I was surprised to hear a landscape professional, at a convention I attended last weekend, repeat a common myth about wood mulch, namely that “mulch robs soil of nitrogen.” In the interest of promoting the soil boosting and water saving benefits of mulch we need to send this common misconception into the bad idea chipper/shredder.
It’s true that if you mix a lot of carbon, such as wood chips, into soil the amount of nitrogen available to plants will decrease. This is because soil organisms use nitrogen to process carbon. But this happens only if you incorporate mulch into rather than on top of soil. When you top dress your soil with mulch some nitrogen at the surface will be locked up, but this actually works to your benefit by inhibiting weed seed germination. Far from reducing nitrogen, as mulch decomposes it will actually increase the nitrogen content of your soil. This is one of the many benefits of using wood chips over inorganic mulches such as gravel.
I sometimes get asked what kind of mulch to use in a vegetable garden. I use straw since it’s inexpensive and easy to clean up at the end of the summer growing season. I wouldn’t use wood chips on vegetable or other annuals since they might get churned into the soil even though I don’t ever till or double dig. Wood chips are for perennials.
Now, my Root Simple friends, go forth and tell people that mulch does not rob the soil of nitrogen!
For more information about mulch which includes a discussion of other mulch misconceptions such as allelopathy and termites, see Washington State University Extension’s publication “Using Arborist Wood Chips as Landscape Mulch.”