Many gardening enthusiasts use mulch thanks to its myriad benefits. It helps retain moisture, regulate soil temperature, and suppress pesky weeds. However, as time goes by, you might start to notice something intriguing happening to your mulch; its structure is changing.
This natural process can make you wonder: does mulch decompose?
Continue reading this article to discover whether or not mulch decays and explore the factors that influence this process.
The short answer is that it depends. As you might know, mulch has two types: organic and inorganic.
Organic mulch is made of 100% natural components. For that reason, it decomposes and adds value to the soil. Some of its common types include bark, grass clippings, and leaves.
Inorganic mulch, on the other hand, can be natural or synthetic. It lasts longer than the other type. Examples of inorganic mulch include rocks, pebbles, and rubber.
Now, you might wonder: why doesn’t inorganic mulch decompose?
You see, for decomposition to occur, microorganisms need to break down and feed on available nutrients. They do so by using many enzymes.
However, environmental factors, like temperature and moisture, should be optimal for this chemical reaction to take place.
Synthetic materials have no nutritional value. That’s why bacteria and mold can’t grow on them. As for rocks, they’re not nutritious enough—only certain groups of microorganisms can inhabit them.
Generally, it can take anywhere from one to seven years, or even more, for mulch to fully decompose.
However, the decaying process starts as soon as you add this organic material to the soil. Once the environmental conditions are optimal, microorganisms will start breaking down the mulch and feeding on it.
That said, the decomposition period can vary depending on the mulch type. Here’s a table explaining this in further detail:
|Mulch Type||Decomposition Time|
|Bark mulch||4-7 years|
|Wood shavings||2-7 years|
|Cocoa shell||2-4 years|
|Grass clippings||1-6 months|
Aside from mulch type, other factors can affect how long organic material decomposes. These include the composition and climatic conditions. The soil microorganism type also plays a role in the decaying period.
Here’s a detailed explanation of each element:
Almost all organic soil materials contain carbon and nitrogen. In fact, microorganisms need these elements to flourish and start the decomposition process.
The difference is, however, in the percentage. Scientists describe this relationship using the C:N ratio.
The higher the carbon content, the wider the C:N ratio and, thus, the slower the decomposition rate. That’s because microorganisms spend a lot of energy trying to break down excess carbon sources.
On the other hand, a higher nitrogen content causes the C:N ratio to become too low. As a result, decomposition happens at a higher rate.
The problem is that it can cause nutrient depletion and starvation. You want to mix different mulch types to reach a C:N ratio of 25-30:1.
That’s the sweet spot for having an ideal decomposition rate and fertile soil.
Cellulose and lignin content are other plant components that affect mulch decomposition.
For those wondering, these materials are fibers that make up a major part of plant cell walls. However, they differ in structure.
Cellulose is made of glucose. These units bond together, forming a long chain. Such a simple structure is easy to biodegrade.
That’s because microorganisms can access the linkage quickly using a few enzymes.
Lignin, on the other hand, is a different story. It has a complex, cross-linked structure with multiple bond types.
Consequently, soil organisms need to use different enzymes to break down this molecule—a process that takes multiple steps. So, the higher the lignin content in the mulch, the slower the decomposition rate.
Climatic conditions, particularly temperature and moisture, have a major influence on mulch decomposition rates.
For starters, soil microorganisms require optimal environmental conditions to flourish. Likewise, the enzymes work best in a particular temperature range.
Generally, bacteria and fungi prefer warm temperatures between 77ºF and 86ºF. Enzymes also work best within a similar temperature range.
As the temperature increases, soil decomposers become more active. Consequently, the decomposition rate increases.
Likewise, the higher the moisture content in the soil, the higher the microbial activity. That’s because water helps dissolve minerals, making them easier for bacteria and fungi to ingest.
Even materials with high lignin content can decay faster as the temperature and moisture range increase and reach optimal levels.
From the above, you can see that microorganisms play a pivotal role in decomposition. However, not all soil organisms work the same way.
For instance, fungi are the best decomposers for complex compounds, particularly those with high lignin and cellulose content. As a result, soils with a higher mold population can decompose mulch faster.
Aside from that, microbial diversity also influences decomposition rates.
As surprising as it may sound, having different decomposer species in the soil can be counterproductive. That’s because these organisms compete for the available sources, eventually dying.
Mulch and compost serve different purposes. So, they aren’t interchangeable.
The former is added as a top layer to regulate the temperature and retain moisture. It also helps with weed control by preventing the seeds from reaching the soil.
Compost is a mixture of decayed organic matter. Its primary purpose is to fertilize the soil and increase nutrient availability.
For that reason, compost is typically used beneath the mulch layer.
That said, you can add decaying organic mulch to your compost bin. The bacteria will break it down further and release its nutrients.
Generally, I don’t recommend composting dyed mulch unless you’re sure it’s 100% natural.
Red and black are among the common dyes used to color mulch. The former is made of iron oxide, a layer that forms rust. As for the latter, it comes from carbon.
Additionally, some mulch dyes come from vegetables, making them 100% organic.
These chemicals are typically harmless. However, manufacturers can add preservatives and other artificial additives, which can be toxic to your soil.
Yes! You should remove the mulch before adding compost. That’s especially true if you use organic products.
As mentioned earlier, compost fertilizes the soil. So, you need to mix it into the potting mix. Alternatively, you can add it beneath the mulch, and the water will wash the nutrients down to the roots.
However, nutrient depletion isn’t the only problem that can occur when composting over mulch. Blooming weeds can be an issue as well.
As you know, these pesky plants are fast growers. Provide their seeds with a fertile layer, and you can expect them to exploit it, stealing all the nutrients from plants.
As you can see, organic mulch does decompose, but the pace depends on several factors, including the mulch type and composition. Other factors, like climatic conditions and microorganisms, also play a role.
Depending on these factors, complete mulch decomposition can span from a couple of months to years.
On that note, remember to change the decaying layer, especially when composting, to avoid nutrient depletion and weeds. That way, you can ensure your plants thrive in a healthier environment!
Ben has a bachelor’s degree in construction engineering. When not constructing or remodeling X-Ray Rooms, Cardiovascular Labs, and Pharmacies, you can find him at home with wife and two daughters. Outside of family, He loves grilling and barbequing on his Big Green Egg and Blackstone Griddle, as well as working on projects around the house.
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