Compost that smells bad, is too wet or too dry, or isn't breaking down: the diagnosis

Print

Healthy compost doesn't ask for much: it smells like the forest floor, it shrinks week after week, and it ends up looking like rich soil. When something is wrong, it lets you know quickly, through a smell, a texture, or a pile that has stopped changing. Don't worry: almost all of these signals come down to just three things you can adjust: air, water, and the balance between green and brown materials.

If you use a worm composter, the causes are similar but the remedies are different: see our guide Bad smells in the worm bin instead.

A rotten-egg or sewer smell: the compost lacks air

This is the most common problem. The microorganisms that make good compost breathe: they need oxygen. When air stops circulating, other microorganisms that live without oxygen take over. They produce foul-smelling gases, but also methane, a powerful greenhouse gas.

Air is almost always lacking for the same reason: too much water, plus materials that are too fine and pack down. A thick layer of fresh grass clippings, a big bucket of juicy food scraps, or several weeks of rain on an open bin is enough to clog every air pocket in the mix.

What should you do?

• Turn the pile to loosen it up and let air in, with a pitchfork or an aerator such as our Brass'Compost;
• mix in coarse brown materials: wood chips from shredded branches, dead leaves, straw, torn brown cardboard;
• if the compost is really soggy, spread some of it out in the sun for a few hours, or mix in drier compost or dry soil;
• close the lid, or cover the pile with a tarp, during very rainy spells.

Then check the result with the squeeze test: a few drops should bead up between your fingers without dripping. We explain this test and how often to turn your compost in our guide Compost moisture and aeration.

Slimy, compacted compost that leaks liquid

A close cousin of the previous problem, this one shows up before you smell it: a sticky, greenish mass, sometimes with liquid pooling at the base of the composter. The most common culprit is too many fresh grass clippings: dumped in on their own in a thick layer, they form a dense mat that air can't get through.

The fix is the same: add wood chips or dead leaves, then mix well. As for grass clippings, get into the habit of spreading them out to dry in the sun before adding them, or set some aside to mulch your vegetable garden and around the base of shrubs. The right balance between "green" and "brown" is explained step by step in our guide Green and brown materials.

An ammonia smell: too much nitrogen-rich material

A sharp smell, reminiscent of certain household cleaners, signals too much nitrogen. The mix contains a lot of green materials (grass clippings, food scraps, fresh manure) and not enough carbon-rich materials to balance them. The microorganisms can't use it all, and the excess nitrogen escapes into the air as ammonia. That's fertilizer lost for your garden.

The remedy comes down to two steps: add brown materials, then mix. If you mow a lot, save the extra clippings for mulching. Another possible, more surprising cause: lime, which we cover below.

Dry, fibrous compost that has stopped changing

The opposite situation: you lift the lid and find shriveled leaves, twigs, and peels, almost intact, weeks after you added them. The mix lacks water, and often green materials too. Without moisture, the microorganisms slow down, then simply stop working.

• Water while turning, so the whole pile gets moist and not just the surface;
• add moist green materials: peels, vegetable tops from the garden, a thin layer of grass clippings;
• cut up or shred pieces that are too big, since they break down much more slowly;
• in summer, check the moisture more often.

Compost that doesn't heat up: should you worry?

Not necessarily. The explanation is simple: in a backyard composter, the heat produced escapes easily, and composting usually happens without any noticeable rise in temperature. You'll still get good compost; it just takes a little longer.

If, on the other hand, nothing seems to have changed for weeks, go through the possible causes:

• volume: a small pile or a half-empty composter doesn't hold heat well; it's better to save up your materials for a few days and add a bigger batch at once;
• the mix: too much brown slows everything down, too much green smothers the pile;
• water: too dry or too wet, the result is the same: everything grinds to a halt;
• air: compacted compost works at a snail's pace;
• the season: in winter, activity naturally slows down, and that's normal (see our guide Composting through the seasons).

One last scenario: your compost did heat up, but it no longer warms up again, even after a thorough turning. This is usually a sign that it has stabilized and is nearing the end of the process. Check its maturity with the tests in our guide How can you tell if your compost is ready?

Willy's tip

When fixing compost that is too wet, don't just spread the brown material on top: mix it in all the way down, with a pitchfork or an aerator. A dry layer sitting on a soggy mass only hides the problem.

Quick reference: symptom, cause, solution

• Rotten-egg or sewer smell: lack of air and too much water. Turn, add coarse brown material, protect from rain.
• Slimy mass, leaking liquid: too many grass clippings or too much green at once. Add wood chips, mix, dry clippings before adding them.
• Ammonia smell: too much nitrogen, or lime added. Add brown materials, skip the lime.
• Dry, fibrous, nothing happening: too much brown, not enough water. Water while turning, add green, cut up large pieces.
• Doesn't heat up: small volume, cold season, or unbalanced mix. Add bigger batches, rebalance, be patient.
• No longer warms up after turning: compost has probably stabilized. Test its maturity.

Lime: a bad idea that sounds good

You'll still read that you should add lime to compost to make it healthier. It's pointless, and even counterproductive: lime does nothing for your compost. Finished compost is already slightly alkaline, and lime encourages ammonia to be released: you lose nitrogen, and you create the very smell you were trying to avoid.

The same caution goes for wood ash: sprinkled in small amounts, it's no problem; added in large quantities, it makes the mix far too alkaline.

What about compost activators?

Let's be honest: when the air, water, and green/brown balance are right, compost works perfectly well without an activator, and a shovelful of finished compost or garden soil already brings in the helpful microorganisms. The rare cases where an activator can give things a boost are covered in our guide Compost activators: useful or not? If you'd like to try one, our Kit to make your own Bokashi & Compost activator lets you make it yourself.

Key takeaways

Strong smell: too much water or too much green, so add brown and air. Dry compost that isn't changing: too much brown, so add water and green. Either way, a good turning is often enough to get everything going again.

In summary

Compost that smells bad is usually short on air, because it's too wet or overloaded with green materials: some brown material, a good turning, and a closed lid in rainy weather take care of most cases. Dry, stalled compost needs the opposite: water and green materials. A lack of heat isn't a problem in itself in a backyard composter, as long as the volume keeps going down. Forget the lime, keep a supply of dry material close at hand, and your composter will soon smell like the forest floor again.

Related products

Maze 180L Dual-Chamber Rotating Composter

Effortless outdoor composting

$166.55 €149.17

Best Seller

Bokashi Composter Confort

3.7-gallon (14-liter) capacity, sold individually or as a set of 2

$45.59 €40.83

Preorder

245L Maze Rotating Composter with Dual Chambers

Effortless outdoor composting

$231.67 €207.50

Compostys wooden composter | 305 liters

PEFC-certified spruce wood

$129.32 €115.83

Other topics in the guide

Vermicomposting30 fact sheets →Bokashi24 fact sheets →EM microorganisms26 fact sheets →