How to Compost
Turn kitchen scraps and yard waste into finished compost in a backyard bin, with EPA's list of what to leave out, the browns-to-greens ratio, and the signs that a pile is ready to use.
You will end up with a backyard compost pile that does not smell or draw animals, and with dark, crumbly compost to work into your garden beds. Composting keeps food scraps and yard waste out of a landfill, where organic material breaks down without oxygen and generates methane, which EPA calls a powerful greenhouse gas.
EPA reports that food makes up more of what goes into US landfills than any other material, at 24.1%. Count yard trimmings, wood, and paper alongside it and those organic materials make up 51.4% of landfilled municipal solid waste. Those shares come from the 2018 landfill data that EPA's composting page charts. Wasted food on its own is responsible for 58% of landfill methane emissions, and landfills are the third largest source of human-caused methane in the country. Composting diverts that waste and turns it into a rich soil amendment for free.
Microorganisms do the breaking down. EPA lists the four things they need as browns for carbon, greens for nitrogen, water, and air, and steps 3 to 5 manage them. If you have no yard, a worm bin indoors does the same job, and FAQ 4 covers it.
A compost bin or a designated spot in your yard. You can buy a tumbling composter (easiest to turn), build a bin from pallets, or keep an open pile in a spot that meets the rules in step 1. A pitchfork or garden fork for turning the pile. A long-stemmed compost thermometer if you want a hot pile, which step 7 explains. A small container with a lid for collecting kitchen scraps between trips to the bin. A stock of browns kept beside the bin, such as a bag of dry leaves, for covering each load of scraps. A garden hose that reaches the pile.
Choose a spot, then a bin
Choose the spot first. NC State Extension says to keep the pile more than 6 feet from your house or any other wooden structure, on flat ground protected from flooding and from runoff into wells or surface water, and within reach of a garden hose. For food safety, it also says to put the pile downhill from your vegetable garden and as far from it as you can. EPA adds good drainage, easy access all year, and a spot that is not right up against a fence. University of Maryland Extension says to keep bins away from trees, because their roots grow into the compost. Shade helps a pile hold moisture, according to NC State, although EPA notes that a pile breaks down in sun or shade. Then choose the container. A three-sided bin made from pallets or wire mesh is cheap and effective. A tumbling composter sits on a frame and rotates, which makes turning easier. Maryland warns that an enclosed tumbler can quickly turn soggy and airless if you add too many kitchen scraps and not enough browns. Size decides whether the pile can ever get hot, because a pile has to be big enough to insulate itself. Maryland puts the minimum at 3 feet by 3 feet by 3 feet, which is 27 cubic feet, and says not to build a pile over 5 feet high, because the weight compacts it and cuts off the air. NC State Extension says much the same, a pile 3 feet high and at least 3 feet across so it becomes self-insulating and holds heat. Anything smaller still composts. It just composts cold, and step 7 explains why that distinction matters. For apartments or small spaces, use vermicomposting (worm composting in a small bin that fits under a sink). A countertop electric unit dries and grinds food rather than composting it, as FAQ 4 explains.
Learn what goes in and what stays out
Compostable materials fall into two categories: greens (nitrogen-rich) and browns (carbon-rich). Greens include fruit and vegetable scraps, coffee grounds and paper filters, paper tea bags, fresh grass clippings, and plant trimmings. Browns include dry leaves, straw, dried grass, and untreated wood chips, plus paper and cardboard with conditions attached. EPA's home composting guidance spells those conditions out: shredded paper should be non-glossy and uncolored, cardboard should carry no wax coating, tape, or glue, tea bags should have no staples, and eggshells go in crushed. University of Maryland Extension adds that many tea bags contain plastic. Cut those open, add the leaves, and throw the bag away. EPA's avoid list starts with meat, fish, and bones; cheese and dairy; fats, oils, and grease; and cooked food beyond small amounts. EPA marks all of these as attractants for animals. EPA also says to keep out compostable foodservice ware and compostable bags, herbicide-treated plants and grass, aggressive weeds and weeds that have gone to seed, and diseased or pest-infested plants. Its list runs on past that to treated or painted wood, pet waste and cat litter, dryer lint, glossy paper, and produce stickers. EPA gives the same reason for the food items and for compostable ware and bags. Backyard piles generally do not get hot enough to break them down fully, while commercial composting facilities can. NC State Extension adds black walnut leaves and twigs, which release substances that can harm plants, and wood ash beyond a handful or two.
Heads up: Never add meat, fish, or dairy to a home compost bin. NC State Extension says they create odors and attract rodents and flies. A commercial composting facility can take them. If your town collects food scraps, EPA says to check first that its facility accepts them.
Add two to three parts browns for every part greens
Start the pile with a 4- to 6-inch layer of bulky browns such as twigs and wood chips, which EPA says absorbs extra liquid and lets air reach the base. The ideal ratio is roughly two to three parts browns to one part greens by volume. That ratio is EPA's own, worded as adding at least two to three times the volume of browns to the volume of greens. It provides the right balance of carbon and nitrogen for the microorganisms that break everything down. Too many greens and the pile becomes slimy and smelly (excess nitrogen produces ammonia). Too many browns and the pile stays cool and breaks down slowly, because there is not enough nitrogen to fuel decomposition. Kitchen scraps arrive all year while dry leaves arrive in autumn, so keep a bag of dry leaves, shredded newspaper, or cardboard near your bin. Keep paper to no more than 10% of the pile by weight, NC State Extension says, because its low nitrogen slows decomposition. Every time you add kitchen scraps, bury them under 4 to 8 inches of dry leaves or other browns, as EPA says to. NC State says that brown top layer keeps out flies and filters odors. EPA builds the pile in layers like lasagna, while University of Maryland Extension says mixing the browns and greens usually works better. Either way works, as long as the scraps stay covered.
Keep the pile moist like a wrung-out sponge
Your compost pile needs moisture to support the microorganisms doing the work. The ideal moisture level is like a wrung-out sponge. If you squeeze a handful of compost material, a drop or two of water should come out. More than that is too wet. No moisture at all is too dry. Cover the pile with a tarp during heavy rain, which University of Maryland Extension says keeps it from getting too wet and stops nutrients from washing out. Food scraps carry their own moisture, so in wet months add extra browns to soak up the excess. In dry weather, do the squeeze test once a week, add water while you turn the pile when it comes up dry, and keep the tarp on loosely to slow evaporation, as NC State Extension advises.
Turn the pile to let air in
Turning introduces oxygen, which the decomposing organisms need to work efficiently. Without oxygen, the pile goes anaerobic (airless decomposition), which is slow and produces sulfur compounds that smell like rotten eggs. Use a garden fork or pitchfork to move material from the outside of the pile to the center, and from the center to the outside. This ensures everything gets exposed to the hot, active center of the pile where decomposition happens fastest. A tumbling composter makes this easy because you just spin it. How often depends on the pile. For a hot pile, NC State Extension says to turn it about once a week, or whenever the center feels cool to the touch, and University of Maryland Extension says to turn it before the temperature drops below 100 degrees Fahrenheit. A cold pile needs far less, and Maryland suggests once or twice a year. Turn a pile made mostly of autumn leaves in mid-November, before it freezes, and then leave it alone until spring, because Maryland says turning in winter lets too much heat escape. If you cannot turn the pile, push a broom handle into it in several places to let air in, as NC State suggests.
Fix a pile that smells, stays cold, or draws animals
If the pile smells like rotten eggs, rancid butter, or vinegar, NC State Extension says it is too wet, short of air, or carrying too much nitrogen. Turn it and mix in dry leaves, straw, or wood chips. If it smells like ammonia, add more browns. If the pile will not heat up, it is too small, too dry, or short of nitrogen. Make it larger, add water while you turn it, and mix in greens such as grass clippings or food scraps. A pile that is warm and damp in the middle but nowhere else is too small, so add more material and moisten it. If you see fruit flies, bury food scraps in the center of the pile instead of leaving them on top. If animals dig into the pile, NC State says the scraps are not covered well enough or meat or dairy went in, and FAQ 1 below covers pest-proofing the bin. A healthy compost pile should smell like earth or have no noticeable smell at all.
Harvest finished compost
Compost is ready when it looks and smells like dark, crumbly soil. You should not be able to identify any of the original materials. NC State Extension adds a temperature check, a pile less than 10 degrees warmer than the air around it. EPA puts a well-maintained pile at about 3 to 5 months and says an untended one may take a year. A hot pile is faster, and it comes with a condition. University of Maryland Extension says hot composting usually means filling the bin all at once and then adding nothing more, and that such a batch can be ready to harvest in 6 to 8 weeks. NC State also describes a hot pile that you add to as materials become available. For a hot pile, the temperature is the point rather than a side effect. EPA puts the working range at 131 to 160 degrees Fahrenheit and says pile temperatures must stay at 131 degrees or above for several days to meet the standards for reducing pathogens. NC State Extension splits that finer, with over 130 degrees killing most pathogens harmful to people and pets and over 140 degrees destroying most weed seeds. A long-stemmed compost thermometer pushed into the middle of the pile is how you find out which side of those numbers you are on. Without one, push your hand into the pile. Maryland says a pile that feels at least as hot as hot tap water is doing fine, and one that feels cool needs turning. A cold pile (one left to sit with little turning) takes about a year, according to both Maryland and NC State. NC State puts the cost plainly, saying the decay is a lot slower and that seeds and pathogens will not be destroyed. For leaves, coffee grounds, and vegetable trimmings that is perfectly fine. It is also the reason not to put diseased plants, weeds that have gone to seed, or anything you would rather not see again into a pile you never intend to heat. When the pile no longer heats up after you turn it, let it cure before you use it. Maryland gives a hot batch 2 weeks, and EPA says at least 4 weeks. NC State says to cover the curing pile and keep it slightly damp. To check your own batch, NC State suggests sowing a few radish seeds in the compost. If they sprout and grow normally, it is ready. Harvest from the bottom of the pile where material has been decomposing longest. Screen out any large pieces that have not fully broken down and toss them back into the active pile. Clemson Cooperative Extension says to work a 2- to 3-inch layer of compost into an in-ground bed. For a raised bed, it says compost should make up 10 to 20% of the total volume of soil, and that too much causes problems just as too little does. Potted plants and lawns benefit too, NC State says, but compost often does not supply everything plants need, so test the soil and fertilize to the result.
Start tonight with a lidded container for kitchen scraps. EPA says the counter, under the sink, the refrigerator, or the freezer all work. When it fills, empty it into the bin and bury it under dry leaves or shredded paper. If the pile stalls or starts to smell, step 6 matches each symptom to its fix, and FAQ 2 below covers winter, FAQ 1 pests, and FAQ 4 worm bins. When the first batch is ready, our guide on how to start a garden shows how to work it into a new bed.
Questions we get asked about this
Answers from experience, not a textbook.
EPA says a well-built, properly maintained pile should not attract pests or rodents. Rats are attracted to meat, dairy, cooked food, and exposed food scraps, so keep meat and dairy out, add cooked food only in small amounts, and bury every load of scraps under browns. For a bin, EPA says to add a lid and, where needed, a floor, with no holes or gaps larger than 1/4 inch. If animals still get into an open pile, NC State Extension says to enclose it in 1/4-inch hardware cloth.
Yes, but expect it to slow down a lot. University of Maryland Extension says composting microbes stay active in small numbers at temperatures just above freezing, and that kitchen scraps buried in garden trenches break down much more slowly from November through March. Keep adding scraps through the winter and bury each load under browns. Leave the pile unturned, because Maryland says turning in winter lets too much heat escape. Insulating the bin with straw bales or bags of leaves helps it hold heat, which is one of the fixes NC State Extension gives for a pile that will not heat up. Decomposition picks up again as the weather warms.
No. NC State Extension says not to add worms to a compost pile, because a working pile should be too hot for them, and earthworms move in on their own once it cools and cures. Worms belong in a worm bin, which FAQ 4 covers. To add microorganisms, NC State suggests tossing a handful of garden soil onto each layer as you build. If a pile has trouble heating, it lists small amounts of meal-form nitrogen sources, such as alfalfa, soybean, cottonseed, or blood meal, dusted over the greens. The fixes in step 6 come first.
Yes. Vermicomposting uses red wiggler worms in a small bin that fits under a kitchen sink or in a closet. The worms eat food scraps and bedding and turn them into vermicompost, which University of Maryland Extension calls an excellent soil conditioner and a rich source of plant nutrients. A properly maintained worm bin should not smell or attract pests, EPA says. You size the bin by surface area. Maryland puts a 2.2 square foot bin at 2 to 3 pounds of kitchen scraps a week, started with at least 1 pound of worms. Maryland says the worms work best between 55 and 75 degrees Fahrenheit and die at freezing temperatures. EPA gives 59 to 77 degrees as the range to aim for. If the bin lives in a garage or on a balcony, EPA says to insulate it with blankets or straw through the colder months. An electric countertop unit is a different machine. EPA says these appliances grind and dehydrate food scraps and produce a dried food scraps mixture rather than compost, which you can then add to a compost pile or a collection program.
Garden Starter
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How to Compost
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