The Mulch Robs Nitrogen Myth

2008-08-12_Mulch

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.”

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10 Comments

    • I haven’t had that problem. But I did grow Sonora wheat one year and it comes back every winter!

    • Your “straw” was more likely hay which does carry many seeds. Straw is only the stem of grains, few if any seeds.

    • As a farmer, you are right in observing oats or wheat growing from your straw. Sometimes we fail to fully thresh or separate the heads of grain from the crop and a little of it, or even weed seeds, end up in the bale of straw, only to grow up through the mulch of straw.

    • Ruth Stoat goes over this problem extensively in her books.

      Basically, don’t fluff the straw up – put it down as flakes. Then when it sprouts, turn it over to smother the growth.

  1. Wondered what your opinions on ramial wood chips. I’m considering using it in the garden, even if it gets churned in to the soil.

  2. Does anyone really mean the nitrogen-robbing thing? I always figured it was an excuse for putting and keeping land in lawn rather than something useful.

    I’ve done a lot of gardening with wood chips (over a cardboard mulch layer) and never had a problem with nitrogen deficiency.

  3. Even though I’ve read multiple times that woodchips-robbing-your-soil-of-nitrogen is a myth, I wonder if there’s something that’s being missed. In my own garden, I’ve noticed many times that if I mulch an area with fresh wood chips (as in, wood that’s been chipped recently), the plants in that area kind of stop growing for 6-12 months. It’s happened in different parts of my yard with multiple kinds of perennials, such as lavendar, citrus trees, sages, penstemon, raspberries, and more. However, once they start growing again, they take OFF and usually get really robust and lush. If I use wood chips that have sat in my driveway for a year (which are then partially decomposed and have a bunch of mycelium growing), I don’t get the same stunted growth. I don’t really understand why, but I’ve just stopped mulching garden areas with fresh wood chips to get around the problem. I’m curious to know if anyone else has this happen.

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