Four easy ways to fix smelly compost

Smell is one of the most powerful influences on how we understand and navigate our world, particularly when it comes to food. We’re attracted to the good, and we instinctively avoid the bad.

When it comes to a home compost pile, people can be concerned that it may result in a variety of not-necessarily-good smells wafting around. But before you write off composting in a bid for olfactory peace, know this: Healthy compost piles don’t actually smell bad. Imbalances do.  

Below, we explore the reasons for a smelly compost pile, how to identify the problem, and what to do to get your pile back into balance (and keep those odors at bay!). 

Should compost smell bad? 

Call it magic or call it science (hint: it’s science), but compost done right doesn’t actually smell bad. Of course, “bad” is a spectrum and it’s subjective. But the sour perfume of a dumpster cooking in the middle of summer is universally recognizable as bad. And let us be clear: This is not how your compost pile should smell. 

The stink that emanates from something like a dumpster or landfill is the result of what’s known as anaerobic decomposition, or decay without oxygen. When organic materials like kitchen scraps are left to stagnate, they can generate gasses with unpleasant smells, like hydrogen sulfide (rotten eggs) and putrescine (rotting meat). 

A healthy outdoor compost pile shouldn’t smell bad at all. Instead, compost generates a mellow, earthy smell, similar to that of soil you’d find on the forest floor.

4 causes of smelly compost — and how to fix them

Smells are signals: If your compost smells bad, consider it your cue to investigate. A bad-smelling pile is always the result of insufficient airflow.  There are usually four main reasons behind this: 

1. The browns and greens are out of balance.

2. There’s not enough circulation.

3. Rotting animal products are souring the pile.

4. The pile is too wet.

Here’s how to identify each issue and how to fix it. 

1. The browns and greens are out of balance.

In order to transform food scraps into the nutrient-dense soil matter known to gardeners as “black gold,” you’ll need a balanced combination of carbon-rich “brown” and nitrogen-rich “green” ingredients. That means adding a few handfuls of carbon-rich “browns,” like dry leaves, hay, wood chips, shredded newspaper, and untreated cardboard.  Greens include wet and nutrient-rich material such as food scraps, coffee grounds, and grass clippings. The brown material provides carbon and typically adds structure.  The structure stops the pile from collapsing and allows air to flow through

A compost pile that has too much green material is very often too wet with insufficient airflow to allow for aerobic composting.  When a pile goes anaerobic, or doesn’t have enough air, it starts to stink.  

The fix: Correct this problem by mixing in more brown materials, aiming for a 2:1 or 3:1 ratio of browns to greens. If 2:1 is leading to that hallmark sour smell, try bumping up your ratio to 3:1.  You can keep things balanced by adding a handful or two of leaf litter or preferred brown material every time you add food scraps to the pile.  Adding coarse browns- such as twigs or wood chips is a great way to increase the flow of air through your pile

The microbes that make the compost are a little finicky.  They don’t like things to wet but they also don’t like things too dry.  A good test to see if you have the moisture right in your pile is to grab a handful and squeeze it.  If water comes out your pile is too wet, if it won’t form a ball, it is too dry.  Too many browns can make a pile too dry and also limit the nitrogen needed for the microbes to do their work.   

2. There’s not enough airflow.

If you’re maintaining a solid balance of greens and browns but your compost pile smells like rotten eggs, odds are you have an airflow issue. Aerobic decomposition is key to the composting process. 

The fix: Circulate! Regularly turning the pile introduces oxygen to the mix, and allows beneficial microorganisms to thrive, so you’ll want to give the pile or compost drum a few turns every time you add to it. To streamline the process, you could dedicate a certain day each week to loosening things up, so you have a set routine that’ll become second nature. 

3. Rotting animal products are souring the pile. 

Meat and dairy in a pile, even if rancid, smell great to a range of critters including rats and raccoons.  That is the reason you often see guidance recommending that you leave these materials out of the pile.

The fix: Generally speaking, it’s best to leave meat and dairy out of your compost pile — unless you can pre-process them with a dehydrating device, like Mill. The Mill food recycler heats food scraps — including meat, bones, and dairy — to a high enough temperature to destroy pathogens.  It also grinds them into small particles, effectively diluting the ‘scent’ and making them less attractive to critters when you add the ground scraps to your pile. Mill’s dry grounds are shelf-stable and ready to be added to your pile at any time.

If you don’t have a Mill or another dehydrating device, your best option is to send meat and dairy to a city-run composting program, if that’s available to you. If not, your meat and dairy scraps may need to head to a landfill. But rest assured — if you’re composting everything else, you’re making a big dent in your emissions footprint. 

4. The pile is too wet.

If you’ve noticed the odors from your compost pile increasing after a particularly rainy stretch, you’re not imagining things. This is more common with open-air compost piles, rather than tumblers or closed containers, but too much moisture can compact your pile and turn things slimy, creating the same conditions as you find with too little circulation. 

The fix: The solution is two-fold: Add more brown material to create textural aeration and absorb some of the excess liquid, and give the pile a good turn to incorporate. Repeat as needed.  

If you live in a particularly wet climate, or expect a rainy season, prepare your pile for the season by either pre-emptively building (or re-building) it on top of wood pallets for better drainage, or by loosely covering it with a tarp or breathable top, ideally stretched across a frame to allow for airflow. You may even choose to do both. 

If your pile gets drenched, consider aerating to help it dry. You can lay down a tarp next to it, pitchfork everything onto the tarp, and let it dry. Then, move everything back into your bin, tumbler, or pile. 

How to avoid smelly compost

Smelly compost is an exception, not the norm, and you can prevent it with attentive cultivation and regular maintenance. Healthy compost piles of the outdoor variety require a 2:1 or 3:1 ratio of brown and green materials, good circulation through turning or tumbling, and time. Once you’ve balanced the carbon, nitrogen, and oxygen, you can sit back and let the process unfold — sans the stench.

If your food scraps are getting smelly in your kitchen compost bin — before they ever hit your pile — consider a Mill food recycler. Mill has a state-of-the-art odor filtration system that ensures there are no smells at all. It comes with activated charcoal filters that utilize two million square meters of surface area to absorb any and all odors. Even better, they’re made from upcycled coconut shells, which otherwise often go to waste.

Mill makes things even easier by pre-processing all of your food scraps before they get to your outdoor pile: first by heating food scraps to kill off pathogens, and then grinding it into coarse, dry grounds. The dry grounds are easier for microbes to break down, and easier to blend with your preferred backyard browns, speeding up the composting process overall.

1. Cornell Waste Management Institute: “Compost Chemistry.” 1996. https://compost.css.cornell.edu/chemistry.html

2. Process Sensing Technologies: “The Importance of Oxygen Measurement in Aerobic Composting.” https://www.processsensing.com/en-us/blog/oxygen_measurement_in_aerobic_composting.htm