Mulch depth from waste wood is a complicated topic with several important factors to take into consideration. Some have tried to simplify the topic by creating blanket “rules” for mulch depth and by doing so create an illusion of environmental responsibility. However, soils and sites are unique, so blanket rules are rarely the best answer for all situations.
Wood = Carbon
First, it helps to remember that wood is about half carbon, and about 40% oxygen, with the remaining made up of hydrogen, nitrogen, and trace elements the tree absorbed during its lifetime. Either decomposition or burning of the wood releases carbon, primarily carbon dioxide (CO2) into the atmosphere.
There’s a big difference in speed, though. Burning wood releases CO2 as quickly as the wood is consumed. CO2 is released more slowly as wood decomposes in the ground, as bacterial and other natural processes turn the cellulose and other wood components into CO2 & water, plus releasing the trace elements back into the soil.
Pile burning is never perfectly clean, either. No matter how seasoned the wood piles, no matter how ideal the air mixing, burning wood releases smoke and this has air quality impacts. Governments have strict rules around burning of waste wood, especially near communities, to make sure health impacts are mitigated.
But what about soil nitrogen?
A lot of concern around mulching stems from what is called “nitrogen depletion”. In short, plants need nitrogen available in the soil to grow, and adding too much carbon for decomposition can temporarily deplete the supply of nitrogen for plants as it gets used up in the decomposition process. That simplification though will lead you to think that any decomposing wood in soil is a bad thing, which is not the case. Wood has been decomposing in soil for as long as there have been trees.
A study of the effect on nitrogen levels of introducing wood into soil found that it takes a lot of wood to start to deplete nitrogen in soils. In fact, they found there is no significant effect until the proportion of wood mixed with soil reached 10%. Nitrogen depletion, when it happens, lasts only as long as it takes for the wood to decompose so it ends up being a temporary effect.
Temporary depletion of soil nitrogen isn’t all bad. If the objective is to discourage fast-growing vegetation, for example underneath a powerline, mixing ample wood fibre into the soil can help slow down natural regrowth that would need to be mowed soon anyways.
Other environmental Benefits of mulch
The responsible handling of waste wood isn’t the only environmental benefit of wood mulch. Wood mulch is a handy way to reduce water velocity from rainfall and/or surface runoff, reducing the severity of erosion and the likelihood that soil is transported away from the site and into watercourses. Mulch absorbs water to help keep soils properly hydrated, and is a way to enrich mineral soils with low organic content. Mulch can also reduce rutting from light vehicle traffic.
So, what depth is OK?
What we’ve learned is, the exact depth of mulch isn’t as important as the volume that gets mixed into the soil. With that in mind, consider this general prescription for your next project:
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- Always remove as much merchantable wood from the project site as possible, so that it gets used rather than wasted
- Now, look at how much waste wood is left on site and compare it to your available soil volume. If you’re going to end up higher than 10%, consider horizontal grinding for bioenergy production to reduce the waste wood loads
- Now you can mulch without the worry of nitrogen depletion
