My compost heap just sits there. What am I doing wrong?

A stalled heap almost always comes down to one of three things: the carbon-to-nitrogen ratio, moisture, or oxygen. Fix those and decomposition takes care of itself.

Ratio first. Composting organisms want roughly twenty-five to thirty parts carbon to one part nitrogen. In practice that means somewhere between two and three parts by volume of dry brown material — cardboard, dry leaves, straw, woody prunings — to one part green material such as grass clippings, kitchen scraps and fresh weeds. A heap of nothing but grass clippings goes anaerobic and slimy; a heap of nothing but autumn leaves sits inert for two years. Most stalled heaps are short of brown material, because greens are what accumulate weekly.

Moisture should be that of a wrung-out sponge. Too dry and the microbes go dormant; too wet and air is excluded. If the heap is dusty, water it as you turn. If it is sodden and smells of ammonia or sulphur, add dry browns and turn it.

Oxygen comes from turning and from structure. Turning every two to four weeks in the active phase makes an enormous difference, both by aerating and by moving the cool outer material into the hot centre. Building the base of the heap on coarse woody material helps air enter from below.

Two other factors: size and particle size. A heap smaller than roughly a cubic metre struggles to retain heat. And chopping or shredding material before adding it can halve the time to finished compost, because decomposition happens at surfaces.

Given all four conditions, a well-managed heap finishes in three to six months rather than two years.