How to compost at home

Why compost

In a healthy landscape, there's no such thing as trash, only food for the next thing. Composting puts that idea to work in your kitchen and yard.

There's a lot to work with. According to EPA figures for 2018, the most recent national data, food made up 21.6% of U.S. municipal solid waste and yard trimmings another 12.1%. Together that's about a third of what households and businesses threw away, and almost all of it can become soil.

The recipe: browns and greens

Compost needs two kinds of material. Greens are rich in nitrogen: fruit and vegetable scraps, grass clippings, coffee grounds, and crushed eggshells. Browns are rich in carbon: dry leaves, twigs, and shredded plain paper or cardboard.

Sources differ on the exact mix, so treat any ratio as a starting point. The EPA suggests at least two to three times as much brown material as green, and covering fresh food scraps with 4 to 8 inches of browns. NC State Extension suggests about two parts brown to one part green. Oregon State University puts it in chemical terms: anywhere from 20 to 40 parts carbon to 1 part nitrogen composts well. Too little carbon, and nitrogen escapes as ammonia, which is the smell of a pile gone wrong.

Hot or cold?

There are two ways to compost, and both work. A cold pile is simply added to over time. It's easy, but slow, and NC State Extension notes it won't get hot enough to kill weed seeds or pathogens.

A hot pile is built all at once and kept moist and turned. NC State recommends a pile about 3 feet wide, deep, and tall; University of Minnesota Extension says 3 to 5 feet each way. Size matters because a small pile can't hold its heat.

The temperatures are what make it worth the effort. NC State Extension says a pile needs to pass 130°F to kill most pathogens harmful to people and pets, and 140°F to destroy most weed seeds. Oregon State uses 131°F for at least three days, and warns that the cooler outer layers won't get there unless you turn them in. Past about 160°F, the heat starts killing the organisms doing the work.

Moisture and turning

A pile should feel like a wrung-out sponge, damp but not dripping. NC State puts that at about 40 to 60% moisture; Oregon State aims a little wetter.

Turning brings in air and moves cooler material to the hot center. NC State suggests turning a hot pile weekly; University of Minnesota Extension says once or twice a month is enough for a more relaxed pile.

What to leave out

Some things don't belong in a home pile. The EPA, NC State, and University of Minnesota all advise keeping out meat, fish, bones, dairy, fats, and grease, which attract animals and cause odors.

• Pet waste, which can carry parasites and disease.
• Diseased plants and weeds that have gone to seed.
• Treated wood and glossy paper.
• "Compostable" cups and bags, which the EPA notes backyard piles generally don't get hot enough to break down.

How long it takes, and how to tell it's done

University of Minnesota Extension says a well-managed pile can be ready in two to four months in warm weather, while an untended one may take a year or more. The EPA gives a similar range.

Finished compost is dark brown or black, smells like earth, and you can't recognize what went into it. NC State says it should stop heating up and sit within about 10 degrees of the air temperature. The EPA recommends letting it cure for at least four more weeks before using it.

No yard? Try worms

Worm composting works in an apartment. The EPA and NC State recommend red wigglers, Eisenia fetida, not worms dug from the garden or bought as bait. Start with about a pound, roughly a thousand worms.

Keep the bin between about 59 and 77°F, with damp shredded paper, cardboard, or leaves for bedding. Bury food under the bedding, and skip citrus, onions, garlic, meat, and dairy. NC State notes a pound of red wigglers can eat up to two pounds of scraps a week, and the EPA says castings are ready to harvest in about three to six months.