Homesteading, Gardening, and Off-Grid Living

Why Does Mulch Smell so Bad (And How to Fix It)

By: Zac Friedman

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Healthy mulch has a natural earthy smell. Bad mulch can smell like rotten eggs, vinegar, or manure. Mulch can develop a bad odor when it doesn’t get enough oxygen. This causes anaerobic decomposition. When this happens, microorganisms produce toxins such as acetic acid, ammonia, and hydrogen sulfide that build up in the mulch. When mulch goes sour, it can harm your plants.

In this guide, I’ll explain why some mulch smells bad. I’ll explain how mulch is supposed to smell, how to determine whether or not your mulch is bad, and how to fix sour mulch smells. 

Why Does Mulch Smell Bad and How to Fix It pin

Why Does Mulch Smell Bad?

Mulch can emit a foul smell due to anaerobic conditions that occur when there is a lack of oxygen in the piles of mulch. When mulch is stored in large piles, the materials at the center of the pile can get compressed, preventing the circulation of air. 

Under these conditions, anaerobic microorganisms thrive and start to break down the organic matter within the mulch. The decomposition process under anaerobic conditions leads to the production of various toxic byproducts such as methanol, acetic acid, ammonia gas, and hydrogen sulfide gas.

These toxic chemicals are the main culprits behind the bad smell. Methanol and acetic acid can give off a pungent, sour odor. Hydrogen sulfide gas has a rotten egg odor. Ammonia gas contributes a sharp, unpleasant smell. Together, these compounds can make the mulch smell rancid or sour like vinegar, rotten eggs, or manure. 

A wheelbarrow next to a pile of mulch

How Is Mulch Supposed to Smell?

Mulch should not have a strong or unpleasant odor. Instead, mulch should smell fresh and earthy, like freshly cut wood or fresh compost. This natural scent is a good indicator that the mulch is in good condition and ready to use.

Different types of mulch have different odors. For example, cypress mulch has a citrusy smell. Cedar mulch has a woody smell. Pine bark mulch has a sweet smell. 

The earthy smell of mulch is attributed to the natural decomposition process of the organic materials such as wood chips, bark, grass clippings, and leaves. With adequate aeration and the right moisture levels, the beneficial microorganisms begin a healthy decomposition process. 

If the mulch has an offputting odor, it is probably bad. Mulch should have a pleasant aroma. It should not have a strong smell. It should have a good mulch smell. 

A close up view of mulch

How to Tell if Mulch is Bad

Simply sniffing the mulch is the best way to determine if it is of good quality. Fresh mulch smells earthy and pleasant. If it smells good, it’s probably fine to use. If it has a foul odor, it could be toxic to plants. When it passes the smell test, it’s probably good. 

Another way to assess mulch quality is by performing a pH test. This can tell you whether the mulch is sour or properly composted. 

Sour mulch tends to have a lower pH, meaning it is more acidic. The pH of sour mulch is usually around 1.8-2.5. This acidic nature is a consequence of the production of toxic byproducts like acetic acid during the decomposition process.

To compare, properly composted mulch exhibits a more neutral pH, typically ranging from 5 to 7. 

This pH is indicative of a well-aerated, oxygen-rich decomposition process where beneficial microorganisms thrive. These conditions allow for the breakdown of organic materials into nutrient-rich compost. 

A neutral pH in mulch promotes a conducive environment for plant growth. It ensures that plants have access to essential nutrients without the risk of exposure to harmful acidic compounds.

To test your mulch, you can use a pH test kit or meter. If you find that the mulch is sour, you shouldn’t use it, as it could harm plant roots and soil health.

For info on testing your mulch at home, check out this helpful guide

Mulch around the base of a tree in front of a house

Can I Use Sour Mulch?

The presence of these toxic compounds not only makes the mulch unpleasant to handle. Using sour mulch in your garden is not recommended due to its potential to harm plants.

Sour mulch contains toxic byproducts such as methanol, acetic acid, and other organic acids. When this toxic mulch is spread around plants, it can create a hostile environment for the plants.

The harmful effects of bad-smelling mulch on plants are usually noticeable very shortly after applying the mulch. Usually within 24 hours. The plants may show symptoms such as leaf scorch, wilting, yellowing of the leaves, browning of the leaves, defoliation, and even death of the plants. These are similar symptoms that fertilizer burn, drought, or overuse of pesticides may cause. 

Newly planted plants and low-growing plants are the most susceptible to the detrimental effects of sour mulch. This vulnerability is due to their root systems being more exposed to the mulch, increasing the risk of absorbing the toxic substances present.

The vapors from the toxins can also harm the plants. These toxins can create stress conditions for the plants, hindering the ability of the plants to grow and flourish.

Despite the adverse effects of sour mulch on plants, it’s worth noting that the vapors emitted by sour mulch are typically not harmful to humans or animals. The smell is simply unpleasant. The primary concern remains with the well-being of the plants and the overall health of your garden. 

Before using mulch, it’s important to ensure that it has been well-composted and aerated. It should be free of any unpleasant odors. If the mulch is bad, don’t use it. 

Why Does Mulch Smell Like Manure?

Mulch may sometimes smell of manure. This is due to its composition of composted materials such as wood chips, bark, and plant matter. The natural breakdown of these organic mulches during the composting process can emanate a manure-like odor.

This is generally considered normal. This odor is a byproduct of the microbial activity involved in the decomposition process, where microorganisms work on the organic materials, converting them into a rich, humus-like material.

However, an excessively strong manure smell could be a red flag. This could indicate that the mulch may be undergoing anaerobic decomposition. In this case, it’s important to be cautious. Using the mulch could adversely affect the health of your garden.

If you are uncertain about the quality of the mulch, conducting a pH test is the best option. A pH test can help determine whether the mulch is too acidic. Ensuring the mulch has a balanced pH is crucial for promoting a healthy garden.

A yellow flower with mulch around the base

How to Fix Sour Mulch

Sour mulch doesn’t necessarily need to be thrown away. Bad mulch can be easily fixed and transformed into a beneficial addition to your garden. 

To fix sour mulch, begin by spreading it out thinly in a dry, sunny area. Turn or stir the mulch periodically. You want to make sure the mulch isn’t too compacted or too wet. A rake is a good tool for this. 

This treatment will expedite the process. It ensures that all parts of the mulch are exposed to air and sunlight.

It can also help to spray the mulch down with some water. This can wash away some of the harmful compounds. If you do wet the mulch, make sure the mulch doesn’t get compacted. Also, make sure it stays aerated.

Another thing you can do to help oxygenate the mulch is to poke holes in the piles. You can use a pitchfork, a spade, or your hand to do this. 

This process allows for aeration. Oxygen is essential for turning the conditions from anaerobic to aerobic. The harmful compounds will dissipate over time.

Generally, it takes 24 hours to 3 days for the undesirable compounds in the mulch to dissipate, and for the mulch to lose its foul odor. It’s best to give it 3 days so you’re sure the mulch is safe to use. 

If sour mulch has already been applied to your garden, it’s advisable to remove it as soon as you realize that it is sour to prevent potential harm to plants and soil.

How to Avoid Having Your Mulch Go Bad in the First Place

Ensuring that mulch remains fresh and conducive to plant health starts with proper storage and application techniques. 

To prevent mulch from turning sour or bad, you should store your mulch in piles that are not larger than 4-6 feet high. This size allows for better aeration, reducing the risk of creating anaerobic conditions conducive to souring.

The mulch piles should also be turned frequently. Regular turning and fluffing of the mulch allows for exposure to oxygen and prevents compaction that could restrict oxygen flow. This prevents the growth of anaerobic bacteria and reduces the production of harmful and smelly compounds in your mulch.

When turning your mulch, make sure you reach the middle and bottom of the piles. These areas are most likely to get compacted and start decomposing anaerobically.

Choose a storage area that is well-ventilated and avoid storing mulch in airtight bags. Enclosed or restricted conditions hinder the free circulation of air, leading to the accumulation of moisture and undesirable compounds. Proper ventilation facilitates the escape of excess moisture and encourages a healthier decomposition process.

When it comes to applying mulch in your garden, apply mulch in a thin layer, no more than 2-3 inches deep. A thicker layer of material might lead to compaction, restricting the essential flow of oxygen.

Proper application ensures that the mulch performs its roles effectively, such as maintaining soil temperatures, retaining soil moisture, and suppressing weed growth, without negatively impacting plant health or soil condition.

A commercial mulch operation
Avoid storing your mulch in large piles to prevent it from going sour. Alternatively, turn it frequently to keep it aerated.

Some Information About Aerobic Vs Anaerobic Decomposition

The decomposition of mulch impacts the mulch quality.

Aerobic decomposition is a process that occurs in the presence of oxygen. It involves microorganisms such as bacteria, fungi, and protozoa. These microorganisms actively break down organic matter in the mulch, converting it into beneficial compost. 

Aerobic conditions ensure a healthier, more efficient mulch decomposition, typically resulting in a mulch that emits a fresh, earthy odor. This odor is indicative of its richness in nutrients vital for plant growth and soil enhancement.

Adequate aeration, proper moisture levels, and regular turning of the mulch piles are essential practices that promote aerobic decomposition. This prevents the mulch from souring or developing foul odors.

Anaerobic decomposition occurs in the absence of oxygen. This often results from overly moist conditions, inadequate aeration, or overly compacted mulch piles. 

In anaerobic conditions, different sets of microorganisms dominate, leading to slower decomposition rates and the production of less favorable byproducts such as methane, acetic acid, and hydrogen sulfide gases.

This contributes to the sour or rotten smell of the mulch. Anaerobically decomposed mulch is less conducive to plant growth, as it may harbor compounds detrimental to plants and soil health. For example, it is acidic. This can lower the pH of the soil. 

Understanding the dynamics of aerobic and anaerobic decomposition is important when it comes to managing mulch. Prioritizing practices that promote oxygen circulation within mulch piles can significantly enhance the quality of mulch, maximizing its benefits. When it comes to mulch, you want to avoid anaerobic decomposition. 

FAQ About Stinky Mulch

Why does mulch smell like ammonia?

Mulch can emit an ammonia-like smell due to the decomposition process of organic materials, such as leaves, bark, and wood chips, under certain conditions.

Specifically, this happens when there is an excess of nitrogen-rich green materials in the mulch, and it decomposes anaerobically. Microorganisms break down these materials, producing ammonia gas (NH3) as a byproduct, which contributes to the strong, pungent odor.

The presence of an ammonia smell in mulch is not ideal. It indicates an imbalance in the composting materials and a lack of proper aeration. Besides being unpleasant, the ammonia odor signifies that the mulch might contain substances that could be harmful to plants and affect soil health negatively.

Why Does Mulch Smell Like Rotten Eggs?

Mulch can occasionally emit a smell resembling rotten eggs. This phenomenon typically results from anaerobic decomposition. 

A rotten egg smell emanating from mulch is generally a bad sign. It indicates that the mulch might contain compounds harmful to plants, such as hydrogen sulfide. These substances can lead to root rot or other plant diseases. 

Why Does Mulch Smell Like Vinegar?

Mulch might sometimes emit a strong vinegar-like odor due to the type of decomposition process it undergoes. This scent is typically a result of anaerobic decomposition.

In this environment, certain microbes proliferate and produce acidic compounds like acetic acid (which has a vinegar-like smell) as a byproduct of their metabolic activities. 

This often occurs when mulch is piled too thickly, preventing air from reaching the inner layers. It can also happen when it retains too much water.

Having a vinegar-like smell emanating from your mulch is not a good sign. Mulch in this condition may affect the pH of the soil, making it more acidic, and could inhibit plant growth.

Mulch in a flower bed

Final Thoughts About Foul Smelling Mulch

Understanding how to identify bad mulch, why your mulch might smell bad, how to fix it is essential. By ensuring proper aeration and appropriate moisture levels, the risk of your mulch going bad is low. If your mulch does go bad, it is easily fixable. 

There are many benefits of mulch for gardeners, from soil enrichment to weed suppression and moisture retention. With proper care, you can keep your mulch fragrant and fully optimized for plant health. 

Have you had mulch go sour? Share your experience in the comments to help other gardeners!

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