Composting Basics: Material Ratios and Odor Control

How compost forms

Compost is organic matter such as vegetable peelings, fallen leaves and dry grass that microbes have broken down into something like soil. For microbes to do their work, they need the right amounts of food, air and moisture. When that balance is right, the pile turns slowly into soil with almost no smell; when it's off, it either rots and stinks or just sits there unchanged. A compost bin that smells bad usually has too little air and too much water. It's easiest to think of composting as keeping conditions comfortable for the microbes.

Browns and greens

Compost materials fall into two broad groups. Dry, crisp browns mainly supply carbon and create air pockets. Moist, fresh greens supply nitrogen and moisture and speed up decomposition. A pile of only greens turns slimy and smelly, while a pile of only browns breaks down very slowly. Think of it as covering every addition of greens with a generous amount of browns. The smaller you chop materials, the more surface the microbes can reach and the faster they break down.

• Browns: dry leaves, dry grass, shredded cardboard
• Greens: fruit and vegetable scraps, coffee grounds, grass clippings
• Generous browns, with greens always covered
• Chop large pieces before adding them

What to leave out

Small home compost often doesn't get hot enough, so leave out materials that are hard to break down or cause problems. Meat, fish, bones, dairy and greasy food easily attract strong odors, insects and animals. Diseased or pest-infested plants can carry pathogens and eggs that survive and make their way back into the garden. Weeds that have gone to seed can also leave viable seeds. Salty, heavily seasoned foods like kimchi and stew broth can build up salt in the soil that harms crops, so keep them out. Pet waste is also safer to exclude for hygiene reasons.

Starting small on a balcony

Even without a yard, you can start with a single lidded bin. Make small holes in the bottom and sides for air and drainage, and set it on a tray. Line the bottom with dry leaves or paper, and every time you add greens, cover them with browns or old soil so no food scraps are exposed. Covering well alone cuts down flies and odor dramatically. Mix it once or twice a week to work in air. When the bin is full, stop adding and let it mature, and start collecting in a second bin; that rotation is easy to manage. Worm composting is another option, but it requires its own temperature and feeding management.

Managing moisture and air

The right moisture level is when a handful squeezed in your fist feels damp like a wrung-out sponge but doesn't drip. If it's too dry, decomposition stalls, so sprinkle in a little water; if it's too wet, mix in more dry browns. Air goes in by turning. Leave the pile unmixed for too long and the bottom loses air and starts to smell sour or rotten. A large outdoor heap gets quite warm inside when decomposition is active, but a small bin doesn't heat up much, so it takes longer to finish.

When things go wrong

When compost misbehaves, the cause is almost always one of three things: the material balance, moisture or air. Read the symptom and correct the causes one at a time. Fruit flies appear most when food scraps sit uncovered, so always cover new additions and close the lid. A bit of white mold is common during decomposition and nothing to worry about.

• Rotten smell: too wet, too little air; add browns and turn
• No change: too dry or too many browns; add water and greens
• Flies: exposed scraps; cover and close the lid
• Ammonia smell: too many greens; add browns

Checking it's finished and using it

Compost is finished when you can no longer recognize the original materials, it's dark brown and it smells like forest soil. Another sign is that it no longer warms up after mixing. Mixing unfinished compost straight into pots means decomposition continues in the soil, which can damage roots or attract odors and insects, so let it mature fully first. Even finished compost isn't a replacement for soil; it's an amendment you mix into potting mix in part. Separately from home composting, the rules for disposing of food waste are set locally, so follow your area's guidance.