Compost

Compost

Compost, partially decomposed organic material used in gardening to improve soil and enhance plant growth. Compost improves the movement of water, dissolved nutrients, and oxygen through the soil, making it easier for plant roots to absorb these vital substances.

A versatile material, compost benefits virtually any soil type. Clay soil, for example, has tiny, tightly packed particles that hamper the flow of water, nutrients, and oxygen. Compost reconfigures the clay into larger, more loosely packed particles. The larger spaces between the particles improve the flow of water, oxygen, and nutrients to roots. In addition, the roots are able to penetrate deeper into the soil and contact more nutrients. Compost also improves sandy soil, where the large spaces between loosely packed particles enable water and its dissolved nutrients to drain too quickly for optimum root absorption. Compost soaks up and holds these substances so that the roots have more time to absorb them. Compost also adds small amounts of zinc, copper, boron, and other vital nutrients to soils.

MAKING COMPOST

Compost is made by harnessing the natural decomposition process carried out by certain species of microorganisms. These microorganisms, primarily bacteria and fungi, live in intimate association with their food supply—on the surface of dead plants, in soil, or on or in animal waste. By breaking down these materials with their digestive enzymes, the tiny creatures release and absorb the nutrients within. For home gardeners, making compost is simply a matter of collecting food for microorganisms in one place and letting them go to work.

A broad range of organic matter, including manure from plant-eating animals, grass clippings, and dead leaves or garden plants, provides a veritable feast for microorganisms. For optimal decomposition, the combined starting materials should have an appropriate carbon to nitrogen ratio, preferably 30 parts carbon to 1 part nitrogen. Leaves, straw, and paper, called brown materials, have a high carbon to nitrogen ratio, about 300 to 1, while grass clippings, kitchen scraps, and manure, called green materials, have a low carbon to nitrogen ratio, about 15 to 1. For the best mix, green materials should be added in abundance; brown materials should be used more sparingly. Materials that should not be used to make compost include manure from meat-eating animals, because it may contain disease-causing organisms that can harm humans who eat plants grown in the compost. Meat should be avoided since it may attract rodents. Fatty foods such as cheese also should not be added to the compost pile, as they are hard for most microorganisms to digest.

The starting materials are heaped into a pile—in a home garden, the pile is typically about a meter high and a meter wide (about three feet high and three feet wide); on farms, composting is done on a larger scale. The pile may sit loose on the ground or it may be enclosed using a variety of materials, including wire fencing, wood boards, cinder blocks, or widely stacked bricks.

MANAGING COMPOST

A variety of techniques may be used to increase the rate of compost decomposition. One technique is to cut the starting materials into 10- to 15-cm (4- to 6-in) pieces to increase the surface area on which the microorganisms act. Increased surface area accelerates decomposition, much like a large ice chunk melts faster if broken up into small pieces. The microorganisms in the compost pile also thrive when oxygen and moisture are present. Fluffing the compost pile every week or so with a pitchfork or other tool introduces oxygen into the pile, and sprinkling water on the pile when it dries out provides the necessary moisture.

In a well-managed compost pile, the microorganisms eat and reproduce rapidly, and heat is released as a byproduct of their intense biochemical activity. The heat in the pile kills most plant diseases and weed seeds that may have been present on the starting materials. The increased heat may also kill the microorganisms doing the decomposing as well, especially those at the center of the pile where temperatures may climb to 90° C (200° F). Mixing the materials well about once a week prevents lethal temperature increases by distributing the heat evenly throughout the pile.

The time it takes microorganisms to decompose the starting materials in compost varies. Factors include the size of the pile, the techniques used to manage the pile, and the nature of the starting materials—green materials decompose readily, while brown materials take longer to break down. In an actively managed compost pile, microorganisms use up their food supply and become less active after about six weeks. Then the pile slowly cools, signaling the near-final stages of decomposition. If the materials in a compost pile are relatively large, if the pile is not kept moist, and if oxygen is not introduced, microorganism activity is slow and the pile does not heat up. Depending upon the climate, it may take months or years for decomposition to occur.

No matter how long decomposition takes, when in its final stage, the compost pile is about half its original size and resembles dark soil. The material in the pile is now called humus—although the terms humus and compost sometimes are used interchangeably. Humus is the highly beneficial material that is added to the garden soil. Once in or on the soil, it continues to decompose at a very slow rate, releasing ammonia, carbon dioxide, and salts of calcium, phosphorus, and other elements that are beneficial for plant growth.

Humus can be added to the soil at any time of year. It can be worked into the soil, where its benefits take effect most rapidly, or it can be left on the soil surface. Humus can be used year after year, and there is never danger of adding too much, since this remarkable substance only enhances soil and encourages plants to thrive.

Cities compost on a large scale to reduce yard waste so that it does not take up space in landfills. Industries compost hazardous materials because the activities of the microorganisms help break down toxic substances into less-harmful or harmless materials. Many municipalities provide information on composting as part of their programs to reduce the amount of solid waste entering their landfills. County or regional offices of the state Cooperative Extension Service also have information on composting.