Over the last ten years, I’ve only experienced a handful of instances where compost smelled really bad.
What’s more, those have usually been times when I’ve been experimenting with far-out methods. Most normal people don’t experiment with different ways to compost fish, or try different compost mixtures with black bag composting!
Of course, compost does have a specific odor. However, as we’ll see later, that can be quite pleasant.
That said, if you do have an unpleasant smell, it’s a useful indicator that something is wrong. So in this article, we’ll cover what different smells mean, how to fix problems with smelly compost – and how to prevent bad odors occurring in the future. Do note that the tips in this article are for outdoor composting. If you’re concerned about inside smells, see our guide to stopping kitchen compost bins from smelling for tips.
Does all compost smell?
There’s always some smell in a compost heap, but it isn’t usually a bad smell. I find that a well-constructed compost heap can smell quite pleasant – a bit like a forest floor after a light shower of rain. It may also have a slightly musty odor, which is normal, especially if the compost is damp.
These smells indicate that aerobic (oxygen-loving) bacteria are efficiently breaking down the organic material.
However, problems arise when the compost lacks oxygen, allowing anaerobic (without oxygen) bacteria to take over. In these conditions, you might notice unpleasant odors like rotten eggs, sweat, or even a sweet or overly strong musty smell.
Identifying compost problems by smell
If your compost is already smelling, the first thing to do is try and identify the problem. The table below matches different smells to possible causes and quick solution.
| Smells: | Cause | Quick solution |
| Like sweat or pee | The cause is a gas called ammonia, which is caused by too much nitrogen. | Add more browns (high carbon material such as sawdust) to your compost. |
| Like rotten eggs/a swamp | Hydrogen sulfide, which is caused by excess water/not enough oxygen. | Add dry materials to absorb liquid, and aerate your compost. Turning can also sometimes help if your compost is wet. |
| Like old garbage | Food scraps are sitting on the surface of the pile, or close to the top. | Bury the food scraps, or apply a layer of sawdust or mature compost. |
| Sweet | Too much nitrogen. | Again, add dry brown materials. |
| Musty | This is not necessarily a problem – it’s quite common for compost to be a little bit musty. However, a strong musty smell can be caused by too much moisture or by too many woody materials. | If damp, add more dry materials. If the heap is high in woody materials and is not damp, add green materials such as freshly mown grass. |
Solutions in-depth
Cause: Too much nitrogen
To create the perfect conditions for composting, you need to add two types of material.
- Greens: These are materials high in nitrogen, such as freshly cut grass, tea leaves, or manure. Note that not all greens are green in color. For example, coffee grounds are considered greens due to their high nitrogen levels.
- Browns: These materials have higher carbon levels when compared to greens. Examples include brown leaves, dried grass and shredded paper.
If you have an excess of greens and not enough browns, you may experience a smell reminiscent of rotten eggs or ammonia. This is a strong indicator that the compost is releasing ammonia gas.
Solution: Add more brown materials
I had this problem with an electric composter I have been trialing. Following the instructions – just throwing food waste in – led to an excessively sweet smell which my family did not appreciate!
To solve this, I simply added plenty of sawdust – after a couple of applications, the smell disappeared.
This is more easily done with a small compost bin or pile than a large one, so do take note of the prevention tips further down this article.
Learn more: The Carbon-Nitrogen Ratio Explained | C:N Ratio Tables
Cause: Lack of oxygen
One of the biggest causes of smelly compost is a lack of oxygen. When your compost runs out of oxygen, the aerobic (oxygen-loving) bacteria stop working. They are replaced by anaerobic bacteria, which don’t need oxygen to break down the material.
These anaerobic bacteria are not only slower, but they also release gases like hydrogen sulfide, which smells like rotting eggs.
Solution: Aerate the compost
The key here is to aerate the compost. You can do this by turning the compost or using a compost aerator.
Learn more: 11 Ways to Aerate Your Compost
Cause: Compost is too wet
One of the reasons compost runs out of air is excess water.
While compost needs some moisture, too much moisture forces the oxygen out of the compost pile, which again makes conditions perfect for odor-causing anaerobic bacteria.
Solutions:
Add a dry amendment: Dry materials can help absorb excess moisture. I find sawdust is ideal, but you can also use materials like shredded paper, cardboard or dry leaves.
Turn the compost: I’ve personally found that turning a damp compost heap can usually reactivate the pile, despite the moisture. The active bacteria then use up some of the moisture.
I prefer this solution for compost heaps which are older than a couple of weeks as any new brown materials will take much longer to break than the existing green materials.
Cause: Rotting food scraps
If you leave smelly food scraps on top of your compost heap, they will rot in the open. They won’t just smell – they will also attract rodents.
Solution: Bury food scraps
Make a hole, bury the food, and cover it up! The smellier the food, the deeper it needs to be buried, as pests can detect food that is buried shallowly and dig it up.
Cause: Wrong materials for your compost heap or bin
In theory, you can compost almost anything that has recently been alive.
However, if your compost heap or bin is not hot, it will not be able to compost some materials before they start smelling (or rodents are attracted).
Some materials should be treated with particular care – in particular, items like fish, guts, dairy products or uncooked meat.
Solutions
- Your immediate concern is probably to stop the smell. Try turning or aerating the compost to speed up the decomposition process. You can also add a layer of straw, sawdust, or even soil to capture and stop the smells.
- To stop this from occurring in the future, adjust the materials you add. Alternatively, learn how to get your compost hot so you can be more adventurous with what you add.
7 prevention tips for odor-free composting
Once you’ve fixed bad-smelling compost, use the following tips to prevent problems with odors in the future.
Cover your compost
Provide protection against rain to stop excess moisture from entering the pile. A closed compost bin or a shelter above your compost heap, is ideal for this. However, if that’s not feasible, a plastic cover will work too. (Ideally, place some sticks under the plastic cover to trap air in.)
Site your compost on a well-drained site
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 itself can become stagnant, leading to further smells. If possible, place the compost bin or pile on soil, which will absorb the water better. If using concrete, try to find an area which has a slight slope to allow water to drain.
Learn more: How to Site a Compost Bin
Use bulking agents for multiple benefits
Bulking agents are dry browns that create air pockets in your compost heap or bin. The oxygen in these pockets helps fast-working aerobic bacteria break down compost material with minimal odours.
They also help absorb excess moisture and their high levels of carbon can help balance out any nitrogen in your heap.
Examples of browns include sawdust, woodchips (ideally already semi-decomposed) and shredded compost. I like to add a layer of whatever bulking materials I have between layers of green material.
Learn more about bulking agents and how they can transform the compost process.
Capture smells with a bio-filtration layer (easier than it sounds)
Adding a bio-filtration layer on the top of your compost is a great way to capture any excess smells. In fact, one study found that using compost as a biofilter can reduce ammonia emissions by 80-99.9%!
This sounds complicated, but it really isn’t. Simply add a layer of materials to absorb smell. I’ve had great success using straw, but other materials commonly used include sawdust and a layer of mature compost.
Add adsorbents to your compost
You can also add adsorbents. These are materials that can bind the gases that cause smells to themselves. Adsorbents that have been tested include zeolite, biochar, and woody peat.
Zeolite is great if you can get it cheaply, as it can also act as a bulking agent. I personally avoid using peat, as it is a scarce resource and its harvesting is harmful to the environment.
Biochar is probably the best-researched compost additive. One study that looked at composting poultry litter found that adding biochar can significantly reduce gases and ammonia released. It also has many other benefits for composting.
Mix microbial agents with compost
Now we do start to get into a complicated area!
The same study linked above also found that adding certain microbes can reduce gases released. They can also help turn nitrogen from a gas form into a form that can be used by plants.
However, getting the right strains and quantities of these microbes is likely to be beyond the average home composter – not to mention adding needless complication and expense. However, there is an easier alternative…
Adding mature compost to your heap or bin
Research has shown that adding 5-10% mature compost to an existing compost heap can significantly reduce odors caused by the composting process. This may be because there is plenty of existing compost bacteria which can help speed up the composting process.
In my own systems, there’s usually some old compost at the bottom of the compost bins, as I don’t manage to scrape every last bit out. That gives me a head start, and as I add the compost pile up, I tend to add the odd spadeful of soil or compost between layers.
Use the right size heap
A huge compost heap can also cause problems. Because it is so large, it struggles to draw air into the compost heap.
Solution: Cornell University advises that the ideal height is 1-3 meters. If the heap is bigger than this, it should be taken apart. You can either turn it into multiple heaps or create windrows, which long are long lines of compost.
Other solutions for odor-free 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.
Trench composting
This has to be one of the simplest ways to compost. Simply dig a trench (or a pit), add your material, and cover it back over. I use this method in the winter, when my compost is usually not very hot, to compost tricky materials like fish guts.
As long as it is deep enough, the material will rot away without producing any smells (or at least any that you can detect!) If you can’t dig the hole deep, place a metal grid over the hole to stop animals burying into the soil.
Here’s a complete guide to trench composting.
Worm bins
Strictly speaking, worm bins produce worm castings rather than compost. Most systems won’t produce as much compost as a typical compost bin, but they are great for small amounts of waste, and are a superb soil amendment.
As long as you treat the system correctly (don’t overfeed and bury food into the bedding) there shouldn’t be any unpleasant smell. You can buy a ready made solution – or see how to make an easy one with the tutorial below.
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.
Do note, these solutions produce pre-compost rather than compost, but this can then be added to your compost or dug into your soil. You can learn more about them with our guide to Bokashi Bins.
Digesters
Digesters use an anaerobic (without air) process to compost food waste. The key with this system is that the base is dug into the ground, allowing the soil around it to absorb the smells.
It’s not the best method for producing large amounts of compost, but if your aim is just to get rid of food waste, it’s a good solution.
Unfortunately, there are limited options available, and the only one I’m familiar with is the Green Cone system.
Compost tumblers
I’m not the biggest fan of tumblers. They can be an expensive way to make a small amount of compost, and unless you get an insulated one they take longer than a good compost bin or pile. But the turning does mean you add plenty of air to the material, which helps control odors.
Electric composters
Electric composters come in two different kinds – those which grind and dehydrate food waste, and those which use heat, air and bacteria to make actual compost. I’ve used the second type and it proved surprisingly effective – but you do still need to take some care to use it properly to avoid smells.
Also see:
Conclusion
The tips here will help you both fix bad-smelling compost, and prevent future problems with odor.
If you’re nervous, remember there’s no harm in playing it safe. So if you have the room, try to site your compost away from your – and your neighbour’s – house. If you’re unable to do this, consider using a closed compost bin and/or the biofiltration level we mentioned above.
Finally, please don’t let concerns about odor put you off. Once you’ve got the hang of it, composting is a simple process which rarely causes issues with smell.
FAQs
Can you use smelly compost?
No. Smells indicate that the compost is not stable and that there are problems with it. These smells also occur during the composting process and before the compost is stable. It’s best to either fix the compost, or leave it to mature. Alternative, you can use it for trench or pit composting. Simply dig a hole, place your compost in it, and cover it back over, so it can continue the process of breaking down.
Are compost fumes toxic?
In rare cases, yes. Compost can release gases like hydrogen sulfide and Benzene, which can be harmful in enclosed spaces or in large industrial composting setups. However, for small home composting systems, especially outdoors, the risk is minimal. Always ensure your compost is well-aerated and located in a ventilated area.
Can adding essential oils or fragrances help mask compost odor?
Essential oils may alleviate odors in the short term. However, they don’t address the root cause of issues such as lack of oxygen or excess moisture. It’s best to address these problems directly by aerating the compost or adding dry, brown materials.
Can excessive use of wood chips or sawdust contribute to a specific odor?
Yes, excessive use of wood chips or sawdust can create a musty smell if the carbon-rich materials overwhelm the compost. To avoid this, balance wood chips or sawdust with nitrogen-rich greens like grass clippings or vegetable scraps. A proper balance keeps decomposition active and prevents odors.
Can compost smell attract pests or rodents?
Yes, strong smells, especially from food scraps like meat or fish, can attract pests like rats or mice. To deter pests:
- Bury food scraps deep in the compost.
- Cover with a layer of sawdust or mature compost.
- Avoid adding items like raw meat, dairy, or oily foods unless using specialized methods like trench composting.
What features should you look for in a compost bin to minimize odors?
A good compost bin for minimizing odors should include:
Drainage: A base that allows excess moisture to escape, especially if you’re composting on concrete.
Proper airflow: Look for aeration valves or vents to allow oxygen circulation.
Rain protection: A lid or cover to keep the pile from getting too wet.
Sources
Gu, W., Sun, W., Lu, Y., Li, X., Xu, P., Xie, K., Sun, L., & Wu, H. (2018). Effect of Thiobacillus thioparus 1904 and sulphur addition on odour emission during aerobic composting. Bioresource Technology, 249, 254-260. https://doi.org/10.1016/j.biortech.2017.10.025
Health and Safety Authority, (2014), Composting Information Sheet
Michel, F. C. Jr., & Reddy, C. A. (1998). Effect of Oxygenation Level on Yard Trimmings Composting Rate, Odor Production, And Compost Quality In Bench-Scale Reactors. Compost Science & Utilization, 6(4), 6-14. https://doi.org/10.1080/1065657X.1998.10701936
Park, K. J., Choi, M. H., & Hong, J. H. (2002). Control of Composting Odor Using Biofiltration. Compost Science & Utilization, 10(4), 356-362. https://doi.org/10.1080/1065657X.2002.10702098
Rosenfeld, P. E., Clark, J. J. J., Hensley, A. R., & Suffet, I. H. (2007). The use of an odour wheel classification for the evaluation of human health risk criteria for compost facilities. Water Science and Technology, 55(5), 345-357. https://doi.org/10.2166/wst.2007.197
Zhu, P., Shen, Y., Pan, X., Dong, B., Zhou, J., Zhang, W., & Li, X. (2021). Reducing odor emissions from feces aerobic composting: additives. RSC Advances, 11, 15977-15988. https://doi.org/10.1039/D1RA00355K