must be piled into a windrow or heap large enough—at least one cubic yard—to insulate the heat generated by the microbes. This heat drives the thermophilic stage, where temperatures must reach between 131 and 170 degrees Fahrenheit. Maintaining a moisture content around 40 to 60 percent is necessary to keep the microorganisms active.
Aeration is accomplished by regularly turning the pile, which introduces oxygen and ensures all material is exposed to the high temperatures. For commercial operations, the temperature must be maintained above 131°F for a specific duration, such as 15 days, with at least five turnings. This methodology ensures the material is fully processed and enters a final curing stage before being considered finished compost.
Key Differences Between Raw and Composted Manure
The primary difference between raw and composted manure lies in the safety and stability achieved through the thermophilic stage. The sustained high temperatures eliminate harmful pathogens, such as E. coli and Salmonella, which can be present in raw animal waste. This heat also destroys the viability of most weed seeds, preventing their introduction into the garden or field.
Composting stabilizes the volatile nitrogen compounds found in fresh manure, significantly reducing the risk of “fertilizer burn.” In raw manure, nitrogen is largely ammonia, which can volatilize into the air or be overly concentrated for plants. The composting process converts this volatile nitrogen into more stable organic forms, which are released slowly as microbes break down the finished product in the soil.
The physical and chemical transformation greatly improves the material’s handling characteristics. Raw manure is often heavy, wet, and possesses a strong, unpleasant odor due to high ammonia content. Composted manure is lighter, has a reduced volume, and the intense odors are largely eliminated, making it easier and safer to transport and apply.
Practical Applications in Gardening and Agriculture
Composted manure is used primarily as a soil amendment, functioning to improve the physical, chemical, and biological properties of the soil rather than acting as a fast-release fertilizer. Its high organic matter content creates stable soil aggregates, which enhances water infiltration and retention, making the soil more resilient during dry periods.
When applying composted manure, it is recommended to spread a thin layer, often between a half-inch and one inch deep, and incorporate it into the top six to eight inches of the soil. This method helps prevent the loss of soluble nutrients and ensures the organic matter is mixed where it can benefit plant roots. Application is best done before planting a new bed or as a top dressing around established plants.
The stable nutrients in compost, including nitrogen, phosphorus, and potassium, are released gradually over time, providing a sustained source of fertility. This slow-release characteristic reduces the risk of nutrient runoff, a concern with synthetic fertilizers, while contributing to the long-term health and structure of the soil.