Does Backyard Compost Smell? A Healthy Heap Smells of Earth

Good compost smells sweet and earthy. Ammonia means too much green, rotten eggs means too wet and airless — both are fixed by adding browns and turning.

By Zakaria October 12, 2026 3 min read Save to Pinterest

A green waste bin filled with fresh leafy prunings — Photo: dmoreaurh, Pixabay Content License, via Pixabay

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A healthy heap smells of earth — faintly sweet, like woodland floor, and you have to lean in to notice. Ammonia means too much green. Rotten eggs means too wet and airless. Both are fixed the same afternoon: add browns and turn it.

The short answer

A working compost heap smells faintly sweet and earthy like a woodland floor. Strong smells mean imbalance, not that composting inherently stinks. An ammonia smell means too much nitrogen-rich green material, releasing nitrogen as gas and losing it from the finished compost. A rotten-egg or sour smell means the heap is too wet and compacted and has gone anaerobic. Both are fixed by mixing in dry brown material and turning. Aim for two to three parts brown to one part green.

Healthy = faintly sweet and earthy, detectable only up close
Ammonia → too much green. You are losing nitrogen as gas
Rotten eggs or sour → too wet, compacted, anaerobic
Both fixes are the same: add dry browns, turn it, open it up
2–3 parts brown to 1 part green by volume
Over-brown is merely slow. Over-green smells. When unsure, add brown

Diagnosing by smell

This table answers the question on its own

Smell	Cause	Fix
Sweet, earthy, woodland floor	Working correctly	Nothing
Ammonia	Too much green	Browns + turn
Rotten eggs, sour	Too wet, airless	Turn, break up, add bulky browns, cover
Sickly, putrid	Meat or cooked food	Remove it; bury the rest
No smell at all, nothing happening	Too dry, or too brown	Water it; add greens

The ammonia smell is money leaving the heap

That smell is the nitrogen — released as gas rather than ending up in your compost. Fixing it quickly is not just about the neighbours.

Browns and greens

Roughly two to three parts brown to one part green, by volume. Most people under-add browns, because greens accumulate faster in a garden.

Browns (dry, carbon-rich): shredded cardboard, dry leaves, straw, shredded paper, wood shavings
Greens (wet, nitrogen-rich): grass clippings, food scraps, soft prunings, coffee grounds

When in doubt, add brown

An over-brown heap is slow. An over-green heap smells. One of those is a problem you notice from the kitchen window.

The classic failure

A bin of nothing but grass clippings. They mat into an airless sodden mass and go anaerobic within days — it is the single most reliable way to produce a heap that smells of rotten eggs.

Mix clippings with browns as they go in, or spread them to dry for a day first.

Rats

No meat, fish, dairy or cooked food in an open heap
Bury fresh scraps under at least 15 cm of other material
A closed bin with a lid and a solid base, or wire mesh underneath
Turn it regularly — rodents want somewhere undisturbed to nest

Covering it

In wet climates, yes. Rain saturating a heap is the commonest cause of it going anaerobic. A lid, tarpaulin or sheet of old carpet over the top sheds rain while letting air in from the sides.

In dry conditions the opposite applies — a heap that dries out stops working entirely. The target is damp like a wrung-out sponge, not either extreme.

Common questions

Does backyard compost smell?

A working heap smells faintly sweet and earthy, like a woodland floor, and you have to put your face near it to notice. Any strong unpleasant smell means something is out of balance rather than that composting inherently stinks. The two common faults are an ammonia smell, which means too much nitrogen-rich green material, and a rotten-egg or sour smell, which means the heap is too wet and has gone anaerobic. Both are fixable in an afternoon.

Why does my compost smell like ammonia?

Too much green material and not enough brown. Grass clippings, food scraps and soft prunings are nitrogen-rich, and when there is more nitrogen than the microbes can use it is released as ammonia gas — which is both the smell and a straight loss of the nitrogen you wanted in the finished compost. Mix in plenty of dry carbon material such as shredded cardboard, dry leaves, straw or wood shavings and turn the heap, and it clears within a few days.

Why does my compost smell like rotten eggs?

Because it has gone anaerobic — too wet, too compacted, or both — and sulphate-reducing bacteria have taken over from the organisms you want. A heap of grass clippings alone does this reliably, because they mat into an airless sodden mass. The fix is oxygen: turn it thoroughly, break up the compacted layers, mix in dry bulky brown material to hold air pockets open, and cover it if rain is keeping it saturated.

What is the right brown to green ratio?

Roughly two to three parts brown to one part green by volume is the usual working guide, though most people under-add browns because greens accumulate faster in a garden. Browns are dry and carbon-rich — cardboard, dry leaves, straw, shredded paper, wood shavings. Greens are wet and nitrogen-rich — grass clippings, food scraps, soft prunings, coffee grounds. If you are unsure, add more brown, since an over-brown heap is merely slow while an over-green one smells.

How do I stop compost attracting rats?

Keep meat, fish, dairy and cooked food out of an open heap entirely, since those are the main attractants, and bury fresh food scraps under at least fifteen centimetres of other material rather than leaving them on the surface. A closed bin with a lid and a solid base, or wire mesh underneath, makes a heap far less inviting. Turning the heap regularly also helps, because rodents want an undisturbed place to nest.

Should a compost heap be covered?

In wet climates yes, because rain saturating a heap is the single most common cause of it going anaerobic and smelling. A lid, a tarpaulin or a sheet of old carpet over the top sheds rain while letting air in from the sides. In dry conditions the opposite applies and a heap that has dried out stops working entirely, so the aim is damp like a wrung-out sponge rather than either extreme.

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