Gardeners have used compost for centuries. When materials such asleaves and grass clippings are composted, a microbial processconverts plant wastes to a more usable organic amendment. Grassclippings and leaves can be hauled to municipal or countycomposting facilities as one means of disposal. However, manyhomeowners may find it more convenient and economical to compostthese materials in their own backyards. In either case, thefinished compost can be used as a soil amendment or mulch toimprove most soils for gardens, landscape beds, lawn preparationor even as 15% of a potting medium. This leaflet has been writtento provide guidelines on how to build and maintain a compost pile.
Decomposition of organic material in the compost pile is dependenton maintaining microbial activity. Any factor which slows orhalts microbial growth will also impede the composting process. Efficient decomposition will occur if the following factors areused to fullest advantage.
AERATION: Oxygen is required for microbes to efficientlydecompose the organic wastes. Some decomposition will occur inthe absence of oxygen (anaerobic conditions); however, the processis slow and foul odors may develop. Because of the odor problem,composting without oxygen is not recommended in a residentialsetting unless the process is conducted in a fully closed system. Turning the pile once or twice a month will provide the necessaryoxygen and significantly hasten the composting process. A pilethat is not mixed may take three to four times longer before itcan be used. A well mixed compost pile will also reach highertemperatures which will help destroy weed seeds and pathogens.
MOISTURE: Adequate moisture is essential for microbial activity. A dry compost pile will not decompose efficiently. If rainfall islimited, it will be necessary to water the pile periodically tomaintain a steady decomposition rate. Enough water should beadded to completely moisten the pile, but overwatering should beavoided. Excess water can lead to anaerobic conditions. Waterthe pile so that it is damp, but does not remain soggy. Thecompost will be within the right moisture range if a few drops ofwater can be squeezed from a handful of material. If no water canbe squeezed out, the material is too dry. If water gushes fromyour hand, it is too wet.
PARTICLE SIZE: The smaller the size of organic wastes, the fasterthe compost will be ready for use. Smaller particles have muchmore surface area that can be attacked by microbes. A shreddercan be used before putting material in the pile, and is essentialif brush or sticks are to be composted. A low cost method ofreducing the size of fallen tree leaves is to mow the lawn beforeraking or run the lawn mower over leaf piles after raking. Rakedpiles should be checked to insure that they do not contain sticksor rocks which could cause injury during operation of the mower. If the mower has an appropriate bag attachment, the shreddedleaves can be collected directly. In addition to speeding up thecomposting process, shredding will initially reduce the volume ofthe compost pile.
FERTILIZER AND LIME: Microbial activity is affected by the carbonto nitrogen ratio of the organic waste. Because microbes requirea certain amount of nitrogen for their own metabolism and growth,a shortage of nitrogen will slow down the composting processconsiderably. Materials high in carbon relative to nitrogen suchas straw or sawdust will decompose very slowly unless nitrogenfertilizer is added. Tree leaves are higher in nitrogen thanstraw or sawdust but decomposition of leaves would still benefitfrom an addition of nitrogen fertilizer or components high innitrogen. Grass clippings are generally high in nitrogen and whenmixed properly with leaves will enhance decomposition. Poultrylitter, manure or blood meal can be used as organic sources ofnitrogen. Otherwise, a fertilizer with a high nitrogen analysis(10-30%) should be used. Other nutrients such as phosphorus andpotassium are usually present in adequate amounts fordecomposition.
During the initial states of decomposition organic acids areproduced, decreasing the pH. In the past, small amounts of limehave been suggested for maintaining and enhancing microbialactivity at this time. However, high rates of lime will convertammonium nitrogen to ammonia gas which will lead to the loss ofnitrogen from the pile. Research indicated that lime additionsmay hasten decomposition; however, the loss of nitrogen from thepile often offsets the benefits of lime. In general, lime is notnecessary for degradation of most yard wastes. The pH of finishedcompost is usually alkaline (pH = 7.1-7.5) without the addition oflime. If large quantities of pine needles, pine bark, orvegetable and fruit wastes are composted, additional lime may benecessary.
Many organic materials are suitable for composting. Yard wastes,such as leaves, grass clippings, straw, and non woody planttrimmings can be composted. Leaves are the dominant organic wastein most backyard compost piles. Grass clippings can be composted;however, with proper lawn management, clippings do not need to beremoved from the lawn (see North Carolina Extension BulletinCarolina Lawns, AG-69). If clippings are used, it is advisable tomix them with other yard wastes, otherwise the grass clippings maycompact and restrict airflow. Branches and twigs greater than 1/4inch in diameter should be put through a shredder/chipper. Kitchen wastes such as vegetable scraps, coffee grounds, andeggshells may also be added.
Sawdust may be added in moderate amounts if additional nitrogen isapplied. Approximately 1 lb. of actual nitrogen (6 cups ofammonium nitrate) is required for 100 lbs. of dry sawdust. Woodashes act as a lime source and if used should only be added insmall amounts (no more than 1 cup per bushel or 10 pounds per tonof compost). Ordinary black and white newspaper can be composted;however, the nitrogen content is low and will consequently slowdown the rate of decomposition. If paper is composted, it shouldnot be more than 10% of the total weight of the material in thecompost pile.
Examples of other organic materials that can be used to addnutrients to the pile include: blood meal, bone meal, livestockmanure, non-woody clippings, vegetable and flower garden refuse,hay, straw and lake plants. Livestock manure and poultry litterare nitrogen sources for composting. Approximately 100 pounds ofpoultry litter will provide 1.8 pounds of nitrogen.
Some materials may pose a health hazard or create a nuisance andtherefore should not be used to make compost. Adding human or petfeces cannot be recommended because they may transmit diseases. Meat, bones, grease, whole eggs, and dairy products should not beadded because they can attract rodents to the site. Most plantdisease organisms and weed seeds are destroyed during thecomposting process when temperatures in the center of the pilereach 150-160 F.
Although plants that have been treated with herbicides orpesticides should be avoided for composting, small amounts ofherbicide-treated plants (e.g., grass clippings) may be mixed inthe pile as long as one is careful to allow thoroughdecomposition. Ideally, clippings from lawns recently treatedwith herbicides should be left on the lawn to decompose.
Use of plastic garbage bags is perhaps the simplest way to makecompost. The bags are easy to handle, and require minimalmaintenance. To make compost using this method, 30-40 gallonplastic bags should be alternatively filled with plant wastes,fertilizer and lime. About one tablespoon of a garden fertilizerwith a high nitrogen content should be used per bag. Lime (onecup per bag) helps counteract the extra acidity caused byanaerobic composting. After filling, add about a quart of water. Close tightly. Set aside for six months to a year. Bags can beset in a basement or heated garage for better decomposition duringwinter months. Using garbage bags requires no turning oradditional water after closing. The main advantage of compostingin garbage bags is that it requires little maintenance; however,because oxygen is limited, the process is slow.
The barrel or drum composter generates compost is a relativelyshort period of time and provides an easy mechanism for turning. This method requires a barrel of at least 55 gallons with a securelid. Be sure that the barrel was not used to store toxicchemicals. Drill 6-9 rows of 1/2 inch holes over the length ofthe barrel to allow for air circulation and drainage of excessmoisture. Place the barrel upright on blocks to allow bottom aircirculation. Fill the barrel 3/4 full with organic waste materialand add about 1/4 cup of high (approximately 30%N) nitrogencontaining fertilizer. Apply water until compost is moist but notsoggy.
Every few days, turn the drum on its side and roll it around theyard to mix and aerate the compost. The lid can be removed afterturning to allow for air penetration. Ideally, the compost shouldbe ready in two to four months. The barrel composter is anexcellent choice for the city dweller with a relatively smallyard.
For larger quantities of organic waste, bin type structures arethe most practical. As an example, a circular bin can be made byusing a length of small spaced woven wire fencing and holding ittogether with chain snaps. The bin should be about three to fivefeet in diameter and at least four feet high. A stake may bedriven in the middle of the bin before adding material to helpmaintain the shape of the pile and to facilitate adding water. With this design, it is easiest to turn the composting material bysimply unsnapping the wire, moving the wire cylinder a few feet,and turning the compost back into it.
A very efficient and durable structure for fast composting is athree-chambered bin. It holds a considerable amount of compost,and allows good air circulation. The three chambered bin works onan assembly line idea, having three batches of compost in varyingstages of decomposition. The compost material is started in thefirst bin and allowed to heat up for three to five days. Next, itis turned into the middle bin for another 4-7 days, while a newbatch of material is started in the first bin. Finally, thematerial in the middle bin is turned into the last bin as finishedor nearly finished compost.
To make a three-chambered bin, it is best to use rot resistantwood such as redwood, salt treated wood or wood treated with anenvironmentally safe preservative or a combination of treated woodand metal posts. Unless the wood is treated or rot resistant, itwill decompose within a few years. Each bin should be at leastthree to five feet in each dimension to contain enough volume tocompost properly. Using removable slats in the front offerscomplete access to the contents for turning.
The compost pile should be located close to where it will be usedand where it will not interfere with activities in the yard oroffend neighbors. From the aesthetic point of view, it is best tocompost in a location screened from view of both your property andneighbor's property. Examples of good locations for the pileinclude: near the garden or between the garage and house. Do notlocate the compost pile near a well or on a slope that drains tosurface water such as a stream or a pond. The pile will do bestwhere it is protected from drying winds and in partial sunlight tohelp heat the pile. The more wind and sun the pile is exposed to,the more water it will need. Locating the pile too close totrees may also create problems as roots may grow into the bottomof the pile and make turning and handling the compost difficult.
Organic wastes, such as leaves, grass, and plant trimmings are putdown in a layer eight to ten inches deep. Coarser materials willdecompose faster if placed in the bottom layer. This layer shouldbe watered until moist, but not soggy. A nitrogen source shouldbe placed on top of this layer. Use one to two inches oflivestock manure, or a nitrogen fertilizer such as ammoniumnitrate or ammonium sulfate at a rate of one third of a cup forevery twenty five square feet of surface area. If these nitrogensources are not available, one cup of 10-10-10 fertilizer per 25square feet of surface area will also suffice. Do not usefertilizer that contains herbicide or pesticide.
About a one inch layer of soil or completed compost can be appliedon top of the fertilizer layer. One purpose of adding soil is toensure that the pile is inoculated with decomposing microbes. Theuse of soil in a compost pile should be considered optional. Inmost cases, organic yards wastes such as grass clippings or leavescontain enough microorganisms on the surface to effectdecomposition. Studies have shown that there is no advantage inpurchasing a compost starter or inoculum. One way to insure thatactivator microbes are present in the new compost is to mix insome old compost as the pile is prepared.
Most compost piles should initially be prepared in layers. Thiswill facilitate decomposition by insuring proper mixing. Eachpile ideally should be about 5 feet high. If only tree leaves areto be composted, layering may not be necessary. Fallen leaves canbe added as they are collected. Leaves should be moistened ifthey are dry and since dead leaves lack adequate nitrogen forrapid decomposition, addition of a high-nitrogen fertilizer (10-30% analysis) should be added to speed up breakdown. Approximately 5 ounces (about 1/2 cup) of 10% nitrogen fertilizershould be added for each 20 gallons of hand compressed leaves.
To prevent odors and hasten decomposition, the pile must be turnedoccasionally. Turning also exposes seeds, insect larvae, andpathogens to lethal temperatures inside the pile. Odors may ariseeither from the addition of excessive amounts of wet plantmaterials like fruits or grass clippings, or from overwatering. Aproperly mixed and adequately turned compost heap will not haveobjectionable odors. An actively decomposing pile will reachtemperatures of 130-160 F in the middle.
Reasons for the pile not heating up may be due to: too small apile, not enough nitrogen, lack of oxygen, too much or not enoughmoisture. The pile should be turned when the temperature in thecenter begins to cool. This will introduce oxygen andundecomposed material into the center and subsequently regenerateheating. The composting process is essentially complete whenmixing no longer produces heat in the pile.
Generally, a well managed compost pile with shredded materialunder warm conditions will be ready in about 2-4 months. A pileleft unattended and material not shredded may take over a year todecompose. Piles prepared in the late fall will not be ready foruse the following spring. When the compost is finished, the pilewill be about half its original size and have an earthy smell toit.
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Written by: Larry Bass, T.E. Bilderback, North Carolina Cooperative Extension Service
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