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How to Balance Carbon and Nitrogen in Compost
Heshan Fernando
Co-founder & COO
A compost heap has two common failure modes and they look nothing alike. One smells of ammonia and attracts flies. The other sits there for eight months, cold, looking exactly like the pile of leaves it started as.
Both are usually the same problem: the ratio between carbon and nitrogen is wrong, in opposite directions.
What the ratio is doing
Composting is microbial digestion. The microorganisms doing the work need carbon for energy and nitrogen to build protein, and they consume them in roughly a 30:1 ratio by mass.
Feed them a mix near that ratio and the population grows quickly, generating heat — a working heap reaches 50 to 65°C, which is hot enough to kill most weed seeds and pathogens.
Too much nitrogen and the excess is driven off as ammonia. That’s the smell, and it’s also a waste of the nutrient you were trying to keep.
Too much carbon and the microbes are energy-rich and protein-starved. They work slowly, generate little heat, and the heap takes a year instead of three months.
Common materials sit at very different ratios: kitchen scraps around 15:1, fresh grass clippings near 20:1, dry leaves around 60:1, cardboard 350:1, sawdust upwards of 500:1. Roughly equal masses of kitchen scraps and dry leaves lands near 30:1 — which is why the “half greens, half browns” rule of thumb works reasonably well.
Why people get stuck here
- Volume versus mass. A bucket of leaves weighs almost nothing compared with a bucket of kitchen waste, so equal volumes aren’t equal masses.
- Everything called “brown”. Cardboard and dry leaves are both browns, and their ratios differ by a factor of six.
- Ratio blamed for other problems. A dry heap or a small heap won’t heat regardless of ratio.
- Adding in layers and never mixing. Stratified material doesn’t compost; the interfaces do.
What a working heap needs
A ratio between 25:1 and 30:1
Anywhere in that range works. Precision isn’t required and isn’t achievable with garden materials anyway.
Moisture like a wrung-out sponge
Damp throughout, not wet. Too dry and the microbes go dormant; too wet and the heap goes anaerobic, which is the other source of bad smells.
Enough volume
Below roughly one cubic metre a heap loses heat faster than it generates it. Small heaps still compost — slowly and cold.
Air
Turning introduces oxygen. An unturned heap will get there eventually via slower anaerobic and fungal processes, but it won’t heat and it won’t kill weed seeds.
| Problem | Cause | Fix |
|---|---|---|
| Ammonia smell | Too much nitrogen | Add browns, turn |
| Cold, nothing happening | Too much carbon, dry, or small | Add greens, water, bulk |
| Sour, wet smell | Anaerobic — too wet | Turn, add dry browns |
| Slow but not smelly | Ratio fine, heap too small | Combine into one larger heap |
Common mistakes to avoid
- Adding a whole lawn’s grass clippings in one layer, which mats down and goes anaerobic.
- Using sawdust as a general brown without realising how extreme its ratio is.
- Turning the heap constantly, which releases the heat it’s trying to build.
- Composting cooked food and meat in an open heap, which attracts animals.
- Judging progress by looking at the top rather than the middle.
How to do it with Compost Ratio Calculator
The Compost Ratio Calculator works out the mix and tells you what to add.
- Estimate or weigh what you have in greens and browns.
- Compare the resulting ratio to the 25:1 to 30:1 target.
- Add the recommended material and mix thoroughly rather than layering.
- Check moisture and heap size — ratio alone won’t heat a dry or small heap.
The Royal Horticultural Society’s composting guidance covers the practical side well. Other garden tools are in the tools directory.
Frequently asked questions
Why does my heap smell of ammonia?
Too much nitrogen. The excess is lost as ammonia gas — both the smell and a waste of nutrient. Add browns and turn the heap.
Why is nothing happening at all?
Usually too much carbon, too little moisture, or too small a heap. Below about a cubic metre, a heap can’t retain the heat it generates.
Does the ratio need to be exact?
No. Anywhere from 25:1 to 35:1 works fine. Moisture and heap size matter more than hitting a precise number.
Final thought
Get roughly into range, keep it as damp as a wrung-out sponge, and make it big enough to hold heat. Precision in the ratio is the least important of the three.