Greens and browns: getting the mix right
Almost every compost question comes back to the mix. A heap made only of fresh, wet material turns slimy and smells. A heap made only of dry leaves and cardboard sits there for years. Most of the skill in composting is keeping a sensible balance between the two kinds of material gardeners call greens and browns, and adjusting that balance when the heap tells you something is off.
What counts as green
Greens are materials rich in nitrogen. They tend to be fresh, moist and soft, and they break down quickly. Typical greens include fruit and vegetable scraps, coffee grounds and paper filters, tea leaves, fresh grass clippings, green garden trimmings, spent annual plants and the manure of plant-eating animals such as rabbits, chickens and horses. The colour is only a rough guide. Coffee grounds are brown to look at but behave as a green, because they are rich in nitrogen and break down fast.
Greens feed the organisms that do the early, fast work in a heap. Without enough of them, a heap stays cool and slow. With too many, the extra nitrogen escapes as ammonia, which you notice as a sharp smell, and the wet material packs down and shuts out air.
What counts as brown
Browns are materials rich in carbon. They are usually dry, tough and slow to break down on their own. Typical browns include fallen leaves, straw, shredded newspaper and plain cardboard, paper towels and napkins without grease, sawdust and wood shavings from untreated wood, small twigs and woody prunings, and dried plant stalks. Browns give the organisms energy, and just as important, they give the heap structure. Their stiff pieces hold open the air spaces that keep a heap healthy.
Glossy or coated paper, cardboard with plastic layers, and wood that has been painted, stained or treated do not belong in a home heap. Neither do ashes from coal or barbecue briquettes.
Why the balance matters
Organisms use carbon for energy and nitrogen for growth, and they need far more of the first than the second. Gardening references often express this as a carbon to nitrogen ratio and suggest a target somewhere around twenty five to thirty parts carbon for every part nitrogen by weight. You do not need to weigh anything to use that idea. Because browns are light and bulky while greens are heavy and dense, a mix of roughly two to three parts browns to one part greens by volume lands in a reasonable range for most home heaps. The mix calculator on this site does that arithmetic from the buckets and bags you actually have.
Treat the ratio as a starting point, not a rule. Material varies a great deal. Fresh grass clippings are far richer in nitrogen than vegetable peelings, and straw is drier than autumn leaves that have sat in the rain. The heap itself is the best guide.
Reading the heap
A heap that smells of ammonia or rot, or feels slimy and heavy, has too many greens or too little air. Add browns, mix them through, and loosen the heap. A heap that stays dry, cool and unchanged for weeks has too many browns, too little water, or both. Add greens and water as you turn it. A heap with the right balance feels damp, smells mild and earthy, and shrinks steadily as you watch it over a few weeks.
Keeping browns on hand
The most common reason a heap goes wrong is simple: kitchen scraps arrive every day, but browns arrive in bursts. Many gardeners keep a bag or bin of dry leaves gathered in autumn beside the heap, or a box of shredded cardboard near the kitchen caddy, and add a handful of browns every time they add a layer of scraps. That habit alone fixes most smell problems before they start.
Layering or mixing
Some guides recommend building a heap in neat alternating layers of greens and browns. Layers make it easy to keep track of the mix as you build, but the organisms do not care about layers. What matters is that greens and browns end up in contact with each other. If you add a thick layer of grass clippings, fork it into the browns underneath rather than leaving it as a mat on top. If you add scraps in a pile, cover them with browns so they are not exposed to flies.
Materials to keep out
Some materials are best left out of a home heap even though they will rot. Meat, fish, bones and dairy attract animals and can smell strongly. Oily food and fats slow the heap down and also attract pests. Pet waste from dogs and cats can carry organisms that are not safe in compost used on food gardens. Diseased plants and weeds that have gone to seed can survive in a heap that never gets hot. Check your local council or extension service guidance, since some areas offer collection for these materials instead.