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The Art of Composting

What is Composting?

Composting is the aerobic, or oxygen requiring decomposition of organic materials by micro-organisms under controlled conditions. During composting, the micro-organisms consume oxygen while feeding on organic matter. Active composting generates considerable heat and large quantities of carbon dioxide and water vapor, which are released into the air. Composting reduces both the volume and mass of the raw materials while transforming them into a valuable soil conditioner. Composting is most rapid when conditions that encourage the growth of micro-organisms are established and maintained.

What Happens During Composting?

Raw feed stock materials are piled together. McFarlane's grinds up and recycles all the yard debris that is brought in by you, the customer. Initial mixing of raw materials introduces enough air to start the process. Almost immediately, the micro-organism consumes oxygen and the settling of the materials expels air from the pore space. As the supply of oxygen decreases , aerobic decomposition slows and may eventually stop if the oxygen is not replenished. Aeration is provided either by passive air exchange (natural convection and diffusion) or by forced aeration (blowers or fans). Regular turning or rotating of the pile supplies the aeration needed for composting.

Since the release of heat is directly related to the microbial activity, temperature is a good process indicator. Temperature increase resulting from microbial activities are noticeable within a few hours of forming a pile or windrow. The temperature of composting materials typically follow a pattern of rapid increase to 120-140 degrees Fahrenheit and finally to ambient air temperature. This characteristic pattern of temperature over time reflects changes in the rate and type of decomposition taking place as composting proceeds.

During the active composting period, the temperature falls if oxygen becomes scarce, because microbial activity decreases. The temperature rises again after turning of forced aeration. If oxygen is available and the microbial activity is intense, the temperature can rise well above 140 degrees Fahrenheit. At this point many micro-organisms begin to die or become dormant. With the decreased microbial activity, the temperature may then stabilize or even fall. Cooling the pile by turning or forced aeration helps to keep the temperature from reaching these damaging levels.

A curing period usually follows the active composting stage. While curing, the materials continue to compost but at a much slower pace. The rate of oxygen consumption decreases to the point where the compost can be piled without turning forced aeration.

The composting process does not stop at a particular point. Material continues to break down until the last remaining nutrients are consumed by the last remaining organisms and until nearly all of the carbon is converted to carbon dioxide. However, the compost becomes relatively stable and useful long before this point. Compost is judged to be "done" by characteristics related to it's use and handling such as carbon to nitrogen ratio, oxygen demand, temperature and odor. Factors affecting the composting process includes oxygen and aeration; nutrients (Carbon to Nitrogen), moisture, porosity, structure, texture and particle size, pH, temperature, and time. The carbon, chemical energy, protein and water in the finished compost is less that that of the raw materials. The volume of the finished compost is 50% or less of the the volume of the raw material.