Greens and browns: finding the right balance in your compost (C/N ratio)

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One compost heap smells pleasantly of the woodland floor, another gives off a whiff of the sewer: the difference very often comes down to a single setting, the balance between "greens" and "browns". It is the most important step in composting, and it is much simpler than it looks.

In this guide, we explain what is meant by green and brown, what the famous carbon/nitrogen ratio is, and above all the practical rule to apply day to day, with no scales or calculator.

Green = nitrogen and moisture, brown = carbon and structure

The micro-organisms that transform your waste need two main elements: carbon, which gives them energy, and nitrogen, which allows them to multiply. Waste falls roughly into two families depending on what it provides.

Greens (nitrogen-rich)

These are fresh, soft, moist materials: peelings, fruit and vegetable scraps, coffee grounds, grass clippings, young shoots, faded flowers. They break down quickly. On their own, they compact, lack air, release liquid and end up rotting.

Browns (carbon-rich)

These are dry, hard, fibrous materials: dead leaves, straw, small shredded branches, untreated sawdust, brown cardboard, paper. They break down slowly. On their own, they stay dry and almost nothing happens.

Colour is only a guide: grass clippings dried in the sun become rather "brown", and coffee grounds, although brown, behave like a green. The right criterion is simple: fresh and moist, or dry and fibrous?

The C/N ratio, explained simply

The C/N ratio compares the amount of carbon and the amount of nitrogen contained in a material. The higher it is, the more "brown" the material. The ideal mix is generally put at around 30 to 1: roughly thirty parts carbon to one part nitrogen.

Here are some indicative values for common materials. They are ranges, as the same waste varies with its freshness and origin.

• Food scraps: 11 to 15 to 1
• Grass clippings: 15 to 25 to 1
• Manure: 10 to 30 to 1
• Fruit waste: 25 to 45 to 1
• Dead leaves: 40 to 80 to 1
• Sawdust: 100 to 750 to 1
• Paper: 200 to 800 to 1
• Wood: 200 to 1,300 to 1

You can see straight away why peelings on their own rot (not enough carbon) and why a pile of sawdust does nothing (not enough nitrogen). By mixing the two families, you naturally get closer to the right zone.

Did you know?

The C/N ratio is a tool for understanding, not for measuring out. Nobody weighs their peelings before throwing them away, and C/N is calculated by weight, whereas practical advice talks about bucket volumes. Simply remember the order of magnitude: you need noticeably more "brown" than you might think.

The practical rule: one bucket for one bucket

For everyday use, aim for a mix of roughly 50% moist waste and 50% dry waste. Put very simply, that means: one bucket of dry material for one bucket of moist waste. This is the rule we recommend to get started.

Other advice goes further towards brown: some recommend two to three volumes of brown for one volume of green, particularly to deter rodents, while others speak of 25 to 50% green material. These differences are not contradictory: very wet kitchen waste needs more brown than slightly dry grass clippings.

Our view: start with "bucket for bucket", then adjust by observing your compost. If it smells bad or becomes slimy, add brown. If it stays dry and does nothing, add green and a little water. The squeeze test will quickly tell you whether the moisture level is right.

Willy's tip

Keep a bag or bin of dry material right next to the composter: dead leaves, shredded woody material, torn cardboard. That way, every time you empty your kitchen caddy, the right step is within easy reach, without having to fetch anything.

Coarse and fine, dry and moist: the other balances

Green and brown are not everything. In practice, three mixes need to be right at the same time.

• Carbon and nitrogen: this is the green/brown balance described above.
• Moist and dry: moist waste on its own suffocates the heap, dry waste on its own stalls it. The mix keeps the moisture of a wrung-out sponge.
• Coarse and fine: coarse pieces (shredded material, small branches) create air pockets; fine materials (grass clippings, sawdust, peelings) compact easily. Too much coarse material and the compost dries out; too much fine and it clogs up.

The simplest approach is to lightly mix each new addition into the layer already in place, rather than piling up thick layers of a single type of waste. Alternating layers work on paper, but they are hard to keep up in real life.

Where to find "brown" when you are short of dry material?

In summer, or in a small garden, it is often brown material that runs short. A few ideas:

• Autumn's dead leaves: store several bags somewhere dry and they will last you all year.
• Brown cardboard: egg boxes, tubes, packaging boxes with no tape or glossy ink, torn into pieces.
• Shredded material from your hedge trimmings, or the kind some local councils make available: ask at your local recycling centre.
• Straw, and sawdust or shavings from untreated wood (never treated wood or chipboard).
• Kitchen roll and paper tissues.

In a flat or in a wormery, cardboard plays exactly the same role as a sponge and regulator: we explain this in our guide What can you feed earthworms?

Correcting an imbalance

Your compost talks to you. Here are the most common signs and how to correct them:

• Rotten egg smell, slimy and compacted material: too much green and not enough air. Add coarse brown material and mix.
• Ammonia smell: too much nitrogen, often after a big lawn mow. Work in leaves or cardboard.
• Dry, fibrous material, nothing happening: too much brown and not enough water. Add peelings or fresh grass clippings, and water lightly.
• Very slow decomposition with no smell: often too much carbon, or simply the winter cold.

A tool such as the Brass'Compost compost aerator helps work dry material into the heart of the mix without having to turn everything over. For a fuller diagnosis of smells and stalled compost, see our guide Compost that smells bad or isn't breaking down.

In summary

Greens provide nitrogen and moisture, browns provide carbon and structure. Specialists aim for a C/N ratio of around 30 to 1, but in the garden one rule is enough: one bucket of dry material for one bucket of kitchen waste, mixing large and small pieces. Always keep a stock of brown material near the composter, then adjust by smell and feel.

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