Wood chip mulch and nitrogen

6 years ago

I'm in the process of replacing my grass lawn with perennial beds. The grass has been dug out and I received a load of Valley Oak arborist chips which has shredded leaves, barks, chips, all 1" to 4" big. It smells great!

I read about how wood chip mulches hold nitrogen hostage and I started this post to try to understand the whys and consequences of this. Does anyone know enough to explain this?

I am planning on laying down builders paper, watering, laying down 1-2" of chips, watering, laying down another 1-2" of chips, watering, laying down a third layer of chips. I live in an area that is somewhat windy but the mulch seems to stay in place according to the test I did yesterday. Is there something I can add now to help with the nitrogen issues? Horse manure compost I can get readily. I also can ask neighbor's gardener for their grass clippings but they seem to blow away for 2-3 days until thatching.

I am planning on putting in landscape plugs end of September.

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gardengal48 (PNW Z8/9)

6 years ago

The ONLY issue with nitrogen tie up and wood chips is if the wood chips are incorporated into the soil. If applied as a mulch or top dressing, then it is a non-issue :-)

The reasons for this is, as with any organic matter, it requires the consumption and digestion by soil organisms to create decomposition. Because of their durability and the oils, lingens and turpens they contain, wood products are harder for the soil biology to digest and require nitrogen input for both the necessary populations as well as the digestive/decomposition process. This they pull from the soil and whatever other sources they can access immediately. When spread on the soil osurface, there is only some minor N unavailability at the soil/mulch interface but not enough to create issues for any plant with a properly established root system.

btw, any N unavailability due to wood products being incorporated into the soil is very temporary. Once the soil organisms have done their work, all that N is returned to the soil, via both their waste and dying bodies :-))

4-6" is the most frequently recommended thickness for a good weed suppressing mulch. Less than 4" is pretty much a wasted effort and more than 6 inches can lead to altering the gas exchange at the soil surface and smothering of plant roots.

If you plan on planting into a heavily chipped or barked planting bed before the mulch has broken down you may want to use something like blood meal or alfalfa meal to supplement N for the new plantings until they get established......it is hard to plant in a mulched bed without getting the mulch mixed into the soil :-))

btw, the Soils, Compost and Mulch forum would have been the ideal place to post this question. It has been discussed many, many times!!

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Comments (6)

CasaLester RTP, NC (7b)
6 years ago
We are great fans of wood chips, as this freely available material has been very useful in improving soil quality of vast areas of former clay desert in our garden.

This issue with nitrogen is one of competition. Unlike sand or clay, wood is not a biologically inert material but undergoes decomposition by various bacteria and fungi, which take up some of the available nitrogen. But these amounts are not particularly large as the decomposition process is slow. Adding more nitrogen will accelerate decomposition by providing more food for the decomposing organisms.

In addition to leaves and various forms of compost, coffee is a great source of good slow-release nitrogen, and bags of several pounds of used grounds can be obtained at any Starbucks.

Here's more information on wood chips.
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bonitapplebum
Original Author
6 years ago
CasaLester RTP, NC (7b) Thank you. Should the coffee be applied before the mulch or on top?
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While I do think Lee Reich knows a lot about gardening and making soils healthy I do know hwe is way off base in that article he wrote for Fine Gardening magazine. the link gardengal provided, by Linda Chalker-Scott, is much better and the link provided by feijoas has information I have written here many times over the last 25 plus years. Many times I have asked for loads of fresh wood chips that have included leaves or pine needles only to be told That I did not want that stuff because it would rob my soil of Nitrogen. The leaves or pine needles, still green, will have a lot of Nitrogen in them and I have seen piles of wood chips with green leaves mixed in spontaneously combust because of the heat generated by the bacteria that are digesting them. The newspaper will suppress "weed" growth by blocking access to light by any plants tht might try to grow under the paper. There is no need to put any kind of lime on the wood chips to "counterbalance" any "acidity" myth tells us might be present. The wood chips will not "rob" Nitrogen from the soil.
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I don't think a small reduction in the amount of available nitrogen in a woodland is a bad thing. Invasive plants tend to thrive in nutrient rich, disturbed areas. Adding a mulch to stabilize soils and reduce the available nitrogen might help slow the spread of invasive plants and help you get an upper hand. I have heard of people who are restoring natural environments using "reverse fertilization" by spreading sawdust on the soil to reduce the level of available nutrients and thus limit the spread of invasive plants. Most of our native plants are adapted to low levels of nutrients. The same is true of most native plants in other parts of the world - it is the few ruderal plant species, adapted to quick growth in disturbed, nutrient-rich soils that tend to become invasive species when they get to areas to which they are not native. Also, I'll add that I think brush piles are a great addition to any woodland, regardless of the surroundings. I have several small patches of woodland amid brushy fields, a powerline right of way that is thick with brush, and some weedy fields that are quite dense. Even with all of the brush around here, brush piles are heavily used and seem to attract more birds to my yard than I would have otherwise. They are particularly well used by birds that visit my feeders in the winter.
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Dan, wood chip mulch is notorious for its impact on plants; it suppresses weeds and it has enough impact on plant growth to spawn a theory of 'nitrogen tie-up'. Yet, it also appears to be beneficial as 'ramial mulch'. Using normal composting practice, it breaks down slowly and won't be a positive addition to soil for many months. Sarah has some freshly chipped material that is the definition of 'ramial', and it's a unique opportunity for her to make something special with it. If she covers the pile while it's still fresh, I promise it will heat up and break down into material that is s precursor to humus. Otherwise, she has a large pile of chips that will take forever to compost. From what I read, lignin is very tough to break down, but the cellulose at least has a chance. Once the woody material is retted, I expect it can be used as a compost pile ingredient, mixed with grass and shredded leaves, or as a top-dressing/mulch. The lignin will feed soil fungi, the broken-down cellulose feeds the soil bacteria. If Sarah is willing to experiment by covering the pile and making observations, I think it will be instructive for everybody. I'm actually a little jealous, but I'll have my own ramial mulch to play with soon. I'll update as I make my own observations. Paul M.
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I teach classes on this and the scientific evidence simply does not support it!! One of a series of articles published by a horticultural PhD (and professor at WSU) offers this: "Concern: Wood chip mulches will tie up nitrogen and cause deficiencies in plants. Evidence: Actually, many studies have demonstrated that woody mulch materials increase nutrient levels in soils and/or associated plant foliage. My hypothesis is that a zone of nitrogen deficiency exists at the mulch/soil interface, inhibiting weed seed germination while having no influence upon established plant roots below the soil surface. For this reason, it is inadvisable to use high C:N mulches in annual beds or vegetable gardens where the plants of interest do not have deep, extensive root systems." Full article here: https://s3.wp.wsu.edu/uploads/sites/403/2015/03/wood-chips.pdf Googling "wood chip mulches and nitrogen availability" will confirm countless articles stating the same thing. The N issue is more myth than reality, as is the concept of mulches acidifying soil. It just doesn't work that way :-))
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CasaLester RTP, NC (7b)
6 years ago
Mixing the coffee in is better as the even distribution will assists a larger volume of soil. But if mixing in is not possible - on established beds, for instance - applying on top works as well.

Another useful component of soil improvement is adding earthworms. We grow them first in leaves (strongly compacted, watered and fertilized) stored in trash cans for at least six months since the time the leaves are collected in fall. Around this time we start releasing the earthworms into the soil by turning it over the spread out leaves or top-dressing with them.
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trickyputt
6 years ago
I do not think you will create soil in the first year. Personally I would dig in 2 inches of compost and some fertilizer as the soil test calls for. Then plant into 2 inches of compost on top of that, and then place 1 inch of chips on that, and just add chips every year as the soil eats them up adding fertilizer on a gentle but regular program. I would not let them get thicker than 2 inches of recognizable chips.
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bonitapplebum
Original Author
6 years ago
trickyputt My main purpose for the wood chips is weed supression so it is okay if I don't have immediate soil. I am planting mostly natives so I thought I should not amend the soil. The area was just grass for decades, so I thought to plant new perennial plugs directly in the existing soil and then fertilize over the years.

Could you explain why I shouldn't have more than 2" of mulch? Most resources recommend 4-6" and some even a foot of mulch.
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Wood chip mulch and nitrogen

gardengal48 (PNW Z8/9)

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Wood chip mulch and nitrogen

gardengal48 (PNW Z8/9)

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CasaLester RTP, NC (7b)

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