Last updated on October 23rd, 2023 at 08:59 pm

You have carefully created a compost and always tried to compost properly. But now you’ve noticed that your compost smells rotten or quite musty and you’re wondering:

In this article, I’ll explain all the possible causes of stinky compost so you can eliminate them as soon as possible.

A compost stinks or smells sour when a rotting process takes place due to a lack of oxygen. During rotting, the waste on the compost is metabolized by microorganisms to produce foul-smelling end products such as butyric or acetic acid or ammonia. Most often, compost rots because it is too wet, gets too little oxygen, or is filled with the wrong waste in the wrong way.

Why Does My Compost Stink? These Are The Reasons

There are a total of four main reasons why a compost pile can stink. Within these main reasons, there are again different causes that make the compost stink.

All of these reasons and causes that can make compost smell foul are explained to you in detail below:

Too wet compost leads to stink
Probably one of the most common reasons for stinky compost is that it is far too moist or wet.

On the one hand, this can cause bacteria to feel very comfortable and multiply greatly. On the other hand, the wetness displaces the necessary oxygen in the compost, causing the waste to rot.

So if your compost has already started to rot due to too much wetness, this process will be accelerated even more, as bacteria feel very comfortable in this warm and humid mixture.

The following causes can lead to your compost being too wet:

Too much wet waste.

If you put too much wet waste on the compost, it will release the water it contains and moisten other waste. If you frequently compost wet or too wet waste, this can water down the entire compost.

Most often, too wet waste is organic waste, such as fermented fruit. But care should also be taken when composting liquids, such as cucumber water.

Also, wet grass or wet leaves in the compost may cause odors.

Incorrect construction of the compost accumulates moisture.
Over time, condensation can also lead to compost that is too wet, causing it to rot and smell.

The normal composting process sometimes produces a lot of condensation due to the heat and moisture.

The condensation should normally be able to escape from the sides of the compost through openings.

However, if the compost is close to a wall or has no air holes or openings on the sides, the condensation can also escape poorly and remains inside, causing the compost to become too wet and smell musty.

Incorrect location of the compost for too wet.

Often compost becomes too wet because it is in the wrong location.

A compost should always be in partial shade, so not in direct sun but also not completely under roof.

However, if the compost pile is placed too shady, too little sun reaches it which heats it up or causes the moisture to evaporate.

Also, the subsoil is important. The texture of the soil beneath a compost pile should be lightly sandy and loose so that seepage can drain away easily.

However, if a compost is on clayey, loamy subsoil, water can sometimes drain poorly, which can lead to waterlogging.

If your compost stands in an open area and is therefore unprotected from wind and weather, you can also cover it during the rainy season – usually in autumn and spring.

This can be done with a light tarpaulin, a reed mat, willow mat, thatched mat* or with wooden slats.

The wrong waste can also lead to a smelly compost.

As mentioned earlier, you should not compost waste that is too wet. In addition, the golden rule in composting is: the mixture makes the difference!

Specifically, this means that you should not put too much of the same material or waste in the compost.

Both dry (such as sawdust or dry leaves) and wet or moist waste (such as rotten fruit or fruit and vegetable residue or moist lawn clippings) should be balanced.

Another reason for a smelly compost can be ginkgo waste. The mirabelle-like fruits that form on the ginkgo tree in the fall contain butyric acid, which smells similar to sweat, vomit or decay.

So if you have a ginkgo tree, you’d better not compost the waste from it and take it to the nearest organics collection center.

Filling it incorrectly can lead to a foul odor in the compost
If you fill your compost incorrectly, this can also be a reason for the stench.

When filling, it is important that the compost is as loose and airy as possible and that it consists of different, alternating layers.

On the one hand, these layers should consist of waste of different moisture content, i.e. dry and rather moist waste mixed together. On the other hand, the structures of the materials should also differ, so sometimes woody, rather coarse but also fine and dense wastes should be mixed.

Especially if only very dense, fine and possibly still moist materials are composted, this can very quickly lead to a lack of oxygen and to rotting and ultimately stench in the compost.

Too little air supply and aeration can make the compost smelly

Last but not least, make sure your compost has the right location or design so that it is always permeable to air.

Most composters (except thermal composters) have vents on the sides or are made of a very coarse mesh material, like classic grid composters or plug-in composters made of wood or aluminum.

This is a good thing, because on the one hand, the living creatures that decompose the waste need sufficient oxygen and air to survive. On the other hand, the draught of air helps to dry out materials that are too moist.

So make sure that you have an air-permeable compost, and secondly, that it is free-standing enough to allow air to circulate.

What happens in the compost that makes it start to stink?

Now that you know what the causes of stinky or moldy compost are, let’s take a closer look.

Maybe you want to know what exactly happens in the compost that makes it start to stink?

Normally, the desired decomposition of waste takes place in compost with the addition of oxygen.

However, if there is too little oxygen or none at all, the desired decomposition process stops and a rotting process occurs, which causes the smell.

Putrefaction always occurs when there is too little oxygen and organic matter, i.e. your kitchen and garden waste, is nevertheless decomposed by microorganisms.

So if your compost is too wet, which usually results in a lack of oxygen, this forms the perfect basis for rot and foul odors.

This is because the end products in the rotting process are both organic matter, such as butyric or acetic acid, and inorganic matter, such as ammonia or hydrogen sulfide.

Therefore, if a compost is too wet, filled incorrectly, and has too little oxygen, it can smell of rot as well as vomit or decay.

Compost smells sour

If your compost smells sour, it is rotting due to lack of oxygen.

This rotting process produces inorganic and organic end products. The organic end products are mainly various acids, such as butyric acid, acetic acid or propionic acid.

So if your compost smells sour, you are smelling these end products that were created when the compost rotted.

Compost smells putrid

If you are wondering why your compost smells putrid, then what your nose is telling you has probably occurred: and that is your compost has started to rot.

Normally, compost rots under oxygenated, aerobic conditions. Rot, on the other hand, can be triggered in the compost by a lack of oxygen, so that the waste decomposes under anaerobic conditions.

If a rotting process takes place, then also appropriate smells develop, which are generally called putrid.

Compost smells of ammonia

If your compost smells like ammonia, then a rotting process is taking place in the compost.

Putrefaction means that the waste on the compost is decomposed by microorganisms under lack of oxygen.

This rotting process produces various waste products, such as butyric or acetic acid, but most importantly ammonia.

Ammonia is a chemical compound of nitrogen and hydrogen and has a strong pungent odor.

So if your compost smells pungently of ammonia, it is rotting instead of decomposing as it should.

Compost smells of decay

If your compost smells like decomposition, then the rotting process has come to a halt due to lack of oxygen and a rotting process is taking place in the compost due to lack of oxygen.

Putrefaction means that organic materials, such as kitchen or garden waste, decompose under a lack of oxygen.

In the process, various metabolic products are released, which are mainly smelly gases.

However, if decomposition takes place with a little more oxygen (such that it is still not enough for the desired decomposition, however), then the organic metabolic products (e.g. acetic or butyric acid) are decomposed to urea, water and CO2.

This process is called decomposition and you will notice this decomposition smell when it takes place due to lack of oxygen in the compost.