Why Does Mulch Smell Like Manure? How Can I Deodorize It?

There are two types of mulch: biodegradable and non-biodegradable. The latter includes materials such as plastic sheeting, pebbles and gravel. Whereas biodegradable mulches are composed of organic matter such as straw, wood chippings and garden compost.

Biodegradable mulches can sometimes take on an unpleasant aroma. Mulch can smell like manure due to biological processes that take place when organic matter is deprived of oxygen, allowing microbes to flourish. The byproduct of these microbes is what causes an unpleasant smell.  

If your mulch smells like manure, or has a bitter smell like it has started to ferment, you should turn the mulch pile to allow an even distribution of oxygen throughout. Turning your mulch pile frequently will prevent the processes that create the bad smells from taking place.

How Do I Stop My Mulch From Smelling?

Compost binIf your mulch has an unpleasant smell then it’s usually because of the way it has been stored. If mulch is left in a pile undisturbed then the deeper parts of the pile won’t have access to oxygen. This lack of oxygen in the mulch leads to chemical reactions that can result in a bad smell.

One way to prevent this from happening is simply to turn your mulch pile regularly. Mix or sift the pile to ensure that it maintains equal access to oxygen.

You might also notice that when you buy bags of mulch from the store, they smell strange upon being opened. The same reasoning applies. You can let the bag air out before spreading it in your garden. If the smell is not too bad, then you can spread it in your garden or vegetable beds and the smell will go away before too long.

How Long Does the Manure Smell Last in Mulch?

At most, the manure smell in your mulch will last for about 2 weeks. However, the drier the conditions, the quicker the mulch will dry out. Dry mulch doesn’t smell because oxygen has access to the material, and the damp conditions in which bacteria thrive are no longer available.

Sometimes it can take as little as a few days to stop smelling. If you live in a hot climate and apply mulch in late spring, chances are that any smell will have disappeared in a day or so.

It’s also dependent on how thickly you lay the mulch. The thinner you spread it, the more rapidly it will dry and the smell will dissipate. Bear in mind though that to be effective, mulch should ideally be spread at least 2 inches thick.

What Kind of Mulch Smells Good?

Essentially, good quality mulch will smell good. If it is healthy, it will smell like freshly cut wood or earthy like soil.

Is the Smell of Mulch Toxic?

Bad smelling mulch is a result of a lack of oxygen, or anaerobic processes, that allow chemicals to build up. Some microbes thrive in these conditions and can produce byproducts such as methanol, acetic acid, ammonia gas and hydrogen sulfide gas.

The quantities of these microbes will not be enough to be harmful to humans. However, they can have a negative effect on plants. If you place bad smelling mulch around young plants or plants that are low to the ground it can impact their growth.

It can also prevent seeds from germinating. As such, it is best to ensure that you turn bad smelling mulch and allow oxygenation to take place through the mulch pile before applying to your garden.

Is Mulch Fermented?

Organic mulches such as straw, if stored improperly, can be prone to processes that are similar to fermentation. You might have smelled silage at some point and noticed the pungent aroma. This happens because silage is wrapped in plastic and therefore becomes anaerobic. This allows acetic acid, ethanol, and methanol to be created.

You want to avoid these processes from taking place, which is why it’s a good idea to frequently turn your mulch pile. You definitely don’t want to spread fermented mulch near your plants as this can cause damage to them, or even death.

How Do You Fix Sour Mulch?

If your mulch has a sour smell and you suspect that it has begun to ferment, you can confirm this by using a pH monitor. The pH of sour mulch will be 1.8 – 2.5. Ideally, mulch will have a neutral pH, with a range of 6-8.

Another sign that your mulch has started to ferment is when it begins to smell like vinegar, ammonia, sulfur or silage.

To fix sour mulch, you must allow oxygen back into the pile. You can do this by turning the mulch pile or spreading it out more thinly. It is good practice to make a habit of turning your mulch pile frequently.

What Is the White Stuff Under Mulch?

If you’ve had a mulch pile sitting undisturbed for some time, you might find that when you dig down into it there are fine white strands at the bottom. This white stuff is likely a harmless wood-decaying fungus.

This type of fungus is common in packed mulch where oxygen is scarce. You can simply turn the pile and break up the strands and no harm will be done. You can use a rake, or any claw-shaped tool, to break it up.

Can Mulch Dust Make You Sick?

Wheel barrowWhen you are breaking up mulch you might notice a fine dust in the air. This will be especially true of mulch that is very dry. Studies have shown that breathing dust from organic matter does not lead to statistically relevant change in lung function. However, if you are concerned about inhaling dust particles, then it’s a good idea to wear a mask when turning a mulch pile or applying mulch to the garden.

Can Mulch Cause Breathing Problems?

For most people, inhaling mulch dust will not cause any long term problems. However, some people might be susceptible to an allergic reaction in response to inhaling mulch dust.

Hypersensitivity pneumonitis is an immune system disorder that can cause symptoms like coughing and shortness of breath.

It is worth noting though that this condition is usually only the result of long and sustained exposure to dust particles. The lungs should heal once exposure has ceased. There is medication available to treat hypersensitivity pneumonitis so consult with your doctor if you notice symptoms.


Mulch can smell like manure if it’s deprived of oxygen for long periods of time. It is good practice to frequently turn your mulch pile to allow even access to oxygen and prevent fermentation processes from taking place.

If you forget and you notice that the mulch pile has started to smell bad, then you can simply turn or sift the pile and the smell should go away within a few days. In damp conditions, the unpleasant odors may take up to 2 weeks to disappear.