Food · Growing
Composting is free fertilizer made from what a household already throws away. The whole process comes down to four ingredients kept roughly in balance: carbon, nitrogen, water, and air.
What this is
Composting is decomposition managed on purpose. Bacteria, fungi, and a succession of invertebrates already present on food scraps and yard waste break the material down into a stable, nutrient-rich soil amendment called humus. The gardener's job is not to add anything alive. The organisms are already there. The job is to keep the mix of carbon, nitrogen, moisture, and air in the range where those organisms work fastest.[1]
There are two broad approaches. Hot composting actively manages temperature, moisture, and turning to sustain thermophilic (heat-loving) bacteria that finish a batch in four to eight weeks. Cold composting is passive: add material as it accumulates, turn rarely or never, and wait six to eighteen months. Neither is wrong. They trade speed for effort, and the choice depends more on how much material a household generates than on any quality difference in the finished product.[2]
Finished compost improves nearly any soil. It loosens heavy clay, helps sandy soil hold water, buffers pH, and delivers a slow release of nutrients that no bagged fertilizer matches. It also reduces household waste headed for the landfill by 20 to 30 percent by weight, since food scraps and yard trimmings make up the largest fraction of what most households discard.[3]
Compost is also the starting point for several other systems on this site: it feeds worm farming, enriches container mixes, and anchors every raised bed a household builds.
The inputs
Almost any organic material will decompose, but not everything belongs in a backyard pile. The list below sorts by what works, what needs caution, and what causes problems that are harder to fix than they are to prevent.
Fruit and vegetable scraps, coffee grounds and filters, tea bags (remove staples), crushed eggshells, dry leaves, straw, hay (see caution below), shredded cardboard and newspaper (no glossy inserts), small wood chips, sawdust from untreated wood, garden trimmings, spent plants at end of season, grass clippings from untreated lawns.
Grass clippings from treated lawns (see herbicide warning below). Manure from herbivores only: horse, cow, chicken, rabbit, goat. Must be aged or hot-composted, never applied fresh to edible gardens. Citrus peels and onion skins (fine in moderation; large quantities slow decomposition). Woody branches (chip or shred first; anything thicker than a pencil decomposes too slowly otherwise).
Meat, fish, bones, and dairy (attract rodents, produce strong odors). Fats, oils, and grease. Dog and cat waste (carry pathogens not destroyed in backyard composting). Diseased plant material (unless you can sustain hot-composting temperatures). Coal or charcoal ash (contains sulfur compounds toxic to plants). Sawdust from treated, painted, or pressure-treated wood. Glossy or coated paper.
Aminopyralid, clopyralid, and picloram are broadleaf herbicides used on pastures, hayfields, and some commercial lawns. Unlike most herbicides, they do not break down during composting, and they survive digestion by grazing animals. Compost, manure, hay, or straw from treated land can carry enough residue to damage or kill tomatoes, beans, peppers, lettuce, and most other garden crops for one to three years after application.[4]
Oregon State Extension warns that these residues make their way into municipal composting systems when treated grass clippings enter the regional waste stream. Symptoms include stunted growth, cupped and elongated leaves, and misshapen fruit. By the time symptoms appear, the herbicide is already in the soil.[5]
The practical rule: if you do not know the treatment history of grass clippings, hay, straw, or manure, do not compost it. Use inputs from your own untreated property or from a source whose practices you can verify. If you purchase bagged compost, look for products that test for persistent herbicides or ask the supplier directly.
The science
Composting microbes need carbon for energy and nitrogen for building proteins. Getting that carbon-to-nitrogen ratio (C:N) close to 25:1 to 30:1 by weight is the single biggest factor in whether a pile heats up and finishes clean or sits inert and smelling. Since nobody weighs their kitchen scraps, the practical version is roughly three parts browns to one part greens by volume.[1]
Dry leaves, straw, shredded cardboard, wood chips, sawdust from untreated wood, torn newspaper, corn stalks, pine needles. Provide the energy source for microbes and the physical structure that lets air circulate through the pile.
Generally dry, brown, or woody. High C:N ratios (40:1 to 500:1).
Fresh grass clippings, vegetable and fruit scraps, coffee grounds, fresh plant trimmings, garden weeds (before they set seed), livestock manure. Higher moisture content, and the nutrient source that fuels microbial reproduction.
Generally wet, green, or fresh. Low C:N ratios (10:1 to 25:1).
Approximate values. Actual ratios vary by age, moisture, and growing conditions.[6]
High nitrogen (greens)
High carbon (browns)
Why the table matters: sawdust at 500:1 is not three parts brown to one part green. A small bucket of sawdust added to food scraps will lock nitrogen and stall the pile for months. Leaves at 60:1 are a far better default brown for most households. Use the table to calibrate: if the pile is too heavy on high-C materials like wood chips, offset with a concentrated nitrogen source like grass clippings or manure, not more food scraps.[7]
Setup
There is no single best composting setup. The right one depends on how much material a household generates, how much space is available, how much effort feels sustainable, and whether rodents are a concern in the area.[8]
Largest yards
The simplest method: pile material directly on the ground or in a wire enclosure. A three-bin system (one filling, one cooking, one finishing) is the gold standard for properties generating both yard waste and kitchen scraps, because it lets a household run continuous batches without interrupting an active pile. Minimum pile size for hot composting is about one cubic yard (3x3x3 feet).[2]
Best for: large yards with steady yard-waste volume. Not ideal where rodents are a problem.
Suburban yards
A bottomless or lidded plastic or wooden bin, typically 3 to 4 feet on each side. Keeps the pile contained and tidy, discourages casual pests, and fits in a corner of a typical suburban lot. Add material from the top, harvest finished compost from a door or hatch at the bottom. Works well for cold composting with occasional turning.
Best for: moderate kitchen and yard waste. Good balance of effort, containment, and output.
Small lots and patios
A sealed, rotating drum on a frame. Tumblers are rodent-proof, keep odors contained, and make turning easy (spin the drum instead of forking a pile). The tradeoff is capacity: most hold 5 to 10 cubic feet, too small for a full hot-composting batch. Ideally, use two tumblers, one filling while the other finishes.[8]
Best for: kitchen scraps in rodent-prone areas. Freezes solid in hard winters (zones 3-5).
No bin needed
Trench composting buries kitchen scraps directly in a garden path or fallow bed, 8 to 12 inches deep. Decomposition happens in place over two to three months; soil organisms do the work. Sheet composting (also called lasagna composting) layers browns and greens directly on the soil surface in fall, left to decompose over winter so the bed is ready for spring planting.[9]
Best for: gardeners who want to skip the bin entirely. No turning, no maintenance, no rodent access.
The methods
4-8 weeks, actively managed
Build the whole pile at once at a minimum of one cubic yard (3x3x3 feet). Below that volume, the pile cannot retain enough heat for thermophilic decomposition. A correctly assembled pile heats to 130-170°F within 24 to 36 hours. Turn every three to seven days to redistribute material and reintroduce oxygen. Keep the mix as damp as a wrung-out sponge, about 40 to 60 percent moisture by weight.[2]
The heat is not added from outside. It is generated by the metabolic activity of billions of thermophilic bacteria consuming the organic matter. When the pile cools and does not reheat after turning, the active phase is over. A curing period of two to four additional weeks stabilizes the compost before use.[1]
Key advantage: sustained heat above 130°F destroys most weed seeds and plant-disease pathogens.
6-18 months, passive
Add material as it accumulates. No need to build the whole pile at once, no minimum size, and no temperature management. Mesophilic microbes, the kind active at ordinary outdoor temperatures, do the work slowly over months rather than weeks. Turning is optional but speeds the process and helps prevent anaerobic pockets.[2]
Cold composting is how most home composters actually operate, even if they intended to hot-compost. It works well for households that generate kitchen scraps daily but do not have enough material to build a full cubic yard of pile at once. The finished product is just as nutritionally valuable as hot compost.
Key tradeoff: does not reliably kill weed seeds or pathogens. Keep diseased plants and seed-heavy weeds out.
The work
Whether hot or cold, the mechanics are the same. Layer browns and greens, keep it damp, and let air in. Here is the sequence for a hot pile built all at once. For cold composting, follow the same layering pattern but add material as it becomes available rather than in one batch.
Level ground with good drainage, convenient to both the kitchen and the garden. Partial shade keeps the pile from drying out in summer heat. Place it on bare soil, not on concrete or asphalt, so earthworms and soil organisms can migrate in from below.
Lay a 4- to 6-inch base of coarse material: small sticks, corn stalks, or straw. This creates air channels at the bottom where drainage and airflow matter most.
Add 3 to 4 inches of browns, then 1 to 2 inches of greens, then a thin sprinkle of finished compost or garden soil (introduces additional microorganisms). Repeat until the pile reaches at least 3 feet high. Water each layer so it is damp but not dripping.[10]
Finish with a layer of leaves or straw on top. This suppresses odor, discourages flies, and insulates the pile.
A correctly built hot pile heats noticeably within a day. After three to seven days (or when the center starts to cool), turn the outside material to the center. A compost thermometer is helpful but not required: if the center feels too hot to hold your hand in for more than a few seconds, it is working. Repeat turning every few days until the pile no longer reheats after being turned. Then let it cure undisturbed for two to four weeks before use.[2]
The test
Using compost that is not fully decomposed can harm plants. Immature compost ties up nitrogen in the soil as microbes continue working, starving plant roots of the nutrient they need most. It can also introduce phytotoxic organic acids. These five indicators, taken together, confirm the compost is ready:
Appearance
Dark brown to black, crumbly, and uniform in texture. No recognizable original materials except possibly large woody pieces, which can be screened out and returned to the next batch.
Smell
Earthy, like forest floor. No ammonia, no sulfur, no sour or rotten smell. If it smells like anything other than soil, it needs more time.
Temperature
Cool to the touch, near ambient air temperature. If the interior is still warm, microbial activity has not finished. Let it continue curing.
Volume
The pile has shrunk to roughly one-third to one-half of its original size. This volume reduction is normal and indicates substantial decomposition has occurred.
The bioassay (germination test)
If any doubt remains, fill a small pot with the compost and plant six to eight lettuce or bean seeds. Water normally. If the seeds germinate within a week and the seedlings grow without curling, stunting, or yellowing, the compost is mature and safe to use in the garden. If growth is poor, return the compost to the pile for further curing. This test also detects persistent herbicide contamination.[5]
What goes wrong
Composting problems are almost always ratio, moisture, air, or material problems. Diagnose by smell and texture, then apply the matching fix. Most recoveries take three to five days.
Too much nitrogen (greens) relative to carbon, often combined with excess moisture. The pile has gone partially anaerobic.
Fix: Add dry browns (shredded leaves, cardboard, straw) and turn the pile immediately to reintroduce oxygen. You may need to add as much brown material as the current pile volume to reset the ratio.
Usually too much carbon, or the material is too coarse for microbes to access quickly. Can also mean the pile is too small (under one cubic yard) to retain heat, or too dry.
Fix: Add nitrogen-rich greens (grass clippings, food scraps). Chop or shred large pieces. Water the pile until it feels like a wrung-out sponge. If the pile is undersized, add more material to reach at least 3x3x3 feet.
Decomposition needs moisture between 40 and 60 percent. Below 30 percent, microbial activity essentially stops.[6]
Fix: Water the pile thoroughly as you turn it. In summer heat, cover the top with a tarp or layer of straw to reduce evaporation. Check moisture weekly during hot, dry weather.
Exposed food scraps on the surface invite pests. Meat, dairy, and fats are especially problematic but even vegetable scraps attract rodents if left uncovered.
Fix: Always bury food waste in the center of the pile under at least 6 inches of browns. Keep meat, dairy, and fats out of a backyard pile entirely. If rodents persist, switch to an enclosed bin or tumbler.
The rhythm
March through May
The best time to start a new pile. Garden cleanup debris, early grass clippings, and overwintered leaves provide a good balance of greens and browns. Turn any pile that sat dormant over winter. Harvest finished compost from last year's batch and work it into garden beds before planting.
June through August
Peak decomposition. Warm temperatures accelerate microbial activity, but the pile dries out faster. Water it regularly. Grass clippings arrive in volume; layer them thinly with browns to prevent matting and anaerobic pockets. A summer-built hot pile can finish in as little as four weeks.
September through November
The highest-volume season. Fallen leaves are the single best brown material for composting: abundant, free, and at a C:N ratio (40-80:1) that blends well with kitchen scraps. Bag or bin extra leaves for use as browns through winter and spring when they are scarce. Shredding leaves with a mower speeds decomposition significantly.
December through February
Decomposition slows sharply below 40°F and stops below freezing. Continue adding kitchen scraps; they will not decompose but they will be in place for a fast start when temperatures rise in spring. Insulate the pile with a thick layer of leaves or straw. Do not turn: the insulating layer is more valuable intact than the marginal benefit of turning in cold weather.
Putting it to work
Once compost passes the maturity tests above, it is ready for the garden. There is no single right way to apply it, but here are the most common uses for a home gardener.
Soil amendment (new beds)
Mix 2 to 4 inches of compost into the top 6 to 8 inches of soil before planting. This is the single most effective thing a gardener can do to improve poor or compacted soil.
Top-dressing (established beds)
Spread a half-inch to one-inch layer on top of existing garden beds in spring or fall. Earthworms and rain will work it down into the root zone over time. No need to till established beds.
Potting and container mix
Blend finished compost at no more than one-third of the total volume with other ingredients (perlite, vermiculite, coconut coir). Pure compost holds too much moisture for most containers and can compact over time.
Lawn renovation
Spread a quarter-inch layer across the lawn in spring and rake it in. This improves soil structure beneath the turf without smothering the grass. Screen the compost first to remove large pieces.
Where it leads
Vermicomposting is composting's indoor, odorless cousin. A good fit when there is no yard for an outdoor pile, or as a complement to one.
Read the guide →
Finished compost is one of the best soil amendments for a new bed, mixed directly into the fill at about one-third by volume.
Read the guide →
The annual cycle that keeps a garden productive, including when and how to apply compost to beds each season.
Read the guide →