Compost is meant to smell earthy, like dirt and fresh soil. Even when it’s working at its best, there should be no bad smell coming from your compost. If you notice a pungent stench coming from the pile, then something needs to be changed.
It’s not necessarily a bad thing if your compost is smelling because it’s nature’s way of saying it’s not working properly. The type of smell it gives off can even tell you what’s going on inside the pile, and by recognizing the difference between each smell, you can get your compost pile back on track.
Why Does Compost Smell Bad?
Compost is alive, full of a complex balance of bacteria and nitrogen-rich materials; it’s a working organism, and will react to different conditions. Different smells coming from your compost will indicate what reactions are going on inside; correctly identifying these can help you improve your compost.
There are many reasons why compost can smell bad. If your compost smells like rotten eggs, it indicates a lack of oxygen has led to anaerobic decomposition. If your compost smells like ammonia or sickly sweet, it suggests there are too many nitrogen-producing greens. If it smells like it is rotting, it may be because of meat or dairy products you’ve put in the compost.
Let’s take a look at these smells in detail, what they mean, and how you can improve your compost:
Why Compost Smells Like Rotten Eggs
Composting occurs when bacteria break down the material in the pile with the help of oxygen. This is called aerobic decomposition. A smell like rotten eggs indicates your organic matter is decomposing anaerobically, without oxygen. Without aerobic bacteria, the mass of airless compost produces hydrogen sulfide, which causes the smell of rotten eggs.
Anaerobic decomposition happens when the compost pile becomes compacted or because the pile has become waterlogged with excess moisture. Either way, there isn’t enough oxygen getting into the pile.
Air circulation is vital to keep your compost in good working order. Turning the pile circulates the air and allows excess water to evaporate while introducing more oxygen. Gardeners who notice this eggy smell also notice it deteriorates when the pile is turned, and water is given a chance to drain.
Another way to aerate your compost bin is to use pine needles. Some believe these will make your compost pile more acidic, but that isn’t correct – or at least, any changes are short-term only. Pine needles are excellent at getting more air into every layer of the pile because they do not compact. So, next time you smell rotten eggs, try turning it over and adding some pine needles to your compost bin to improve the balance of oxygen.
Why Compost Smells Sickly Sweet
A sickly sweet odor in your compost can be caused by not enough oxygen leading to anaerobic decomposition or an overabundance of grass clippings. To maintain the good, earthy smell compost should have, the bacteria inside need a ready supply of oxygen. The solution is often to turn your compost over more regularly or to add more brown material with your grass cuttings.
Just like a rotten egg smell, a sweet odor can indicate aerobic decomposition. The cause is the same, but the different smell depends on what materials are breaking down. A compost bin will also smell sickly when there is too much green material inside. It’s easily done at certain times of the year; when you have an abundance of grass clippings from mowing the lawn regularly, for example. Freshly mown grass clippings smell wonderful on the lawn, but an experienced composter knows not to throw too many on the pile.
Green materials are rich in nitrogen, so make sure that you limit the amount of green material in your compost pile to just 25%.
Why Compost Smells Like Ammonia
A smell like ammonia coming from a composting pile is likely to be caused by too many green materials in your compost leading to excess nitrogen. Green materials include grass clippings, freshly pulled weeds, kitchen scraps, and coffee grounds. You’ll need to reduce the amount of green materials or add more brown to your compost heap.
This imbalance happens when we stray too far from the suggested ratio of greens and browns. In order for effective compost to occur, there should be a ratio of around 3:1 or 4:1 carbon-rich brown materials to nitrogen-rich green materials.
Turn the pile and even spread it out to let the smell evaporate. Then, add more brown material to add more carbon and reduce the levels of nitrogen, and be sure not to add too much green material going forward.
Why Compost Pile Smells Like a Rotting Swamp
If your compost piles stink like they are rotting, they probably are. A common cause is food waste like meat and dairy that are difficult to break down and start to rot. These not only make your compost smell bad, but they also attract rodents and other undesirable pests. It is not recommended to add these items to your compost pile.
A common reason for smelly compost is that we don’t always know what should go into compost bins, particularly when we’re first starting out. It’s tempting to throw all our food scraps and kitchen waste onto the pile, but that’s when problems arise.
The kind of food waste that should be thrown onto a compost pile includes fruits and vegetables, tea bags, plant materials such as trimmings and weeds, and eggshells. What shouldn’t be included are animal products such as meat or dairy products. These materials are difficult to break down and cause the terrible, rotting smell from your pile.
Rotting meat and dairy products will also attract pests like raccoons and rats. They will be attracted to a smelly compost pile in the same way they would be to yard waste. They will also leave their own droppings on the pile, which will make the problem even worse. On that note, you should never put pet waste on a compost pile, either, as this will make the smell worse.
Compost Doesn’t Smell? Check Your Pile is Still Composting!
While some gardeners are asking “why does compost smell bad?” others are wondering why there’s absolutely no scent at all coming from their piles. One of these reasons for this is that the compost heap is no longer alive.
It might be that there are no longer any live microbes in your compost, doing the work of breaking down all the material. You’ll notice this from the way leaves are no longer decomposing after plenty of time in the bin and how even the most organic of your kitchen scraps sit on top of the mound and go moldy and rot rather than decompose.
If this is the case, then you may need to start again, or inject some much-needed nitrogen into the compost heap with the help of green materials like grass, freshly pulled weeds, and coffee grounds.
What Should Good Compost Smell Like?
Good compost should smell healthy, like dirt, without any eggy or sickly aromas. When your compost pile is hot and so is full of the microbes that are breaking down the plant matter and other garden waste and kitchen scraps, you might find that it has more of a scent, but it should only ever smell like an earthy mound of soil, and should never smell bad.
How to Stop Compost Smelling Bad
Now that you’ve identified the reasons your compost is smelling, you’ll have a better idea of how to fix it. If you’ve identified a specific smell and cause, follow the tips in the section above. If you’re not certain what the cause is, here are the best practices you can follow to keep your compost from smelling bad:
- Get the right balance of materials – Your compost pile should have a ratio of 4:1 in favor of browns over greens. Cut back on the grass clippings and up your leaves and other brown materials.
- Aerate regularly – Regular aeration ensures that oxygen can get to all areas of your compost, which ensures aerobic decomposition will happen.
- Only compost the right materials – Composting is all about building up the right kind of bacteria and avoiding the kind that will cause the compost pile to smell. Stay away from meats, dairy, greasy things like takeout food and other matter that will make your compost smell terrible and attract all the pests you can imagine.
- Good drainage is vital – No matter if you do everything else right; if you have poor drainage your compost pile will continue to have smelling issues. Waterlogged compost is one of the prime reasons for anaerobic bacteria taking hold, causing the terrible stench. Don’t compost on a concrete floor, and let excess moisture evaporate with regular turning and aeration.
- Consider a composting tumbler – If turning piles of compost is either too difficult or too much work, consider investing in something that will do the hard work for you. A composter that turns with a handle is a good way of keeping the pile aerated without having to strain your back.