Compost is organic material that has decomposed - and is still decomposing. Dark, fluffy and crumbly, with a deliciously earthy odor, compost can be used as an organic mulch or added to soil, improving soil health and benefiting plants. Compost is different from humus, the decomposed organic compounds in soil. That’s because compost usually contains bits of leaves, twigs and other things that will eventually complete decomposition in the soil, becoming humus.
The ingredients that make compost are often called “browns” and “greens.” Browns are usually darkly colored, organic materials with high carbon content, like leaves, sticks, sawdust and other woody wastes. Greens are organic materials with a higher nitrogen composition. Keep in mind that the word “green” is about nitrogen, not color. Some of the highest-nitrogen organic materials for the composter are darkly hued, like animal manure and coffee grounds.
Leaves are the most common “browns.” Other high-carbon sources for the compost pile are straw, corn stalks, bark, wood chips and sawdust. The smaller the surface area of the organic brown material – shredded leaves compared to whole leaves, for instance – makes it friendlier for the bacteria to do their work. Newspaper and shredded paper can also be composted, but these take so long to break down that paper recycling is
usually viewed as a more responsible way to reuse paper.
“Greens” include farm animal and poultry manures, grass clippings, vegetable scraps and coffee grounds. While most any organic material may be composted, it is not recommended to include grease and other fatty kitchen wastes (like meat scraps) or pet (dog/cat) feces for home composting.
The right mixture of high-carbon browns and high-nitrogen greens, along with water and oxygen, creates a friendly environment for bacteria that specialize in breaking down organic matter. These hardworking “aerobes” are the workhorses of decomposition; they help transform nutrients, especially nitrogen, into forms that are more usable by plants.
Compost piles are often built by layering browns and greens. A layered pile will start to decompose on its own and, over time, become finished compost. The decomposition process is sped up when water is added to the pile and the pile is turned, allowing more oxygen into the pile, helping increasing the oxygen-loving bacteria that transform organic wastes into finished compost.