Composting for Beginners: The Ratio That Makes It Work
2 min read · Last reviewed March 2026
A compost heap that smells bad or never breaks down is missing one of four things. It is never bad luck.
Composting is a managed decomposition process, not a pile of rubbish left to rot. Microorganisms need a rough balance of carbon and nitrogen, enough moisture to stay active, and enough air to avoid turning anaerobic — which is where the smell comes from.
Browns and greens, roughly by volume
| Category | Examples | Role |
|---|---|---|
| Browns (carbon) | Dry leaves, cardboard, straw, wood chips | Energy source; adds bulk and airflow |
| Greens (nitrogen) | Grass clippings, vegetable scraps, coffee grounds | Protein source; drives fast breakdown |
A working ratio by volume is roughly two to three parts browns to one part greens. Too many greens produces a wet, smelly, slimy heap. Too many browns produces a dry heap that barely warms up and takes a year or more to finish.
Moisture: like a wrung-out sponge
Grab a handful from the middle of the heap. It should hold together and feel damp without dripping. Bone dry heaps stall completely; soggy heaps go anaerobic and smell of ammonia or rot. Adjust with water or with dry browns as needed.
Air: turn it or build it in
Turning a heap every one to two weeks reintroduces oxygen and speeds decomposition dramatically — the difference between finished compost in 6-8 weeks versus 6-12 months. A heap that is never turned still composts, just slowly, and is the lower-effort choice for anyone who does not need the compost quickly.
- Hot composting: frequent turning, careful ratios, finished in 6-8 weeks, kills most weed seeds.
- Cold composting: little or no turning, whatever is available goes in, finished in 6-12 months, some weed seeds survive.
Why does my compost smell like ammonia?
Too much nitrogen-rich green material and not enough browns. Add a generous layer of dry leaves or shredded cardboard and turn the heap.
How do I know when compost is finished?
It looks dark, crumbly, and earthy, with no recognisable food scraps, and has cooled back down to ambient temperature after its hot phase.
Run the numbers for your own project
- Composting CalculatorWhat ratio of greens to browns does my compost need?
- Raised Bed Soil CalculatorHow much soil do I need to fill my raised bed?
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- Building Soil That Actually Grows ThingsFeeding plants is a short-term fix. Feeding the soil is what makes a garden easy.
- Reviving a Tired Lawn: A Season-Long PlanA thin lawn is a symptom. Treating the symptom with more seed is why it keeps coming back.