Why is my compost not breaking down?

Compost that is not breaking down is almost always short of one of four things: nitrogen, air, water, or size. A cold pile that has sat unchanged for months is usually too carbon-rich — a pile of leaves or cardboard has the carbon but almost no nitrogen for the microbes to build bodies from. Add a nitrogen-rich green and turn it.

Diagnose it by the symptom

The rest of this page works through each in turn.

Too much carbon, so nothing happens

The most common cause by a distance, and the reason most autumn leaf piles are still leaves the following autumn.

Microbes need carbon for energy and nitrogen to build cells. A pile of dry leaves, straw or cardboard has plenty of the first and almost none of the second, so the population never grows enough to generate heat.

Here is why this is so easy to get wrong. The carbon-to-nitrogen ratios of common browns are far higher than most people assume:

Figures from the Cornell Waste Management Institute composting tables. Cornell's own wording is that these are "representative ranges, not universal values", which is why the range column matters as much as the headline number.

A pile wants to land near 30:1. Cardboard at 580:1 is nearly twenty times too carbon-rich on its own — a fact that surprises people who have been told cardboard is simply "a brown".

Fix: add a nitrogen-rich green. Grass clippings, vegetable scraps, manure or alfalfa meal. Because greens are mostly water, it takes far more of them by weight than people expect.

Work the ratio out rather than guessing, because the popular shortcut of averaging the materials' C:N ratios gives badly wrong answers — the ratio has to be computed from the actual masses of carbon and nitrogen. The compost ratio calculator does that, and tells you how much of a given green would bring the pile into range.

Why does my compost smell like ammonia?

Too much nitrogen. The pile has more than the microbes can incorporate, and the surplus leaves as ammonia gas — which is both the smell and a straight loss of the nitrogen you wanted to keep.

It usually happens after a large single addition: a full mower bag of grass clippings, or fresh manure without bedding. Fresh chicken manure at 7:1 is the most concentrated common material and the easiest to overdo.

Symptoms: sharp ammonia smell, slimy texture, the pile collapsing into an airless mat.

Fix: mix in a carbon-rich brown and turn it. Shredded leaves, straw and torn cardboard all work. Turning matters as much as adding, because a nitrogen-heavy pile is usually short of air too.

A sharp ammonia smell is distinct from a sour, rotten one. Ammonia means too much nitrogen; sour means no oxygen.

Too wet and airless

Composting is aerobic. When a pile is waterlogged or compacted, the organisms that need oxygen die back and anaerobic ones take over — a different, much slower and much smellier process.

Symptoms: sour or rotten smell rather than ammonia, a sodden feel, no heat.

Fix: turn it, adding dry brown material as you go to open the structure. Cover an open pile in a wet climate. If it sits in a hollow, move it.

The target is a wrung-out sponge: damp enough that a squeezed handful holds together, dry enough that no water runs out. That is roughly 50–60% moisture.

Too dry

Less common than too wet, and it stalls a pile just as completely. Microbes live in water films around particles; below about 40% moisture they go dormant.

Symptoms: dusty, unchanged material, often with a dry crust on top.

Fix: water it while turning, layer by layer, rather than pouring water on top where it runs straight off. A pile in full sun in a dry summer may need this several times a season.

The pile is too small

A pile generates heat faster than it loses it only above a certain size, because heat escapes through the surface while heat is produced throughout the volume — and volume grows faster than surface area.

Below roughly a cubic yard (3 × 3 × 3 ft), a pile will compost, but cold and slowly.

Symptoms: a well-balanced, correctly damp pile that still never warms up.

Fix: make it bigger, or accept a cold pile. A cold pile produces perfectly good compost in a year. It just will not kill weed seeds or pathogens, so keep perennial weed roots and diseased material out of it.

Materials that stall or spoil a pile

Keep out: meat, fish, dairy and cooking oils, which attract vermin and go rancid rather than composting. Dog and cat waste, for pathogen reasons. Anything treated with a persistent herbicide — including straw and manure from treated fields, which can damage vegetables years later. Diseased plant material, unless the pile genuinely gets hot.

Be careful with: perennial weed roots and seed heads, which a cool pile will happily propagate. Wood ash, strongly alkaline and fine only in small amounts. Citrus and onion in quantity, which slow things down more than they ruin anything.

Chop everything. Microbes work on surfaces, so a whole brassica stem takes a year where a chopped one takes weeks. This is the cheapest speed improvement available.

Once it is finished, compost is the 30% in what to fill raised garden beds with, and the annual top-up that keeps a bed from slumping.