If you’re considering composting for the first time, you might well be concerned about the smell.
This does become more of a problem if your compost heap or bin is close to your neighbors or to your own house.
Fortunately, it’s not hard to stop compost from smelling bad, and a lot of it comes down to how you construct your heap.
The tips in this article are very much aimed at outdoor bins and piles, as we have already looked at how to stop kitchen compost bins from smelling in a previous article.
Kitchen bins are usually designed to collect compost, rather than for the actual composting process, so the steps you need to take are (mostly!) quite different for a compost heap or bin.
Does all compost smell?
Yes, but it doesn’t always smell bad.
A well-constructed compost heap can smell quite pleasant – a bit like a forest floor after a light shower of rain. It certainly shouldn’t smell bad enough to be offensive.
A strong, bad smell is usually a sign that something has gone wrong.
Why control smell?
There are obvious reasons why you would want to control smell. It can annoy the neighbors, and it can attract unwanted pests such as mice and rats.
But smell also has a purpose. It can let you know that there’s a problem with the compost heap or bin – mainly that it’s switched to a form of composting that doesn’t use air.
Why does compost smell?
When compost has plenty of air (along with certain other conditions), it undergoes aerobic composting. This literally means “with air”.
Odors are minimized, and the composting process is speeded up.
The problem often occurs when there is not enough air in the compost heap. Then anaerobic composting takes place.
Anaerobic microorganisms release gases. These gases include hydrogen sulfide – which smells like rotten eggs!
9 ways to prevent smells
Once we understand the primary cause of the smell, we can take actions to prevent it.
There’s a big overlap here with constructing a good compost heap or filling a compost bin.
That’s because the steps you need to get good compost are very similar to those you need to prevent smells.
Aeration
The first step is to ensure there is enough air in your compost pile.
That doesn’t mean you have to turn it every other day – that’s a surefire way to lose interest in composting.
There are many ways to aerate your compost. These include using a compost aerator and adding bulking materials to your pile.
However, if your compost is smelly because it is too wet, turning it can be a good way to resolve the issue.
I’ve recovered a compost heap that was too wet this way. When the extra oxygen came in after turning, the compost heap heated up again, a sure sign that aerobic composting was taking place. I then used an aerator to keep oxygen in the heap until it cooled down again a couple of weeks later.
Also see: 11 Ways To Aerate Your Compost | The Role of Oxygen in Compost
Add both high-nitrogen and high-carbon materials
The bacteria we want in our compost pile need both nitrogen and carbon to thrive.
If there is not a good balance between the two, the wrong type of bacteria reproduces. These bacteria release excess nitrogen in the form of ammonia, which smells like sweat or pee.
It also affects the final quality of the compost, as it reduces the amount of nitrogen available.
To prevent this, make sure you include both high nitrogen (greens) and high carbon (brown) materials in your compost.
Do note that the words green and brown don’t refer to the color. For example, coffee grounds can be brown, but they are high in nitrogen, so are considered a ‘green’.
There’s a lot of disagreement about the exact ratio, but you don’t need to get it exactly right. Try to add it in a roughly 50:50 mix if you can. It’s also a good idea to always have some high-carbon material, such as sawdust, on hand.
If you have a lot of greens in your compost heap, and it is already smelling, adding more high-carbon materials can often resolve the issue.
Also see: The Carbon Nitrogen Ratio
Mixing or layering materials
The browns and greens also need to be fairly close to each other, so the bacteria can access both the carbon and the nitrogen.
So if you added a load of grass, followed by a load of sawdust, you could quite well get a bad smell.
Many guides advise mixing the material together. This is a good idea in theory. However, I find that when you have a lot of compost material, it adds too much time to the composting process.
If, like me, you are short of time and efficiency is important, try layering your compost instead.
For example, you might put a layer of grass down followed by a layer of sawdust. This process still makes perfectly good compost and mostly avoids odors.
Moisture
Getting moisture levels is imperative for composting.
When you have too much moisture, water enters air pores in the compost, forcing oxygen out. This causes compost to switch to anaerobic composting which, as we’ve seen, causes smells.
If you have too little moisture, the microorganisms that turn organic material into compost will go dormant, die or leave.
If you have a compost pile, it’s worth turning it at least once so you can monitor moisture levels. If it is too wet, you can then add more dry materials to soak up the liquid.
See Moisture Levels In Compost for more guidance.
Place on a well-drained site
Linked to the point above, it’s a good idea to place your compost on a site that is well-drained.
If water gathers in a pool around the bottom of the compost, it can again force air out of the bottom of the compost.
What’s more, the compost quickly uses up the oxygen in the water. The water can become stagnant, leading to further smells.
Learn more: How to Site A Compost Bin
Materials
Some compost materials are trickier than others to handle in the compost heap.
One of them is fresh grass. This is very high in nitrogen – and contains a lot of water. If you use grass, make sure you balance it with a high-carbon material.
Many guides advise against composting fish, cheese, milk, and meat.
That’s not quite correct. Pretty much anything that has been alive recently can be composted.
However, items like this are best left to the experienced/ambitious composter. In a small or cool compost heap, they can cause smells and attract pets.
Manure can also smell in the early stages of decomposition. However, as it breaks down the smell will dissipate.
Also see: Compost Materials: From The Everyday to The Bizarre
Burying compost material
If you do have smelly material (or material that could become smelly), make sure you bury it in the compost heap.
Simply dig a hole and insert the material before covering it up again.
If you have an active, well-constructed pile, and bury the material deep enough, this will usually both stop any smells and prevent pests from coming to your heap. (I’ve even done this with dead chickens!)
Using a layer of straw or mature compost
A great tip I picked up from the Humanure Handbook is to use a layer of straw around the compost.
This has a couple of benefits. It helps insulate the compost, which helps the thermophilic bacteria which break down the compost fast.
It also acts as a biofilter, helping to trap smells before they escape.
Add biochar
Biochar is a type of charcoal that is made by burning materials without oxygen.
What’s more, some research suggests that biochar can help control odors.
As biochar has other benefits for compost, it might be worth considering if you have odor problems.
My compost is already smelling – what do I do?
So far, we’ve focussed on ways to prevent a compost heap from smelling.
Many of these link into stopping a compost heap from smelling, so I’ve put these into a table for quick skimming!
| Cause | Solution |
| Compost is too wet. | Add dry, absorbent materials such as sawdust. Aerate or turn the compost. |
| Too much green material, often signified by a slimy appearance and/or a smell of ammonia. | Mix in brown materials. |
| Brown and green materials have not been mixed together. | Stir or turn the compost, ensuring the browns and greens become well mixed. |
| Pile is compacted, leading to a lack of oxygen. | Turn or aerate the compost. |
| Foods such as meat are rotting on the outside of the pile. | Bury or remove foods, or add a layer of compost, sawdust, straw or similar material. |
| Plastic sheeting is preventing air from accessing the compost. | Place sticks between the compost and the sheeting to ensure air is trapped. |
Four other options for odorless composting
So far we have focussed on controlling smells in a typical bin or outdoor pile. However, there are other options which control smell well.
Worm bins
Strictly speaking, worm bins produce worm castings rather than compost. It’s still great for your garden!
As long as you treat the system correctly (don’t overfeed) there shouldn’t be any unpleasant smell.
Bokashi bins
Bokashi bins use anaerobic organisms to digest the food you put in. They do have a smell, but this is a vinegary smell which many people find quite pleasant.
Again, they produce pre-compost rather than compost, but this can then be added to your compost or dug into your soil.
Digesters
Digesters use an anaerobic (without air) process to compost food waste. It’s slower than aerobic composting, but solutions are often designed so they don’t bother you with smells.
Electric composters
Electric composters process your food into a fertilizer. There’s a bit of controversy over where the final result is actually compost or not, but they do eliminate the problems of smells.
Play it safe!
The tips here will help to prevent odors.
Still, it’s quite normal for a compost heap to go through at least some anaerobic composting.
To play safe, it’s best to site your compost a little bit away from your house or your neighbor’s garden.
That means if something does go wrong, you won’t have to deal with the smells, and you won’t be encouraging pests into your house.