Why Your Compost Is Not Breaking Down

Diagnose it by what the pile is doing

A stalled heap is almost always short of one of four things: nitrogen, water, air or bulk. Illinois Extension sets out the symptoms in a way that makes the diagnosis quick.

What you have	Cause	Fix
Damp, sweet-smelling, will not heat	Lack of nitrogen	Mix in fresh grass clippings, manure or fertiliser
Bad odour	Not enough air, not enough water, or too small	Turn it; add dry material if too wet
Dry in the centre	Lack of nitrogen	Moisten and turn
Warm only in the middle	Pile is too small	Add material and rebuild into a bigger pile
Large items still intact	Low surface area	Remove, chop or shred them

Work through that table before changing anything else.

The ratio behind it

Composting organisms need carbon for energy and nitrogen to build cells. The working target is a carbon-to-nitrogen ratio of about 25:1 to 30:1.

Above roughly 40:1, decomposition slows sharply — there is not enough nitrogen to process the available carbon, and the pile sits inert.

In practice this is a browns-and-greens problem. Autumn leaves, straw, cardboard and woody prunings are carbon. Grass clippings, kitchen scraps, coffee grounds and manure are nitrogen. A heap built entirely from autumn leaves will still be recognisably autumn leaves next spring.

Size matters more than people expect

Missouri Extension gives the working range: at least 3 feet by 3 feet by 3 feet, and no larger than about 5 feet cubed.

Below the minimum, a heap cannot hold the heat it generates and never gets hot. Above the maximum, air cannot reach the centre and it goes anaerobic.

This single factor accounts for a great many failed compost bins. A small plastic bin with a week’s kitchen scraps in the bottom is not a compost heap yet — it is a collection of material waiting to become one.

Air and water

Both, in moderation.

Moisture should be like a wrung-out sponge. Too dry and biology stops; too wet and it goes anaerobic, which is what produces the sour smell.

For air, Missouri’s guidance is to firm each layer as you add it “but do not compact it so much that air cannot move freely through it.” Turning is the reliable way to reintroduce air, and it is the single most effective thing you can do to a slow heap.

When slow is fine

A cold heap still composts. It just takes a year or more instead of a few months, and it will not reach temperatures that kill weed seeds or pathogens.

If you are not in a hurry and you are not composting weeds gone to seed or diseased material, a slow heap is a legitimate choice rather than a failure. Decide which you are running before you troubleshoot it.