Feeding the microbes the right diet
Compost is made by bacteria and fungi breaking down organic matter. Like any living thing, they need a balanced diet: carbon for energy and nitrogen to build proteins. They work fastest when the pile contains about 25 to 30 parts of carbon for each part of nitrogen, by weight. This is the C:N ratio.
- Too much nitrogen (C:N below 20): the excess escapes as ammonia, the pile smells and turns slimy.
- Too much carbon (C:N above 40): the microbes run short of nitrogen and decomposition slows to a crawl.
Greens and browns
| Material | Typical C:N | Typical nitrogen |
|---|---|---|
| Vegetable scraps | 15 to 20 | 2 to 3% |
| Fresh grass clippings | 15 to 25 | 2 to 4% |
| Coffee grounds | 20 | 2% |
| Horse manure with bedding | 25 to 30 | 1 to 2% |
| Dry leaves | 40 to 80 | 0.5 to 1% |
| Straw | 60 to 100 | 0.5 to 0.8% |
| Cardboard | 300 to 400 | 0.1 to 0.2% |
| Wood chips | 400 to 600 | 0.1% |
Values vary with the season, the plant and its moisture, so treat them as estimates.
How the calculator works
Because the nitrogen content differs between materials, you cannot simply average the ratios. The calculator weighs each material by its nitrogen:
Browns per unit of greens = N_green × (C:N_green − target) ÷ [N_brown × (target − C:N_brown)]
Worked example
You have 50 lb of kitchen scraps and fresh grass (C:N 15, 2.5% nitrogen) and a pile of dry leaves (C:N 60, 0.8% nitrogen). For a target of 30:
- Browns per pound of greens: 2.5 × (15 − 30) ÷ [0.8 × (30 − 60)] = 1.56.
- Dry leaves to add: 78.1 lb.
- C:N of the mix: 30.0.
By volume, dry leaves are so light that this can mean three or four times as many buckets of leaves as of scraps.
Tips for a healthy pile
- Moisture: the pile should feel like a wrung-out sponge. Greens bring water; browns soak it up.
- Air: turn the pile every week or two, or build it over coarse sticks, so it does not go anaerobic and smelly.
- Size: a pile of about 3 × 3 × 3 ft holds enough heat to reach 130 to 150 °F, which kills most weed seeds.
- Shred leaves and cardboard; smaller pieces break down much faster.
- Avoid meat, dairy, oily food and pet waste in home piles, as they attract pests and may carry pathogens.
Finished compost is dark, crumbly and smells of earth, usually after two to six months in an actively managed pile.