A compost pile is a pile of organic wastes, animal wastes or fertilizers, and soil meant to decompose at an accelerated rate. At a mature stage, the compost decomposes into a dark brown or black substance called humus that is well used as a soil conditioner and fertilizer. Adding compost to soil can help soil to drain or retain water, enrich the soil with nutrients, and introduce beneficial microorganisms. Composting also reduces the solid waste stream.
Compost piles accelerate the natural rate of decomposition of organic materials by manipulating the waste to be especially conducive to consumption by microorganisms. This revolves around finding the proper ratio of carbon to nitrogen in the compost pile. Microbes need carbon from wastes such as leaves or wood for energy and nitrogen from green wastes for protein in order to thrive and grow. These microorganisms also need proper amounts of oxygen and water, so wastes added to the compost pile ought to be moist but not so wet as to block aeration. The pile should also be constructed so that oxygen can reach a large area of the pile.
Factors that might seriously affect this balance, such as extreme rain, sun, or wind, can adversely impact decomposition. Picking the proper location for this pile is important in fostering a good balance for a compost pile. The location should be level and have good drainage to prevent too much water sitting stagnantly in the pile. Avoiding areas with exposure to too much sun or wind keeps the pile from drying out, but it is important to be careful of using trees as shelter because they may grow roots into the pile and use up the nutrients. Sheltering compost piles with trees can work nicely, however, if the pile is built on a stone or brick foundation, but not cement, as it prevents aeration.
The method of layering and constructing the pile determines how active or passive the pile is. Free compost piles kept in a yard will decompose at a slower rate and be ready in six to twelve months, whereas compost prepared in a bin takes about two months. Most compost piles are set up one part green waste two parts brown materials. In repetition, this can look like organic wastes on the first level, fertilizer or another type of high-heat, high-microbe starter on the second level, and soil on the third level. Wood chips, plants, ashes, hair, newspaper, manure, and many other materials are okay to compost, while coal ash, dairy products, pet waste, meats, pesticide-ridden or diseased plants, and fats and oils are not recommended.
Compost piles have many environmental and practical benefits. The EPA estimates that 23 percent of the United States waste stream is made up of food residuals and yard trimming; materials that, if composted, could be removed from the waste stream. The heat of decomposition can kill pests and pathogens in waste while taking up toxic chemicals. Applied compost can remediate soil, thus increasing crop yields or repairing forestlands, and reduces the economic and environmental cost of chemical fertilizers.