Household Composting

Reprinted from the Solid Waste Management Newsletter , Univ. of Ill. at Chicago, 10/93

Introduction | Biology of Composting | Process | Methods | Uses | Further Reading

Introduction

Composting is not a new technology. Prior to the widescale use of chemical fertilizers, U.S. farmers typically spread composts made from decayed animal manures, plant materials, and other organic products to enrich the fertility of the soil. The renewed interest in organic farming and gardening techniques, such as composting, can be attributed to increased awareness of the potentially adverse environmental and human health effects associated with chemical fertilizers and pesticides. In addition, composting is receiving considerable attention since it can be a practical and feasible altemative to landfilling or incineration.

By composting yard materials and food scraps, homeowners reduce the amount of waste requiring disposal. In addition, a useful soil amendment is produced that, once applied to lawns and gardens, can condition the soil and replenish nutrients. By choosing to compost yard materials at home, homeowners also eliminate the need to purchase special yard waste disposal bags (as well as stickers for the bags) that must be hauled to curbsides or dropoff sites for composting elsewhere. Home composting saves transportation and disposal costs and, ultimately allows homeowners to become actively involved with the management of solid waste.

In addition to backyard composting, homeowners may choose two other options for managing yard debris. One option is to layer various yard trimmings as mulches around plants, trees, and shrubs. Mulches of grass clippings, leaves, or wood chips can reduce the soil temperature, inhibit weeds, and prevent evaporation. A second option is to leave grass clippings on the lawn (called grasscycling) where they will decompose naturally and retum some nutrients back to the soil.

The Biology of Composting

Composting is a controlled biological decay process that involves many species of microorganisms and invertebrate animals. In nature, dead animals and plants decay slowly. Composting involves human intervention to manipulate various organic materials and environmental conditions to speed up this process. Understanding the interrelationships between the biological systems and physical parameters involved in composting is essential to producing a high-quality end product. Such understanding is also necessary to avoid operational problems. A properly constructed compost pile represents a remarkably interactive biological and ecological system. It involves a diversity of species that emerge in response to changes in the nutritional and environmental conditions of the pile. However, it is imperative that a composting system be carefully constructed and maintained to exclude pests such as rodents, raccoons, and flies.

The decomposition of organic material involves both physical and chemical processes. The decomposition of organic materials into smaller compounds results from the activities and appetites of various invertebrates such as mites, millipedes, beetles, sowbugs, earwigs, earthworms, slugs, and snails. The chemical decomposition of organic compounds results predominately from soil microorganisms. These include bacteria, actinomycetes, fungi, and some protozoans. Under the appropriate conditions, complex organic compounds present in food scraps and yard trimmings are enzymatically degraded into smaller compounds, carbon dioxide, and water. Heat is also generated as a by-product of this oxidative process. Composting continues as long as appropriate microorganisms are present and adequate environmental conditions are maintained. Once the decay process is completed, the nondegradable organic matter becomes the characteristic humus-like material called compost.

Factors Affecting the Compost Process

Providing adequate sources of carbon and nitrogen is important since these elements are required by microorganisms for growth and cell division. Carbon also serves as an energy source for soil organisms. Ideally, the ratio of these nutrients in the compost system should approximate 30 parts carbon to 1 part nitrogen (by dry weight). This allows for the most rapid and efficient degradation of the organic material.

Food scraps and yard trimmings can be classified into two categories according to their carbon and nitrogen content:

• "Green" materials, such as fresh grass clippings, manure, garden plants, and kitchen scraps contain large amounts of nitrogen.
• "Brown" materials, such as dried leaves and plants, branches, and woody materials have a high carbon content but are relatively low in nitrogen.

Through experimentation and patience, a suitable formulation of green and brown materials will be derived. This is part of the art and science of backyard composting. For example, mixing together equal volumes of fresh grass clippings ("greens") and dry leaves ("browns") is a simple "recipe" for novice backyard composters. Refer to Table 1 for a listing of various organic materials for inclusion and exclusion in backyard composts. Note that even human and animal hair clippings (which are good sources of nitrogen), soiled paper (e.g., paper towels, napkins) and shredded nonrecyclable paper can also be included in a backyard compost.

Aeration refers to the amount of oxygen in the system. Microorganisms in the compost pile can degrade organic materials either aerobically (with oxygen) or anaerobicalIy (without oxygen). The types of organisms active in the pile and the metabolic processes used to degrade organic compounds are related to the oxygen content of the system. Aerobic degradation is preferred for rapid composting Air can be introduced into the pile by turning the materials periodically with a pitchfork. However, care must be taken to avoid compacting the materials, which reduces the porosity of the pile.

Composting can also be performed anaerobically; however this technique is much less popular than aerobic composting. Home composters may be reluctant to practice anaerobic composting since it can produce methane gas; malodorous compounds such as hydrogen sulfide gas, cadaverine, and putrescine; as well as various organic acids and alcohols that may be detrimental to plants. However, anaerobic composting can be beneficial because it improves the nitrogen content of the compost. This is in contrast to aerobic composting where organic nitrogen can be oxidized and released as a gas into the surrounding air.

Maintaining an adequate moisture content is essential since it provides the humidity required by microorganisms for optimal degradation. Water also dissolves the organic and inorganic nutrients in the pile making them available for utilization by soil organisms. A moisture content between 40 to 60 percent by weight throughout the pile is ideal. Exceeding this level slows decomposition and promotes anaerobic degradation. Moisture levels less than 40 percent cause the microbes to slow their metabolic activities and become dormant or die.

Moisture is easily added with a garden hose. In areas with excessive moisture, or during periods of extreme rain or snow, the pile can be covered with a tarp to reduce water infiltration. Measuring the moisture content is part of the art of composting. The optimal moisture level is achieved when the composting matenal feels damp to the touch; that is, when a few drops of liquid are released after squeezing a handful of the material (e.g., as wet as a "wrung out" sponge).

The surface area of the organic material exposed to soil organisms is another important factor in determining the rate of composting. To increase the rate of decomposition, yard tnmmings and food scraps should be shredded, chipped, or otherwise reduced in size. This is especially important for woody materials, large garden plants, and some fruit and vegetable scraps (e.g., citrus rinds, broccoli). Fallen leaves can be shredded by mowing them prior to raking and composting.

Since aerobic decomposition is an oxidative process, considerable heat is generated as a result of the microbial activity in the pile. Soil microorganisms are metabolically active over defined temperature ranges. As the temperature of the pile increases, different groups of organisms become active. Compost piles that contain adequate levels of oxygen, moisture, carbon, and nitrogen as well as matenals of appropriate particle size can heat up to temperatures in excess of 170 deg. F. Temperatures between 90 deg. F and 140 deg. F are typical of a well-operated system and are indicative of rapid composting. Higher temperatures begin to limit microbial activity. Temperatures above 160 deg. F are lethal to most soil microorganisms. Commercially available temperature probes are available from garden centers and garden supply catalogs to monitor the temperature profile of the pile. Although composting will occur without careful temperature control, maintaining high temperatures is necessary for rapid composting as well as to destroy weed seeds, insect larvae, and potential plant or human pathogens that may be present in the composting matenal.

Problems with the composting process can usually be traced to one or more of the factors previously mentioned. In many cases, turning the pile to improve aeration will be sufficient. Otherwise, adjusting one or moreof the other factors will usually correct the problem.

Backyard Composting Methods

Backyard composting is attractive to many homeowners since it can be adapted to fit their lifestyle, income, yardsize, and overall ambition. Home composting can be perfommed by a variety of methods. Typically, these include:

• placing materials in open piles
• burying materials in pits or trenches
• enclosing materials in drums or bins (e.g., holding bins, tuming bins, and worm bins)

In order to heat up properly, compost piles should be at least one cubic yard in size. This provides the minimal insulation required to sustain the high temperatures in the center of the pile. Composting units or bins can be constructed from inexpensive materials with little carpentry or masonry skills. Of course, detailed construction plans are available for making more complicated and durable compost bins. Bins can also be purchased at garden centers and through garden supply catalogs. However, prior to beginning a compost pile it is important to check with local authorities for public health ordinances that restrict home composting; especially the composting of food scraps.

The slow or "cold composting" method is the simplest way to compost yard trimmings and requires minimal work. A cold compost pile can be maintained as an open pile or enclosed in a holding bin. Materials in a cold compost pile degrade slowly since no effort is made to mix and aerate the materials. New material is merely added to the top of the pile. Finished compost (i.e., the material at the bottom of the bin or pile) is generally produced within one to two years. Since it involves slow decomposition, cold composting is not recommended for the composting of food scraps or diseased plants.

Materials can also be buried in holes or trenches and allowed to decay naturally. This is a useful way of composting food scraps. Note that food scraps must be covered with at least six inches of soil to discourage animals and insects from gaining access to the materials.

The fast or "hot composting" method requires more work on the part of the homeowner. Finished compost can be produced in six to eight weeks. This method requires that materials be composted in the correct carbon and nitrogen proportions. Food scraps should be buried in the central and hottest portion of the pile. A properly constructed hot compost pile will heat up rapidly (i.e., over 100 deg. F within the first week) due to microbial activity in the pile. Maintaining this rapid degradation rate requires frequent mixing to aerate the pile and control its moisture content. Unless it is properly constructed and maintained, a hot pile can "short-circuit" and revert to a much slower cold composting system.

Worm composting or vermicomposting can be accomplished in bins or pits and adapted for indoor or outdoorcomposting. It can be ideal for persons with small yards or apartment dwellers who want to derive some of the benefits of composting and reduce solid waste. Worm bins are easy to construct and can be adapted to accom-modate the food scraps generated within the household.

Indoor methods involve placing redworms (not nightcrawlers or field worms typically found in gardens) in a bin containing shredded newspaper and garden soil. Once established in the bin, the worms (which consume their own weight in food each day) will degrade fruit and vegetable scraps as well as coffee grounds added to the bin. Dairy and meat products should be avoided since they can attract animals and insects. The finished compost, called vermicompost, makes an excellent potting soil and soil enhancer.

Finished Compost and its Uses

Caution should be exercised before applying "unfinished" compost to lawns and gardens. Compost is "finished" when the pile cools off (i.e., usually within 1 0 deg F of ambient temperatures). In addition, the size of the pile will have decreased to about one-third to one-half its original volume. Finished compost is dark and crumbly with an "earthy" smell. Applying unfinished compost may be detrimental since it can remove nutrients from the soil as the decay process continues. In addition, unfinished compost may contain substances that are detrimental (i.e., phytotoxic) to seedlings or sensitive plants.

Adding compost to gardens and lawns has a number of benefits:

• Compost is essentially a soil conditioner. It enhances the structure of soil by binding soil particles together. This improves aeration and the ability of the soil to retain water and nutrients. Compost also improves drainage in clay soils and water retention in sandy soils.
• Compost improves the buffering capacity of the soil and minimizes adverse effects to plants due to extreme shifts in soil pH.
• Adding compost to soil attracts earthworrns, which aerate the soil and add additional nutrients to the soil.
• Compost can store nutrients and release them slowly for use by surrounding plants. Note that compost is not considered to be a fertilizer although it does contain low levels of essential plant nutrients.

Further Reading

The following sources are recommended for more extensive and detailed information regarding backyard composting (e.g., approximate carbon-nitrogen ratios for various yard trimmings and food scraps, suggested composting "recipes", compost bin designs, and manufacturers of home composting products):

Backyard Composting: Your Complete Guide to Recycling Yard Clippings (1992, 96 pp.) is available for $6.95 from Harmonious Technologies, P.O. Box 1865-100, Ojai, CA 93024, (805) 646-8030 (Expanded, and reissued in 1994 as: "Keep it Off the Curb: Your Complete Step-by-Step Manual for Home Compost Program Management, 218 pages)

Composting to Reduce the Waste Stream: A Guide to Small Scale Food and Yard Waste Composling ( 1991, document NRAES-43, 44 pp.) is available for $7.00 from the Northeast Regional Agricultural Engineering Service, 152 Riley-Robb Hall, Cooperative Extension, Ithaca, NY 14853, (607) 255-7654

The Rodale Book of Composting ( 1992, D. Martin and G. Gershuny, eds., 278 pp.) is available for $14.95 from the Rodale Press, 33 East Minor Street, Keystone Building, Emmaus, PA 18098, (215) 967-5171

This article was reprinted from "Solid Waste Management," the October 1993 Newsletter (vol 7, no.10) of:
The University of Illinois at Chicago
Office of Solid Waste Management (M/C 922)
School of Public Health
2121 West Taylor Street
Chicago, IL 60612-7260