Composting

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Composting

Composting is the process of arranging and manipulating organic wastes so that they are gradually broken down, or decomposed, by soil microorganisms and animals. The resulting product is a black, earthy-smelling, nutritious, mixture called compost or humus. Compost is usually mixed with other soil to improve the soil’s structural quality and to add nutrients for

plant growth. Composting and the use of compost in gardening are important activities of gardeners who prefer not to use synthetic fertilizers.

Composting is a natural process. In the natural world, organic (carbon-containing material) decomposes. The organic and inorganic constituents of the former plant or animal body become available in soil for plants to take up again. Composting takes advantage of this natural process of decomposition, usually speeding up the process, by the creation of a special pile of organic materials called a compost heap.

The major benefit of compost is its organic content. Humus added to soil changes its structure, its ability to hold oxygen and water, and its capacity to adsorb certain nutrient ions. It improves soils that are too sandy to hold water or contain too much clay to allow oxygen to penetrate. Compost also adds some mineral nutrients to the soil. Dependingon the organic material of the compost and microorganisms present, it can also balance the pH of an acidic or alkaline soil.

History

Prehistoric farming people discovered that if they mixed manure from their domesticated animals with straw and other organic waste, such as crop residues, the mixture would gradually change into a fertile soil-like material that was good for crops. Composting remained a basic activity of farming until the twentieth century, when various synthetic fertilizers were found to provide many of the nutrients occurring naturally in compost.

The steadily increasing population of the world demands increasing quantities of food. To increase productivity, many farmers have come to depend on synthetic fertilizers made in factories from nonrenewable resources. However, regular use of these fertilizers does not improve the structure of the soil, and can, in fact, gradually harm the soil. Also, synthetic fertilizers are expensive, an important consideration to farmers in less developed countries.

It was in an underdeveloped country—India— that modern-day composting originated. Sir Albert Howard, a government agronomist, developed the so-called Indore method, named after a city in southern India. His method calls for three parts garden clippings to one part manure or kitchen waste arranged in layers and mixed periodically. Howard published his ideas on organic gardening in the1940 book An Agricultural Testament.

The first articulate advocate of Howard’s method in the United States was J.I. Rodale (1898-1971), founder of Organic Gardening magazine. These two men made composting popular with gardeners who prefer not to use synthetic fertilizers.

People are attracted to composting for a variety of reasons. Many wish to improve their soil or help the environment. Compost mixed with soil makes it darker, allowing it to warm up faster in the spring. Compost adds numerous naturally occurring nutrients to the soil. It improves soil quality by making the structure granular, so that oxygen is retained between the granules. In addition, compost holds moisture. This is good for plants, of course, but it is also good for the environment because it produces a soil into which rain easily soaks. When water cannot soak directly into soil, it runs across the surface, carrying away soil granules and thus eroding the soil. In addition, the use of compost limits the use of natural gas, petrochemicals, and other nonrenewable resources that are used in making synthetic fertilizers. Composting also recycles organic materials that might otherwise be sent to landfills.

Despite its many benefits, making and using compost does have its disadvantages. Composting releases methane, a greenhouse gas which traps solar heat in Earth’s atmosphere and may contribute to global warming. The kitchen wastes and warmth of compost heaps may attract pests such mice, rats, and raccoons.

Composting on any scale

Composting can be done by anyone. A homeowner can use a small composting bin or a hole where kitchen wastes (often minus meats and fats, which can create objectionable odors) are mixed with grass clippings, small branches, shredded newspapers, or other coarse, organic debris.

Communities may have large composting facilities to which residents bring grass, leaves, and branches to be composted. Such communities often have laws against burning garden waste and use composting as an alternative to disposal in a landfill. Sometimes sewage sludge, the semisolid material from sewage treatment plants, is added. The heat generated in the heap kills any disease-causing bacteria in the sludge. The materials are usually arranged in long rows, called windrows, which may be covered by roofs. The resulting humus is used to condition soil on golf courses, parks, and other municipal grounds.

The largest scale of composting is done commercially by companies that collect organic materials, including paper, from companies and private citizens. Commercial composting is usually mechanized, using large machines called composters. Raw solid waste is loaded onto a slow-moving belt, then is dumped into a device which turns the waste, and compost comes out the other end within a few days or weeks. This in-vessel or container process allows careful control of moisture and air. Some communities are looking toward such mechanized digesters as a way of helping to solve the municipal solid waste problem as more and more landfills close in the future.

Materials to compost

Most organic materials can be used in a compost heap. Meat is often omitted because it can putrefy, giving off bad odors. It can also attract rats and other pests. Soil or finished humus is added to supply the microorganisms needed to make the heap work. To work most efficiently, the materials are layered, with woody materials, grasses,kitchen waste, and soil alternating. Farmyard or zoo manure mixed with straw makes an excellent addition to compost. However, feces from household pets may carry diseases.

A ratio of approximately 25 parts carbon to 1 part nitrogen should be available in the compost heap. If the ratio is quite different, ammonia smells can be given off, or the process may not work efficiently. Chopped-up tree branches, fallen leaves, and sawdust are good sources of carbon. Alfalfa is a good nitrogen source.

How it works

A compost heap needs to have both water and oxygen to work efficiently. In dry weather it may need to be watered. More importantly, however, the compost heap must be turned regularly. The more often it is turned, the more the compost materials are exposed to oxygen, which raises their temperature and increases the efficiency of the process.

Microbiological, chemical, and physical processes occur during composting. Microorganisms break down the carbon bonds of organic materials in the presence of oxygen and moisture, giving off heat in the process.

High temperatures can be achieved most easily in a compost heap that is built all at once and tended regularly. Enclosed bins, often used by city gardeners, may produce humus within a month. An open heap, such as in the back corner of a garden, will probably achieve lower temperatures, but it will still eventually decompose. However, a year or more may be required to produce humus, and it will contain some undecomposed materials. In northern winters, the composting process will slow down and almost stop except at the core of a large, well-arranged heap.

The byproducts of composting can also be used. Some composters run water pipes through their compost heaps and utilize the heat generated to warm greenhouses and even houses. The methane given off can also be collected and used as a fuel called biogas for cooking.

The most heat is given off at the beginning of the composting process, when readily oxidized material is decomposing. Digestion of the materials by bacteria is strongest at that time. Later, the temperature within the pile decreases, and the bacterial activity slows down, though it continues until all the waste is digested. Other microorganisms take over as the heap cools.

The degradative action of microorganisms such as bacteria, protozoa, fungi, and actinomycetes (the latter resemble both bacteria and fungi) change the chemistry of the compost. They produce enzymes that digest the organic material. Bacteria are most important initially and fungi later. If the pile is not turned regularly, the decomposition will be anaerobic and produce foul-smelling odors. By turning the pile, a gardener creates conditions for aerobic decomposition, which does not produce odors.

Some organisms work on the compost pile physically instead of chemically. They tend to arrive only after the pile has cooled to normal air temperature. These organisms include mites, millipedes, sowbugs and pill-bugs (isopods), snails and slugs, springtails, and beetles. Finally, the worms—nematodes, flatworms, and earth-worms—do their part. These animals eat and digest the organic materials, adding their nutrient-filled excrement to the humus. In addition, they give off substances that bind the material in granules or clumps. The movement of these animals, especially earthworms, through the material helps to aerate it.

During the composting process, the material oxidizes, breaking down into proteins and carbohydrates. The proteins break down into peptides and amino acids, then into ammonium compounds. These compounds are changed by certain bacteria into nitrates,

KEY TERMS

Aerobic— Requiring or in the presence of oxygen.

Anaerobic— Describes biological processes that take place in the absence of oxygen.

Decomposition— The breakdown of the complex molecules composing dead organisms into simple nutrients that can be reutilized by living organisms.

Microorganism— Any living thing that can only be seen through a microscope.

Nutrient— Any substance required by a plant or animal for energy and growth.

Organic— Made of or requiring the materials of living things. In pure chemistry, organic refers to compounds that include carbon.

Vermicomposting— Using the digestive processes of worms to compost organic materials.

a form of nitrogen that can be used by plants to make chlorophyll and essential proteins. The carbohydrates break down into simple sugars, organic acids, and carbon dioxide.

Nutrients in humus enter plant tissues by a process called base exchange. In this process, hydrogen ions in the fine root hairs of plants are exchanged for the nutrient ions in the soil moisture. The nutrients are then free to move up into the plant.

Some composters use a somewhat different form of composting, especially during winter. Called vermicomposting, it consists of maintaining worms (preferably redworms, or Eisenia foetida ) in a container filled with a plant-based material (such as shredded corrugated paper, manure, or peat moss) that they gradually consume.

Kitchen waste is pushed into the soil and digested by the worms. Their excrement, called castings, along with partially decomposed waste, can be “harvested” in about four months and used as a nutrient-laden addition to soil. Worm castings are even more nutrient-filled than garden compost.