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Composting

What is Compost?

Compost is a dark, crumbly mixture of decomposed organic matter, such as grass clippings, leaves, twigs, and branches.

How Does Composting Work?

Even the first-time composter can make good quality compost. Like good cooking, composting is part science, part art. Attention to the following parameters will help you get started.

Materials

Anything that was once alive will naturally decompose. However, some organic wastes should not be composted at home.

DO compost these items: grass clippings, leaves, plant stalks, hedge trimmings, old potting soil, twigs, annual weeds without seed heads, vegetable scraps, coffee filters, and tea bags.

Do NOT compost these items: diseased plants, weeds with seed heads, invasive weeds such a quack grass and moring glory, pet feces, dead animals, bread and grains, meat or fish parts, dairy products, grease, cooking oil, or oily foods.

Making It Work

To prepare compost, organic material, microorganisms, air, water, and a small amount of nitrogen are needed.

Organic material is leaves, grass clippings, etc. that you are trying to decompose. Microorganisms are small forms of plant and animal life, which break down the organic material. A small amount of garden soil or manure provides sufficient microorganisms.

The nitrogen, air, and water provide a favorable environment for the microorganisms to make the compost. A small amount of nitrogen fertilizer can add sufficient nitrogen to the compost. You can purchase nitrogen fertilizers at many hardware stores, feed stores, or nurseries.

Air is the only part which cannot be added in excess. Too much nitrogen can kill microbes; too much water causes insufficient air in the pile.

Biology

Bacteria are the first to break down plant tissue and are the most numerous and effective compost makers in your compost pile. Fungi and protozoans soon join the bacteria and, somewhat later in the cycle, centipedes, millipedes, beetles, and worms complete the composting process.

Surface Area

If the microorganisms have more surface area to feed on, the materials will break down faster. Chopping your garden debris with a machete, or using a chipper, shredder, or lawnmower to shred materials will help them decompose faster.

Volume

Compost piles trap heat generated by the activity of millions of microorganisms. A 3-foot by 3-foot by 3-foot compost pile is considered a minimum size for hot, fast composting. Piles wider or taller than 5 feet don't allow enough air to reach the microorganisms at the center.

Moisture and Aeration

The microorganisms in the compost pile function best when the materials are as damp as a wrung-out sponge and have many air passages. Extremes of sun or rain can adversely affect the balance of air and moisture in your pile. The air in the pile is usually used up faster than the moisture, so the materials must be turned or mixed up occasionallly to add air that will sustain high temperatures and control odor. Materials can be turned with a pitchfork, rake, or other garden tool.

Time and Temperature

The most efficient decomposing bacteria thrive in temperatures between 110F and 160F. Thus, the hotter the pile, the faster the composting. If you achieve a good balance of carbon and nitrogen, provide lots of surface area within a large volume of material, and maintain adequate moisture and aeration, the temperature will rise over several days.

Uses for Compost

Compost contains nutrients, but it is not a substitute for fertilizers. Compost holds nutrients in the soil until plants can use them, loosens and aerates clay soils, and retains water in sandy soils.

To use as a soil amendment, mix 2 to 5 inches of compost into vegetable and flower gardens each year before planting.

In a potting mixture, add one part compost to two parts commercial potting soil, or make your own mixture by using equal parts of compost and sand or perlite.

As a mulch, spread an inch or two of compost around annual flowers and vegetables, and up to 6 inches around trees and shrubs.

As a top dressing, mix finely sifted compost with sand and sprinkle evenly over lawns.

U.S. Department of Agriculture
1400 Independence Avenue, SW
Washington, DC 20250
www.usda.gov