The Usual Reasons Compost Smells
A healthy pile smells earthy, like damp soil or leaves. A sour, rotten-egg or ammonia smell points to an imbalance. Most smell problems come down to one of four things: too many wet, nitrogen-rich "greens", not enough air, too much moisture, or food scraps left exposed on the surface.
You do not need to test anything. The kind of smell gives a clue, and each cause has a simple fix.
As a general guide, a rotten-egg or sour smell usually means the pile is wet and short of air. A sharp ammonia smell usually means there is too much nitrogen-rich material. Both are usually fixable by adding browns and mixing.
Getting the Greens-to-Browns Ratio Right
"Greens" are nitrogen-rich materials such as food scraps, coffee grounds and fresh grass clippings. "Browns" are carbon-rich materials such as dry leaves, shredded cardboard, straw and sawdust. A pile that is mostly greens tends to turn wet and smelly.
The EPA recommends adding at least two to three times the volume of browns to the volume of greens. If you have been adding food scraps and coffee grounds all week without any dry material, that is a likely cause.
What Counts as a Brown
Dry leaves, shredded cardboard and non-glossy shredded paper, straw, and sawdust from untreated wood all work. Keep a bag of dry material next to the bin so adding browns becomes part of adding scraps.
What to Hold Back On
The EPA advises against putting meat, fish, bones, dairy and greasy foods in a home compost pile because they can attract animals and cause odors. If smell is already a problem, make sure none of these are in the bin.
Airflow and Turning
Compost breaks down best with oxygen. When a pile sits untouched and compacts, the center can run short of air, and the smell turns sour or sulfurous. The EPA notes that turning and mixing the pile from time to time speeds up decomposition and aerates it.
How Often to Turn
There is no single schedule. Turning every week or two is common for an active pile, and it helps most after you add a lot of fresh greens or if the pile has compacted. Use a garden fork or compost aerator to lift material from the bottom and center to the top, and break up dense clumps.
Signs Your Pile Needs Air, Not Just Time
If you find a slimy, matted layer that smells worse than the surface, the pile is compacted and short of air. Turn it thoroughly and mix in dry browns as you go. Check again after a few days and keep adjusting until the smell is earthy.
Moisture: The Sneaky Culprit
Even with a good ratio and regular turning, a pile that is too wet will smell. The EPA describes the right moisture level as the consistency of a wrung-out sponge.
Squeeze a handful from the middle of the pile. If water runs out, it is too wet: mix in dry browns and turn it. If it crumbles and nothing sticks together, it is too dry, and a little water while you turn will help. Rain is a common cause of soggy piles, so a loose cover that keeps rain out while letting air in can help.
Keeping Pests Out
Smell and pests often go together, because flies and rodents are drawn to exposed, rotting food. The EPA advises covering and burying food scraps in the pile and not adding meat, dairy or greasy foods.
Fruit flies usually breed in an exposed, wet top layer. Covering fresh additions with browns each time reduces them. A secure lid or a layer of dry material on top acts as a barrier.
What to Do Next
A smelly pile usually needs a little more air, more brown material, or less moisture. Turn it, add dry browns, and check it again in a few days. The smell is your guide: when it turns earthy, the pile is back on track.
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