What Is the Carbon-to-Nitrogen Ratio and Why Does It Matter for Composting?

The carbon-to-nitrogen ratio — often shortened to C:N ratio — is the single most important factor governing how quickly and effectively a compost heap works. It describes the proportion of carbon to nitrogen in the composting materials. Composting microorganisms need both carbon (for energy) and nitrogen (for protein building and reproduction) in a specific ratio to thrive — and when the balance is wrong, composting either stalls or becomes smelly and problematic. Understanding this ratio in practical terms, not just as a concept, is the key to consistent, productive composting.

What are "greens" and "browns"?

Greens are nitrogen-rich materials: fresh grass clippings, raw vegetable peelings, fruit scraps, coffee grounds, fresh manure, freshly cut plant material, and nettles. They are typically moist, soft, and decompose quickly on their own. Browns are carbon-rich materials: dry cardboard, straw, dry autumn leaves, wood chips, dry straw, shredded paper, and sawdust. They are dry, tough, and decompose slowly on their own. A productive compost heap needs both in roughly equal proportions to maintain the right conditions for fast, aerobic decomposition.

Reading the heap's signals

A heap that is too nitrogen-rich (too many greens) will be wet, compacted, smells of ammonia or silage, and may become slimy. Fix it by adding browns — cardboard, straw, dry leaves — and turning to reintroduce air. A heap that is too carbon-rich (too many browns) will be dry, show little sign of decomposition, and may be home to woodlice and other slow composters rather than bacteria. Fix it by adding greens — fresh grass clippings, kitchen scraps, nettles, or diluted urine — and wetting the pile if it is dry.

The ideal ratio and why it varies

The theoretical ideal C:N ratio for hot composting is around 25–30:1 carbon to nitrogen by weight. However, this is a guideline rather than a precise target — practical composting by volume (roughly equal amounts of greens and browns) achieves a good result in the vast majority of garden situations. Aim for a mix rather than a precise calculation, and use the heap's behaviour (smell, temperature, moisture) as your guide.