Chapter 10

Compost Management on Organic Farms

Composting Process

Composting is a natural biological process that breakdowns organic material under aerobic conditions into stable compost. During composting, the microorganisms consume oxygen while feeding on organic matter (carbon containing) and break it down into simple compounds and reforming them into new complex compounds. This produces a fiberrich, carbon-containing humus with inorganic nutrients like nitrogen, phosphorus, and potassium. In this process, the amount of humus increases, the carbon-to-nitrogen ratio (C:N) decreases, pH neutralizes, and the exchange capacity of the materials increases. During composting some loss of nitrogen may occur and ammonia can escape from the composting pile. Nevertheless, composting retains most nutrients supplied by the raw materials and stores them within stable organic compounds. The microorganisms, by breaking the chemical bonds of the organic materials, obtain energy for growth. During this process, some of the chemical energy is transformed to heat that increases the pile temperature and escapes to the surroundings. When the microbes have used the available carbon and nitrogen they start to decline in numbers and the heat being produced is reduced, indicating the end of the active composting process. Success with composting depends on providing conditions conducive to the preferential growth of desirable microbes. Under optimal conditions, composting proceeds through three phases: (1) the mesophilic, or moderate-temperature phase, which lasts for a couple of days; (2) the thermophilic, or high-temperature phase, which can last from a few days to several months; and finally, (3) a curing phase.

Initial Mesophilic Phase

If the proper conditions exist, the pile begins to heat up almost right away. This first phase of composting, lasting one to two days, is called the mesophilic stage. In this stage, strains of microorganisms increase exponentially as readily available food sources of the substrate are metabolized.

Second Thermophilic Phase

The next phase in the composting process is the thermophilic stage, which can last for a few days to several months. As active composting takes place, temperatures in the center of the pile climb to about 120 to 150 degrees F (49–66°C). At these temperatures, heat-loving (thermophilic) bacteria vigorously degrade the organic material.

The Curing Stage

In the curing phase, microbial activity slows down and as the process nears completion, the material approaches ambient air temperature. The rate of oxygen consumption declines to the point where compost can be stockpiled without turning. During curing, organic materials continue to decompose and are converted to biologically stable humic substances—the mature or finished compost, which takes on many of the characteristics of humus, the organic fraction of soil.

Time for Curing

There is no clearly defined time for curing. Common practices in commercial composting operations range from one to four months. However, compost piles can cure for as long as 6 to 12 months. Usually, this is done once pile temperatures cool to 105 degrees F (41°C) and high temperatures don’t recur following turning.

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