Division of Environmental Quality Director: Kyra Moore
Compost is a natural fertilizer that can be added to soil to help retain moisture, feed plants, improve the soil around trees and shrubs and top dress lawns. Compost stimulates plant growth through time release nutrients. It also protects the landscape against weather extremes, especially drought, by keeping soils warmer in winter and cooler in summer. Organic matter conserves water by loosening clay soils and binding sandy soils. The healthy soil ecosystem fosters rapid decay of grass clippings, eventually enhancing the soil food chain that supports the wild bird population.
By using food scraps and yard waste to make compost, we keep those materials out of our landfills, where they take up valuable space and release methane, a greenhouse gas. Residents can support composting by becoming compost users, promoting resource recovery by stimulating the market for compost. Using compost rather than chemical fertilizers reduces nitrogen runoff, protecting our lakes and streams.
With these principles in mind, everyone can make excellent use of their organic yard waste.
The compost pile is really a teeming microbial farm. Bacteria start the process of decaying organic matter, breaking down plant tissue. They are also the most numerous and effective composters. Fungi and protozoans soon join the bacteria and, somewhat later in the cycle, centipedes, millipedes, beetles and earthworms do their part.
The more surface area the microorganisms have to work on, the faster the materials will decompose. It's like a block of ice in the sun - slow to melt when it's large, but melting very quickly when broken into smaller pieces. Chopping your garden wastes with a shovel or machete, or running them through a shredding machine or lawn mower, will speed the composting process.
Anything growing in your yard is potential food for these tiny decomposers. Carbon and nitrogen, from the cells of dead plants and dead microbes, fuel their activity. The microorganisms use the carbon in leaves or woodier wastes as an energy source. Nitrogen provides the microbes with the raw element of proteins to build their bodies.
Everything organic has a ratio of carbon to nitrogen (C:N) in its tissues, ranging from 500:1 for sawdust, to 15:1 for table scraps. A C:N ratio of 30:1 is ideal for the activity of compost microbes. Fresh grass clippings, with a C:N ratio of 20:1, have too much nitrogen. Brown tree leaves have too little: 40:1. The proper ratio of grass to leaves may vary but should not exceed 1 part grass for 1 part leaves. Layering can be useful in arriving at these proportions, but a complete mixing of ingredients is preferable for the composting process. Other materials, such as weeds and garden wastes, can also be used. Generally, brown materials such as fallen leaves and sawdust are high in carbon, while green materials such as grass clippings and weeds are high in nitrogen. If you save your fall leaves in bags or piles, they will provide an excellent source of carbon to mix with your grass clippings the following spring and summer. Though the C:N ratio of 30:1 is ideal for a fast, hot compost, a higher C:N ratio (i.e. 50:1) will be adequate for a slower compost.
A large compost pile will insulate itself and hold the heat of microbial activity. Its center will be warmer than its edges. Piles smaller than 3 feet cubed (27 cu. ft.) will have trouble holding this heat, while piles larger than 5 feet cubed (125 cu. ft.) don't allow enough air to reach the microbes at the center. These proportions are of importance only if your goal is a fast, hot compost.
Moisture and Aeration
Virtually all life on earth needs a certain amount of water and air to sustain itself. The microbes in the compost pile are no different. They function best when the compost materials are about as moist as a wrung-out sponge. Extremes of sun or rain can disrupt the moisture balance in your pile. Therefore, you may need to add water to your compost pile if it is too dry or cover the pile if it is receiving too much moisture. It is also important for compost materials to receive adequate aeration. Compost piles comprised primarily of grass clippings do not allow air to circulate through the pile. By mixing leaves or small limbs and twigs with your grass clippings, you provide spaces for air to circulate through the pile.
Time and Temperature
Generally speaking, the hotter the pile, the faster the composting. If you use materials with a proper C:N ratio, provide a large amount of surface area, a sufficient volume and see that moisture and aeration are adequate, you will have a hot, fast compost (hot enough to burn your hand!). If you just want to deal with your yard waste in an inexpensive, easy way, the holding unit (discussed on the inside) will serve you well.
How to Compost
Choose Your Method
Passive composting bins are simple containers or open piles for yard wastes and are the least labor and time-consuming way to compost. For building plans and ideas, see Wood and Wire Cage-Type Composting Bin - PUB0213 and Circle Compost Bin - PUB1085.
- Which Wastes?
- Non-woody yard wastes are the most appropriate.
- Place the holding unit or pile where it is most convenient. As weeds, grass clippings, leaves and harvest remains from garden plants are collected, they can be added to the unit or pile. Chopping or shredding wastes, alternating high-carbon with high-nitrogen materials, and providing adequate moisture and aeration will all speed the composting process.
- Advantages and Disadvantages:
- For yard wastes, a holding unit or pile is the simplest method. The unit or pile is portable; it can be moved to wherever needed in the garden. The method can take from six months to two years to compost organic materials, so you only need to be patient.
- Holding units can be made of circles or hardware cloth, old wooden pallets or wood and wire. Sod can also be composted with or without a holding unit by turning sections of it over, making sure there is adequate moisture and covering it with black plastic. For aesthetic reasons, the open pile should be placed in a place inconspicuous to you and your neighbors.
Active composting bins are a series of three or more bins that allow wastes to be turned on a regular schedule. Turning units are most appropriate for gardeners with a large volume of yard waste and the desire to make a high-quality compost. For building plans and ideas, see Wood and Wire Stationary 3-Bin System - PUB0214.
- Which Wastes?
- Non-woody yard wastes are appropriate.
- Alternate layers of high-carbon and high-nitrogen materials to about a 30:1 ratio. Add more nitrogen materials if the pile doesn't get hot, and keep the pile as moist as a damp sponge. Check the pile temperature regularly. When the heat decreases substantially (5 to 10 days), turn the pile into the next bin. Then make a new pile in the original bin. Repeat the process each time the pile in the first bin cools. After two weeks in the third bin, the compost should be ready for the garden use. See the Rodale Guide to Composting in your library for more information on hot composting.
- Advantages and Disadvantages:
- This method produces a high-quality compost in a short time but requires more effort and space than a holding unit.
- The unit can be built of wood, concrete blocks or a combination of wood and wires. Another type of turning unit is the barrel composter, which tumbles the wastes for aeration.
Choose Your Materials
Some food wastes that may be composted with your yard waste are vegetable and fruit scraps, coffee grounds (including the filter), tea leaves and tea bags and eggshells. Yard wastes that can be composted include:
- Grass clippings.
- Small twigs.
- Non-spreading weeds.
- Old plants/potting soil.
- Wood chips.
Some organic wastes that should not be composted include:
- Meat, fish or dairy products.
- Diseased or insect infested plants.
- Weeds gone to seed.
- Weeds that spread by runners (morning glory, quack grass, buttercup...).
- Dog and cat wastes.
- Large branches.
- Pressure treated, painted or preserved lumber.
The following chart is a guide to more efficient composting using a turning unit.
|Compost has bad odor||Not enough air||Turn the pile to loosen it.|
|The center of the pile is dry||Not enough water||Moisten materials while turning the pile.|
|Compost is damp and warm in the middle but nowhere else.||Too small||Collect more material and mix the old ingredients into a new pile.|
|The heap is damp and sweet-smelling but still will not heat up.||Lack of nitrogen||Mix in a nitrogen source like fresh manure, bloodmeal or ammonium sulfate.|
Compost can be used to enrich the flower and vegetable garden, to improve the soil around trees and shrubs, as a soil amendment for house plants and planter boxes and when screened, as part of a seed-starting mix or lawn top-dressing. Before they decompose, chipped woody wastes make excellent mulch or path material. After they decompose, these same woody wastes will add texture to garden soils.
Just as fresh organic matter can be used as a mulch, so can compost at any stage of maturity. Spread around shrubs, trees and in the garden, compost can be used as a concentrated mulch. Most people, however, believe once the time and effort has been invested to make a true compost, it is best to use it in ways other than as a mulch.
Like some wines and cheeses, compost improves with age. For a person unfamiliar with the composting process, it is often difficult to tell when compost is "cured" or ready to use. Fresh compost reacts with soils differently than well-aged compost and should be used with discretion. Uncured compost mixed directly into gardens or planter mixes can "burn" plants through a stress condition called "phytotoxicity." Fresh compost, like fresh manure, can also rob the soil of nitrogen temporarily while it finishes its curing process.
If the compost looks dark, crumbles in the hand, can be screened through a 1/2 inch screen, and has a pleasant odor, it is probably "cured." Age is not a good indication of stability since the rate of decomposition is determined by nutrient balance, mixing, moisture and aeration. Some composts from active processes are more stable at one month than many stockpiles years old. Aging a compost an additional six months even after it seems cured is a good insurance policy.
Stable compost can be blended into soil mixes and is suitable for most outdoor planting projects. It is typically mixed with other ingredients such as peat moss, shredded bark, sand, or loamy topsoil when used as an outdoor planter mix. Mixing ratios vary, but 10% compost is considered to be a minimum, 30% optimum and 50% maximum in planting shrubs and trees.
Stable and cured compost probably has its greatest value when rototilled directly into the soil. One cubic yard of compost covers 108 square feet at three inches, 216 square feet at two inches and 324 square feet at one inch. The rule of thumb is to spread compost no more than one-third the depth of the rototiller. A one-inch layer of compost should be tilled in three inches, a two-inch layer tilled in 6 inches, and a three-inch layer tilled in nine inches. Two or more passes with the tiller helps blend the compost with the topsoil and break up any clumps of material.
Nothing in this document may be used to implement any enforcement action or levy any penalty unless promulgated by rule under chapter 536 or authorized by statute.
For more information
Waste Management Program
Division of Environmental Quality
P.O. Box 176
Jefferson City, MO 65102-0176