Compostagem C:N Ratio

C:N ratio: the 25–30:1 target

The microbes that break down organic matter burn carbon for energy and use nitrogen to build their proteins. Get the balance right and things move fast: the ideal carbon-to-nitrogen ratio is 25–30:1 by mass for hot composting, with the pile hitting 55–65 °C within days. Drop below 20:1 and you smell ammonia from the extra nitrogen; go past 40:1 and decomposition grinds to a halt. Browns (carbon): dry leaves 60:1, paper/cardboard 170:1, straw 80:1, sawdust 500:1. Greens (nitrogen): fresh grass clippings 20:1, animal manure 15:1, kitchen scraps 15–30:1, coffee grounds 20:1. Example: mix 1 kg dry leaves (60:1) with 1 kg grass clippings (20:1) and you average around (60+20)/2 ≈ 40:1, so toss in a little more grass or scraps to land near 30:1. If you'd rather eyeball it by volume, the usual guideline is 3 parts browns to 1 part greens.

Applications

This shows up everywhere from backyard bins to Bokashi fermentation (which cares less about C:N), organic farming, regenerative gardening, and vermicomposting, where worms do best around 20–25:1. At the other end of the scale, municipal operations track windrow C:N day by day.

FAQ

My pile smells bad. What's wrong? An ammonia smell means too much nitrogen, so mix in browns. A rotten or sulfur smell means it's gone anaerobic, so turn the pile to let oxygen back in.

Can I compost meat or dairy? Keep them out of open piles, where they draw pests and stink. Bokashi or a closed digester can deal with them instead.

How long to finished compost? With the C:N dialed in, hot composting takes 4–8 weeks. Leave it to cold composting and you're looking at 6–12 months.

Moisture matters too? Definitely. Shoot for 50–60% moisture, the point where a squeezed handful releases a couple of drops. Too dry and the microbes stall; too wet and it turns anaerobic.