Compost troubleshooting: what the smell, the heat and the squeeze test tell you
Almost every stalled heap fails for one of four reasons, and each has a tell you can smell or feel. Work through them in order and you can fix it in an afternoon.
A compost heap that has stopped working looks exactly like a compost heap that is working slowly. Both are a pile of stuff in the corner of the yard. The difference only shows up months later, when one of them has turned into something you would happily dig into a bed and the other is still, unmistakably, an orange peel.
Nearly every stall traces back to one of four things: the mix, the moisture, the air, or the size of the heap. They have different symptoms and different fixes, and you can usually work out which one you have in the two minutes it takes to open the pile and put your hand in it. Do it in that order, because a couple of the fixes make the other problems worse if you apply them blind.
Smell it before you touch it
Odor is the fastest diagnostic you have, and it is specific enough to be genuinely useful. Lift the cover, lean in, and pay attention to what comes off the pile.
- Sharp and ammoniac, like a neglected litter tray: too much nitrogen. The heap has more green material than the microbes can use, and the surplus is escaping as ammonia gas. Add dry browns and mix.
- Sour, or like rotten eggs: the pile has gone anaerobic. Water has filled the spaces that should hold air, usually because it is too wet, too compacted, or both. Turn it and open it up.
- Almost nothing, or a faint mushroomy smell like a forest floor: the chemistry is fine. Your problem is somewhere else, and it is probably dryness, cold, or a heap built entirely out of twigs.
The mix, in rough volumes
The organisms doing the work burn carbon for energy and use nitrogen to build themselves, and they want far more of the first than the second. The number usually quoted is somewhere around twenty-five or thirty parts carbon to one part nitrogen across the whole heap. Nobody actually calculates this at the bin. In practice it comes out as two or three times the volume of dry brown material to fresh green material, layered or mixed rather than dumped in slabs.
| Dry leaves, straw, plain cardboard | Brown. High carbon, slow on its own |
|---|---|
| Grass clippings, kitchen scraps, soft weeds | Green. High nitrogen, breaks down fast |
| Coffee grounds | Green, despite the color |
| Woody prunings and twigs | Brown, and very slow unless you shred them |
| Sawdust from untreated wood | Brown, and so carbon-heavy it is easy to overdo |
Grass clippings are the classic way to get this wrong. A mower-bag's worth tipped in as a single layer mats down into a wet green slab that air cannot get through, and the middle of it goes slimy and sour while the rest of the heap sits there doing nothing. The same clippings forked through a pile of autumn leaves will heat up within a day.
Moisture, measured by hand
A working heap feels like a wrung-out sponge. Take a handful from a foot inside the pile and squeeze: you want a drop or two of water to appear at the edge of your fist, not a trickle running down your wrist, and not nothing at all. The bacteria live in the thin film of water around each particle, so a dry heap is not slow, it is stopped.
Re-wetting has to happen from the inside. Hosing the top of a dry pile mostly runs off the surface and out the sides. Fork the heap out onto a tarp, water each layer as you rebuild it, and check the middle again the next day. Going the other way, a sodden heap needs dry material forked through it rather than a drying-out period: shredded cardboard, torn-up egg boxes and dry leaves all soak up the excess and restore air spaces at the same time. If it sits under open rain, put a lid or a sheet over the top while leaving the sides open.
Air, and what turning actually does
Turning gets talked about as a chore with mystical benefits. It does three concrete things. It puts oxygen back into a pile that has used it up, it evens out wet and dry pockets, and it moves the cool, barely-touched material from the outside of the heap into the hot middle where the work happens. That last one is why an unturned heap often composts perfectly well in a core the size of a basketball and not at all in the outer six inches.
You do not need a schedule. Turn when the heap cools down, or when it smells sour, and otherwise leave it alone. If turning is not realistic, build the airflow in from the start: a base layer of coarse chopped prunings, no compaction, and enough browns that the structure holds itself open.
Size, and the honest limits of a small bin
Heat is generated throughout a pile but lost only from its surface, so small heaps shed warmth faster than they can make it. Below roughly a cubic yard, a heap will generally compost cold no matter what you feed it. Cold composting works fine. It is just slower, and it has one real consequence: it never reaches the sustained 130 to 160 degrees Fahrenheit that kills weed seeds and the root fragments of perennial weeds. Keep bindweed roots, quackgrass and anything already in flower out of a cold heap, or you will spread them with the finished compost.
The lever most people never pull is particle size. Decomposition happens on surfaces, and chopping multiplies surface area for free. A whole apple takes months. The same apple quartered takes weeks. Run the mower over your leaf pile before it goes in, cut prunings down to a few inches, and tear cardboard rather than laying it in flat sheets.
Knowing when it is finished
Done compost is dark, crumbly, smells of nothing but earth, and gives up almost no clues about what it used to be. A few surviving twigs and eggshells are normal and harmless. What matters is the stage before that: compost that is nearly there still has microbes actively finishing the job, and they will draw nitrogen out of the surrounding soil to do it, which can leave seedlings planted into it looking pale and stunted. If you are not sure, use it as a surface mulch around established plants rather than digging it in. It will finish where it lies, and nothing you care about goes hungry in the meantime.