You’ve made your first compost pile. You wait, expecting to come back and find a beautiful brown crumbly material. Instead – it stinks!
If you haven’t been composting for long, it can easily happen. Even if you’ve been composting for a while, it’s easy to slip up and make a mistake.
On the positive side, the smell of your compost is a useful indicator.
If it smells right, it’s a sign that things are going well. If it smells wrong, though, it’s a sign that something is wrong.
What’s more, as we’ll see in this article, you can often diagnose the problem from the exact smell you are getting.
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 may also be slightly musty. 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 – usually that it’s switched to a form of composting that doesn’t use air.
Why your compost smells: 6 causes and solutions
Cause: Not enough oxygen in the pile
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 another type of bacteria which doesn’t need much oxygen to break down the material.
These anaerobic bacteria are not only slower, but they also release a gas that 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.
If you have covered the compost with a plastic sheet or tarpaulin, place some sticks between the compost and the sheet to ensure air is trapped inside.
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 smelly bacteria.
Solution:
Add a dry material to the compost pile which can help absorb excess moisture. I find sawdust is ideal, but you can also use materials like shredded paper, cardboard or dry leaves.
I’ve also found that turning the compost can help, and this can be combined with adding dry materials.
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 is considered a green due its high nitrogen levels.
- Browns: These materials have high carbon levels when compared to greens. Examples include leaves, dried grass, shredded paper, cardboard and sawdust.
If you have too many greens, and not enough browns, you may get a smell like rotten fish or urine. This is a sign that the compost is releasing ammonia.
Solution: Add more brown materials
I recently had this problem with an electric composter I have been trialing. Following the instructions – just throw food waste in – lead to a 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.
Learn more: The Carbon-Nitrogen Ratio Explained | C:N Ratio Tables
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 (although that’s mostly because pests will detect it.
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 does not have ideal conditions, it will not be able to compost some materials before they start smelling (or rodents are attracted).
Some materials should be treated with care – in particular, items like fish, guts 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, either adjust the materials you add, and stop adding potentially smelly materials. Alternatively, learn how to get your compost hot so you can be more adventurous with your food.
Cause: The compost heap is too large
A huge compost heap can also cause problems. Because they are too large, these large heaps struggle 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 long lines (windrows) of compost.
6 prevention tips for odor-free composting: From basic to advanced
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.
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.
These ensure bacteria have access to oxygen, which in turn helps reduce the chances of odor. They also help absorb moisture and, as a brown, they are a great source of carbon.
Learn more about bulking agents and how they can transform the compost process.
Capture smells with a bio-filtration layer (easier than it sounds)
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. In fact, one study found that using compost as a biofilter can reduce ammonia emissions by 80-99.9%!
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’d avoid using peat for environmental reasons.
Biochar is probably the best-researched compost additive. One study that looked at composting poultry litter found that adding biochar can reduce 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 composting – not to mention adding needless complication and expense. However, there is an easier alternative…
Adding mature compost to your heap or bin
Both soil and compost are rich in microbes – and if you’re already making your own compost, it won’t cost you anything to buy either.
Research has shown that adding 5-10% mature compost to an existing compost heap can significantly reduce odors caused by the composting process.
Reference table: Causes and solutions by smell
| Smells like | Cause | Solution |
| Sweat or pee | The cause is a gas called ammonia, which is caused by too much nitrogen. | Add more browns like sawdust, shredded paper, dry brown leaves or cardboard to your compost. |
| Rotten eggs/a swamp | Hydrogen sulfide, which is caused by excess water/not enough oxygen. | Add dry brown materials to absorb liquid, and aerate your compost. |
| Your garbage bin after it’s been sitting for a week | 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. |
Best 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.
As long as it deep enough, the material will rot away without producing any smells (or at least any that you can detect!)
Here’s a complete guide to trench composting.
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.
You can buy a ready made solution – or see how to make an easy one with our 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.
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.
Compost tumblers
I’m not the biggest fans of tumblers – many 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 that there is plenty of oxygen in, which helps control odors.
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.
Also see:
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.
FAQs
Can you use smelly compost?
If the compost is smelly, it’s usually a sign it’s not yet stable. So it’s best not to use it with plants or as mulch. However, 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?
Yes, they can be, as compost can release dangerous substances such as hydrogen sulfide and Benzene. If you have a home composting system, it is unlikely to do you harm, especially if it is outside. However, there have been incidents when large amounts of compost in enclosed spaces have released gases leading to death.
Can adding essential oils or fragrances help mask compost odor?
It’s feasible they could help in the short term. However, it is unlikely to help in the long term, as the smells will wear off or be washed away. What’s more, essential oils only mask the problem – they don’t help you fix the core of the composting problems, such as lack of oxygen or too much nitrogen.
Can excessive use of wood chips or sawdust contribute to a specific odor?
While wood chips or sawdust can be used as bulking agents, excessive use can create a woody or musty odor in the compost. It’s important to maintain a proper balance of carbon-rich materials to avoid overwhelming the compost with woody materials.
Can compost smell attract pests or rodents?
Yes, certain odors from compost can attract pests or rodents. It’s important to bury smelly food scraps and ensure proper coverage to deter unwanted visitors.
What features should you look for in a compost bin to minimize odors?
The compost bin should be designed to have good airflow while excluding rain. Some bins do this by having aeration valves at the bottom and base of the bin, which ensures air flows through the bin without sacrificing too much in the way of insulation.
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