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The carbon-to-nitrogen (browns:greens) ratio
Almost every "my compost isn't working" problem comes down to one number: the ratio of carbon to nitrogen, written as C:N. Get it roughly right and the pile heats up, breaks down fast, and smells like a forest floor. Get it wrong and you either have a cold pile that just sits there, or a slimy, ammonia-reeking mess. The good news is you don't have to do chemistry — you just have to balance "browns" and "greens."
What the ratio actually means
The microbes that do the composting eat carbon for energy and nitrogen to build their bodies. By weight, they want roughly 30 parts carbon for every 1 part nitrogen. The practical target for a home pile is a starting mix around 25–30:1:
- Too much carbon (say 50:1 or higher) — not enough nitrogen for the microbes to grow, so the pile stays cool and decomposes slowly.
- Too much nitrogen (say 15:1 or lower) — excess nitrogen is lost as ammonia gas, which is the source of that sharp, eye-watering smell.
Browns vs. greens
You don't measure C:N directly — you mix two buckets of material. Browns are high-carbon and usually dry and woody. Greens are high-nitrogen and usually moist and fresh. Note that "green" is about chemistry, not color: coffee grounds are brown but count as a green.
| Material | Approx. C:N | Type |
|---|---|---|
| Wood chips / sawdust | 200–500:1 | Brown |
| Shredded paper / cardboard | 150–350:1 | Brown |
| Straw | 50–100:1 | Brown |
| Dry autumn leaves | 40–80:1 | Brown |
| Fruit & veg scraps | 15–25:1 | Green |
| Fresh grass clippings | 12–20:1 | Green |
| Coffee grounds | ~20:1 | Green |
| Poultry manure | 6–10:1 | Green |
The shortcut: 2–3 parts brown to 1 part green
You don't need a spreadsheet. By volume, aim for roughly 2 to 3 parts browns for every 1 part greens. That ratio reliably lands most piles in the workable 25–35:1 zone. If your pile smells bad or looks slimy, it's too "green" — add browns. If it's dry and nothing's happening, it's too "brown" (or too dry) — add greens and water.
Worked example
Say you've collected a full kitchen caddy of fruit and veg scraps — about 1 bucket of greens. To balance it toward ~30:1, you'd layer in roughly 2–3 buckets of browns: a bucket of shredded cardboard plus a couple of buckets of dry leaves works perfectly. Mix or layer them, dampen to "wrung-out sponge" moisture, and the pile should start warming within a few days. Each time you add a new caddy of scraps, top it with another 2–3x of browns — which is exactly why keeping a stash of dry leaves or shredded cardboard next to the bin makes composting effortless.
C:N is the big one, but a pile also needs moisture (damp, not soggy), oxygen (turn or aerate it), and size (about a cubic yard to hold heat). If the ratio looks right but nothing's happening, check moisture and air before adding more material.
Gear this guide uses
Balancing browns and greens is easier with a way to shred browns, a way to mix, and a way to confirm the pile is actually heating.
- Turn dry leaves and cardboard into fast-composting browns with a leaf shredder or paper shredder.
- Mix or turn the pile to spread nitrogen evenly with a compost aerator.
- Confirm the ratio is working with a long-stem compost thermometer — a balanced pile should climb into the hot range.
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Sources
- Cornell Composting — Compost Chemistry (C:N targets and the effect of getting it wrong)
- Planet Natural — carbon-to-nitrogen ratio of common materials
- MAWEB — C:N ratio compost guide and material ranges
General guidance for home gardeners. C:N values are approximate ranges — materials vary with source, age, and moisture; treat the 2–3:1 brown:green shortcut as a starting point and adjust by smell and heat.