Composting

Composting is a fermentation process, the break down of organic material aided by an array of microorganisms , earthworms, and other insects in the presence of air and moisture. This process yields compost (residual organic material often referred to as humus ), ammonia, carbon dioxide , sulphur compounds, volatile organic acids, water vapor, and heat. Typically, the amount of compost produced is 40–60% of the volume of the original waste.

For the numerous organisms that contribute to the composting process to grow and function, they must have access to and synthesize components such as carbon , nitrogen , oxygen, hydrogen , inorganic salts, sulphur, phosphorus , and trace amounts of micronutrients. The key to initiating and maintaining the composting process is a carbon-to-nitrogen (C:N) ratio between 25:1 and 30:1. When C:N ratio is in excess of 30:1, the decomposition process is suppressed due to inadequate nitrogen limiting the evolution of bacteria essential to break the strong carbon bonds. A C:N ratio of less than 25:1 will produce rapid localized decomposition with excess nitrogen given off as ammonia, which is a source of offensive odors.

Attaining such a balance of ratio and range is possible because all organic material has a fixed C:N ratio in its tissue. For example, food waste has a C:N ratio of 15:1, sewage sludge has a C:N ratio of 16:1, grass clippings have a C:N ratio of 19:1, leaves have a C:N ratio of 60:1, paper has a C:N ratio of 200:1, and wood has a C:N ratio of 700:1. When these (and other) materials are mixed in the right proportions, they provide optimum C:N ratios for composting. Typically, nitrogen is the limiting component that is encountered in waste materials and, when insufficient nitrogen is present, the composting mixture can be augmented with agricultural fertilizers, such as urea or ammonia nitrate.

In addition to nutrients, the efficiency of the composting process depends on the organic material's size and surface characteristics. Small particles provide multi-faceted surfaces for microbial action. Size also influences porosity (crevices and cracks which can hold water) and permeability (circulation or movement of gases and moisture).

Moisture (water) is an essential element in the biological degradation process. A moisture level of 55–60% by weight is required for optimal microbial, nutrient , and air circulation. Below 50% moisture, the nutrients to sustain microbial activity become limited; above 70% moisture, air circulation is inhibited.

Air circulation controls the class of microorganisms that will predominate in the composting process: air-breathing microorganisms are collectively termed aerobic , while microorganisms that exist in the absence of air are called anaerobic . When anaerobic microorganisms prevail, the composting process is slow, and unpleasant-smelling ammonia or hydrogen sulfide is frequently generated. Aerobic microorganisms will quickly decompose organic material into its principal components of carbon dioxide, heat and water.

The role of acidity and alkalinity in the composting process depends upon the source of organic material and the predominant microorganisms. Anaerobic microorganisms generate acidic conditions which can be neutralized with the addition of lime. However, such adjustments must be done carefully or nitrogen imbalance will occur that can further inhibit biological activity and produce ammonia gas, with its associated unpleasant odor. Organic material with a balanced C:N ratio will initially produce acidic conditions, 6.0 on the pH scale. However, at the end of the process cycle, mature compost is alkaline, with a pH reading greater than 7.0 and less than 8.0.

The regulation and measurement of temperature is fundamental to achieving satisfactory processing of organic materials. However, the effect of ambient or surface temperatures on the process is limited to periods of intense cold when biological growth is dormant. Expeditious processing and reduction of herbicides, pathogens, and pesticides is achieved when internal temperatures in the compost pile are maintained at 120–140°F (55–l60°C). If the internal temperature is allowed to reach or exceed 150°F (65°C), biological activity is inhibited due to heat stress. As the nutrient content is depleted, the internal temperature decreases to 85°F (30°C) or less–one criteria for defining mature or stabilized compost.

Mature or stabilized compost has physical, chemical, and biological properties which offer a variety of attributes when applied to a host soil . For example, adding compost to barren or disturbed soils provides organic and microbial resources. The addition of compost to clay soils enhances the movement of air and moisture. The water retention capacity of sandy soil is enhanced by the addition of compost and erosion also is reduced. Soils improved by the addition of compost also display other characteristics such as enhanced retention and exchange of nutrients, improved seed germination, and better plant root penetration. Compost, however, has insufficient nitrogen, phosphorous, and potassium content to qualify as a fertilizer . The ultimate application or disposition of compost depends upon its quality, which is a function of the type of organic material and the method(s) employed to enhance or control processing.

Compost processing can be as simple as a plastic garbage bag filled with a mixture of plant waste that has had a couple of ounces of fertilizer, some lime, and sufficient water added to make the material moist. The bag is then sealed and set aside for about 12 months. Faster processing can be achieved with the use of a 55 gal (208 l) drum into which 0.5-in (1.3-cm) holes have been drilled for air circulation. Filled with the same mixture as the garbage bag and rotated at regular intervals, this method will produce compost in two to three months. A multi-compartmentalized container is faster and increases the diversity of materials which can be processed. However, including such items as fruit and vegetable scraps, meat and dairy products, cardboard cartons, and fabrics must be undertaken with caution because they attract and breed vermin.

Such methods are designed for individual use, especially by those who can no longer dispose of garden waste with their household waste . Similarly, commercial and government institutions employ natural (low), medium, and advanced technical composting methods, depending on their motivation, i.e., diminishing landfill capacity, availability of fiscal resources, and commitment to recycling . The simplest composting method currently employed by industry and municipalities entails dumping organic waste on a piece of land graded (sloped) to permit precipitation and leachate (excess moisture and organics from composting) to collect in a retention pond. The pond also serves as a source of moisture for the compost process and as a system where photosynthesis can oxidize the leachate. The organic material is placed in piles called windrows (a ridge pile with a half-cone at each end). The dimensions of a windrow are typically 10–12 ft (3–3.7 m) wide at the base and about 6 ft (1.8 m) high at the top of the ridge. The length is site specific. Windrows are constructed using a front-end loader. A mature compost will be available in 12–24 months, depending on various factors including the care with which the organic material was blended to obtain optimum C:N ratio; the supplementation of the material with additional nutrients; the frequency of aeration (mixing and turning); and the moisture content maintained.

Using the same site layout, the next step in mechanization is the use of windrow turners. These turners can be simple aeration machines or machines with the added ability to shred material into smaller particles, while injecting supplemental moisture and/or nutrients. Optimizing the capabilities of such equipment requires close attention to temperature variation within the windrows. Typically, the operator will use a temperature probe to determine when the temperature falls in the range of 100°F (37–38°C). The equipment will then fold the outer surface of the windrow inward, replenishing air and moisture, and mixing in unconsumed or supplemental nutrients. This promotes further decomposition, which is identified by a gradual rise in temperature. Sequential turning and mixing will continue until temperatures are uniformly diminished to levels below 85°F (30°C). This method produces a mature compost in four to eight months.

Two more technologically advanced composting methods are the in-vessel system and the forced-air system. Both are capable of processing the bulk of all solid and liquid municipal wastes. However, such flexibility imposes substantial capital, technical, and operational requirements. In forced air processing, organic material is placed on top of a series of perforated pipes attached to fans which can either blow air into, or draw air through the pile to control its temperature, oxygen, and carbon dioxide needs. This system is popular for its ability to process materials high in moisture and/or nitrogen content, such as yard wastes. Time to produce a mature compost is measured in days, depending on the class of organic material processed. During in-vessel processing, organic material is continuously fed into an inclined rotating cylinder, where the temperature, moisture, and nutrient air and gas levels are closely controlled to achieve degradation within 24–72 hours. The composted material is then screened to remove foreign or inert materials such as glass, plastics , and metals and is allowed to mature for 21 days.

[George M. Fell ]

RESOURCES

BOOKS

Appelhof, M. Worms Eat My Garbage. Kalamazoo, MI: Flower Press, 1982.

The Biocycle Guide to the Art and Science of Composting. Emmaus, PA: JG Press, 1991.

The Biocycle Guide to Yard Waste Composting. Emmaus, PA: JG Press, 1989.

The Rodale Book of Composting. Emmaus, PA: Rodale Press, 1992.

PERIODICALS

Kovacic, D. A., et al. "Compost: Brown Gold or Toxic Trouble?" Environmental Science and Technology 26 (January 1992): 38-41.

Lecard, M. "Urban Decay." Sierra 76 (September-October 1991): 27-8.

Composting

History

Composting on any scale

Materials to compost

How it works

Resources

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.

Resources

Books

Ebeling, Erich. Basic Composting: All the Skills and Tools You Need to get Started. Mechanicsburg, PA: Stackpole Books, 2003.

Koontz, Robin Michal and Matthew Harrad. Composting: Nature’s Recyclers. New York: Picture Window Books, 2006.

Soloman, Steve. Organic Gardener’s Composting. Boston: Echo Library, 2006.

Jean F. Blashfield

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, crumbly 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 .

Nature itself composts materials by continually recycling nutrients from dead organic matter . Living things take in inorganic nutrients to grow. They give off waste, die, and decompose. The nutrients contained in the 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. Depending on 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 has come to require large supplies of food. In order to increase productivity, 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 composting got its big start. Sir Albert Howard, a government agronomist, developed the socalled 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 the 1940 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 the 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 (minus meats and fats) 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 diseasecausing 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—shredded paper, hair clippings, food scraps from restaurants (omitting meats), coffee grounds, eggshells, fireplace ashes, chopped-up Christmas trees, seaweed, anything that originally came from a living thing. Meat is 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.

A compost heap needs to be at least 3 ft (0.9 m) in diameter and about 3 ft (0.9 m) high to work properly. The heap can be just piled on the ground or layered within a shallow hole. It can also be placed inside a small fenced enclosure or even in a large plastic or metal tub with holes cut into it. A smaller compost heap will probably be unable to achieve the high internal temperature-about 120–140°F (49–60°C)-necessary to work efficiently.

The chemical process

The processes that occur within a compost heap are microbiological, chemical, and physical. 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 organisms

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.

Microorganisms, such as bacteria, protozoa , fungi , and actinomycetes (the latter resemble both bacteria and fungi), work to 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 earthworms—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.

The nutrients

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, a form of nitrogen which 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.

Composting with worms

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.

See also Waste management.

Resources

books

Appelhof, Mary. Worms Eat My Garbage. Kalamazoo, MI: Flower Press, 1982.

Blashfield, Jean F., and Wallace B. Black. Recycling. Saving Planet Earth series. Chicago: Childrens Press, 1991.

Campbell, Stu. Let It Rot! The Gardener's Guide to Composting. Rev. ed. Pownal, VT: Storey Communications, 1990.

Culen, Gerald, William Bluhm, Preethi Mony, Janice Easton, and Larry Schnell. Organics: A Wasted Resource? an Extended Case Study for the Investigation and Evaluation of Composting and Organic Waste Management Issues. Champaign, IL: Stipes Publishing, 2001.

Martin, Deborah L., and Grace Gershuny, eds. The Rodale Book of Composting. Rev. ed. Emmaus, PA: Rodale Press, 1992.

Whitehead, Bert. Don't Waste Your Wastes-Compost 'Em: The Homeowner's Guide to Recycling Yard Wastes. Sunnyvale, TX: Sunnyvale Press, 1991.

Jean F. Blashfield

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.

Composting

Decomposed biosolids (e.g., leaves, crop residue, animal waste) have long been used to recycle plant nutrients and enhance soil fertility. It is one of the most ancient of agricultural innovations, as is evidenced by an ancient Telgu proverb "Leaf manure produces luxuriant growth" (Donahue et al., p. 154). Despite its long history, the scientific principles and systematic explanation of the techniques involved were not described until 1935 when Sir Albert Howard, working in Indore, Madya Pradesh, India, described the so-called Indore method of composting in which plant and animal waste is converted into humus . The process was developed between 1924 and 1931 for two reasons: to eradicate parasites from biosolids and maintain soil fertility. It was realized that "improved varieties by themselves could be relied upon to give an increased yield in the neighborhood of 10 percent, improved varieties plus better soil conditions were found to produce an increment up to 100 percent, or even more" (Howard, p. 39). The process involves creating an admixture of animal and plant wastes with a base for neutralizing acidity, and managing the admixture so that microbial processes are most effective at humifying the biosolids. The fermenting processes are allowed to occur in a shallow pit to avoid loss of water. The pit is surrounded by a cutoff drain to prevent run on and by a thatch of roof to keep rains out and reduce the risk of inundation.

Thus, composting is the biological reduction of biosolids into a soil-like, nutrient-rich material. The composted product is safe and easy to handle, and does not induce nitrogen deficiency in recipient plants by nitrogen stabilization in the compost. It suppresses disease infestation by partial sterilization and detoxifies pollutants. Principal types of organic wastes used in composting are animal manure, yard waste, municipal solid waste, paper mill sludge, municipal sewage, and fermentation waste. An important precaution when creating a usable end product is to exclude those materials that contain weed seed or cuttings which may sprout and become weed, or infested material that may spread pathogens to recipient crops. These organic materials are decomposed into humus outside of the soil by a process called humification that normally occurs within the soil. The application of biosolids directly to soil may have adverse impacts on soil quality and plant growth. With decomposition of biosolids and their humification the compost pit minimizes the adverse impacts. Techniques have also been developed for making satisfactory compost from sewage sludge. Of concern here is the risk that heavy metals in municipal sludge will contaminate cropland.

Composting is a hygienic way of recycling nutrients in the organic byproducts of agriculture, urban, and industrial activities. It represents safe storage and easy handling, and is an economic source of plant nutrients. It is an important strategy for handling a significant volume of by-products. The quantity of biosolids available for composting in the United States is large (see table). Properly used, it is a major resource for enhancing soil quality and improving environments. Compost material is principally used for the reclamation of drastically disturbed (e.g., mined) soil and other degraded ecosystems, and for landscaping and agriculture. Rather than cause environmental pollution, properly composted organic material can be a major asset in the enhancement of soil fertility, restoration of degraded soils, and sequestration of carbon. Carbon sequestration implies removal of carbon dioxide from the atmosphere either biotically through photosynthesis as plant products or abiotically by capturing from industrial sources. Subsequent storage of the carbon thus captured into long-lived pools such as soil, forestry products, geological strata, or ocean.

Process of Composting

The humification of organic material under most conditions occurs in three stages:

1. Mesophilic stage. This is the initial stage of decomposition, lasting for about a week, during which sugars and other simple carbohydrates are rapidly metabolized. This is an exothermic process and may cause an increase in temperature by 40°C.
2. Thermophilic stage. This is the second stage, lasting for about two weeks, during which the temperature may rise to about 50 to 75°C. Such a drastic increase in temperature is accompanied by the decomposition of cellulose and other resistant materials. It is important that the material be thoroughly mixed and kept aerated during this stage.
3. Curing stage. The temperature decreases during this final stage and the material being composted is recolonized by mesophillic organisms, which often produce plant-growth stimulating compounds. Mesophillic organisms are usually fungal-dominated and useful to restore bacteria dominated soils.

At the completion of this process, the plant or other organic parts (leaves, roots, etc.) are no longer identifiable in the compost. The humification of organic material is characterized by an increase in concentration of humic acids from approximately 4 to 12 percent, and a decrease in the C/N ratio from thirty in the original material to about ten in the final product.

Composting Techniques

Traditionally, composting has been an important technique for maintaining soil fertility. In developed economies, composting is a commercial enterprise, manufacturing soil products for horticultural and ornamental plants, and organic farming. On a small scale, composting is done in a bin at least 1 m2 at the base and no more than 120 cm (4 feet) high. A 5-cm mesh of woven wire is placed at the base of the bin as a retaining barrier and to facilitate drainage. The bin has an overflow gate at about 90 cm from the base.

Composting material is packed in the bin in approximately 15-cm layers, alternated by 15-cm layers of soil. It is important to flatten the top and create a small depression for water penetration. Small-scale backyard composting is an effective way to recycle food and yard waste.

Precautionary measures that should be taken include the following:

• provide good aeration throughout the pile;
• avoid excessive packing;
• avoid weed seeds, rhizomatous, and disease-infested materials;
• do not use by-products containing heavy metals and other contaminants;
• build pile large enough to generate sufficient heat;
• keep the pile moist at 50 to 70 percent moisture content;
• provide a coarse mesh screen at the base of the bin; and
• mix bulking agents such as wood chips and residue.

The compost is usually ready within three to four months.

see also Agriculture; Biosolids; Recycling.

Bibliography

Barker, A.V. (1997). "Composition and Uses of Compost." In Agricultural Uses of By-Products and Wastes, edited by J.E. Recheigl and H.C. MacKinnon. Washington, DC: American Chemical Society.

Brady, N.C., and Weil, R.R. (2002). The Nature and Properties of Soils, 13th edition. Upper Saddle River, NJ: Prentice-Hall.

Donahue, R.L.; Follett, R.H.; and Tulloch, R.W. (1990). Our Soils and Their Management. Danville, IL: Interstate Publishers.

Howard, A. (1935). "The Manufacture of Humus by the Indore Process." Journal of Royal Society on the Arts 84:25.

Howard, A. (1940). An Agricultural Testament. London: Oxford University Press.

Rechaigl, J.E., and MacKinnon, H.C., eds. (1997). Agricultural Uses of By-Products and Wastes. Washington, DC: American Chemical Society.

Rynk, R., and Richard, T.L. (2001). "Commercial Compost Production Systems." In Compost Utilization in Horticultural Cropping Systems, edited by P.J. Stofella and B.A. Kahn. Boca Raton, FL: Lewis Publishers.

Sopper, W.E. (1993). Municipal Sludge in Land Reclamation. Boca Raton, FL: Lewis Publishers.

Internet Resources

U.S. Composting Council Web site. Available from http://www.compostingcouncil.org.

Rattan Lal

Composting

Composting is the process of arranging and manipulating plant and animal materials so that they are gradually broken down, or decomposed, by soil bacteria and other organisms. The resulting decayed organic matter is a black, earthy-smelling, nutritious, spongy 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 is a method used by gardeners to produce natural fertilizer for growing plants.

Why compost?

Compost added to soil aids in its ability to hold oxygen and water and to bind to certain nutrients. It improves the structure of soils that are too sandy to hold water or that contain too much clay to allow oxygen to penetrate. Compost also adds mineral nutrients to soil. Compost mixed with soil makes the soil darker, allowing it to absorb the Sun's heat and warm up faster in the spring.

Adding compost to soil also benefits the environment. The improved ability of the soil to soak up water helps to prevent soil erosion caused by rainwater washing away soil particles. In addition, composting recycles organic materials that might otherwise be sent to landfills.

Words to Know

Decomposition: The breakdown of complex organic materials into simple substances by the action of microorganisms.

Humus: Decayed plant or animal matter.

Microorganism: A living organism 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 coming from living matter.

Composting on any scale

Composting can be done on a small scale by homeowners using a small composting bin or a hole where kitchen wastes are mixed with grass clippings, small branches, shredded newspapers, or other organic matter. Communities may have large composting facilities to which residents bring grass, leaves, and branches to be composted as an alternative to disposal in a landfill. Sometimes sewage sludge, the semisolid material from sewage treatment plants, is added. The resulting humus is used to condition soil on golf courses, parks, and other municipal grounds.

Materials to compost

Most organic (carbon-containing) materials can be composted—shredded paper, hair clippings, food scraps, coffee grounds, eggshells, fireplace ashes, chopped-up Christmas trees, and seaweed among them. Meat is omitted because it can give off bad odors during decomposition and attract rats and other pests. The microorganisms needed to break down the organic matter are supplied by adding soil or humus to the compost heap. Manure from farm or zoo animals makes an excellent addition to compost. Wastes from household pets are not used because they may carry disease.

How a compost heap works

A compost heap needs both water and oxygen to work efficiently. More importantly, the contents must be turned regularly to expose all areas to oxygen, which raises the temperature of the compost.

The processes that occur within a compost heap are microbiological, chemical, and physical. Microorganisms break down the chemical bonds of organic materials in the presence of oxygen and moisture, giving off heat. Some organisms work on the compost pile physically after it has cooled to normal air temperature. Organisms such as mites, snails, slugs, beetles, and worms digest the organic materials, adding their nutrient-filled excrement to the humus.

The nutrients

During the composting process, organic material is broken down into mineral nutrients such as nitrogen. Plants absorb nutrients through their roots and use them to make chlorophyll, proteins, and other substances needed for growth. Chlorophyll is the green pigment in plant leaves that captures sunlight for photosynthesis, the process in which plants use light energy to manufacture their own food.

[See also Agrochemicals; Recycling; Waste management ]