Hi. Today I’d like to talk about an often debated topic -
does wood chip mulch tie up nitrogen in the soil
and increase nitrogen fertilization requirements?
This debate arises from the fact
that bacteria need nitrogen to decompose organic matter.
And because wood chips have a high carbon to nitrogen ratio,
there’s concern that there isn’t sufficient nitrogen in wood chips themselves
for decomposition to take place,
and bacteria will utilize nitrogen in the soil instead.
It turns out that there’s some debate even among experts on this topic,
but one thing they agree on is that nitrogen depletion occurs only on the surface of the soil,
and as a result does not impact plants with roots that extend below the surface.
This, of course, assumes the wood chips are being used only as mulch
and are not being mixed into the soil,
which would tie up nitrogen below the surface.
So, established perennials would not be negatively affected,
nor would tomato transplants, whose roots are buried well below the surface.
But what about annual vegetable seedlings with very shallow roots?
Will they be affected by possible nitrogen depletion?
There’s some disagreement here among the experts I consulted.
For example, Jeff Gillman doesn’t think the depletion is significant enough to worry about.
In “Decoding Gardening Advice”, he writes
“The truth is that some nitrogen from the top layer of soil is used up when wood chips are initially broken down by bacteria.
But this nitrogen loss is likely to affect only the most tender annuals,
and even they probably will not sustain any permanent injury.”
Gillman goes even further,
and says the advice that extra nitrogen fertilizer is needed when using wood chip mulch is “just wrong”.
Dr. Linda Chalker-Scott, however, is a little more cautious when it comes to small seeded annuals.
In her article “Wood Chip Mulch: Landscape Boon or Bane?”, she states
“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.”
One thing both experts completely agree on, however, is that over time
wood chip mulch adds more nitrogen to the soil than it takes.
According to Gillman, “The confusion over whether wood chip mulches cause nitrogen deficiencies that harm plants persists,
despite numerous studies demonstrating that serious depletion of nitrogen is not taking place.
In fact, many studies have indicated that just the opposite occurs.
As wood mulches decompose, they enrich the soil with many nutrients, including nitrogen.”
This is consistent with our experience.
We’ve never noticed any nitrogen deficiencies when using wood chip mulches,
despite the fact we use no store-bought nitrogen fertilizers.
If you’re still concerned about nitrogen depletion, however,
you can always add fast decomposing high nitrogen materials
like grass clippings or comfrey to your mulch.
They’ll provide nitrogen to aid in the decomposition of the wood chips
and potentially decrease nitrogen depletion on the soil surface.
When using wood chips, we typically apply a few inches,
and are always careful not to mix them into the soil,
as this would cause nitrogen depletion below the surface
where plants need nitrogen.
We use wood chips from the municipal wood chip pile, which contains a wide variety of tree species.
Allelopathic trees that suppress the growth of other plants are not common in our area,
so we use the wood chips without worry.
However, if you live in an area where allelopathic trees like Black Walnut or Eucalyptus are common,
you’ll want to avoid using them as mulch.
When transplanting vegetable seedlings into the garden,
I brush the wood chips aside,
and plant the seedling in the soil with a little bit of compost,
taking care not to mix the chips into the soil.
Similarly, when planting seeds, I create a small furrow in the wood chips,
fill it with compost, and plant the seeds in the compost.
So, I wouldn’t let concerns about nitrogen depletion stop me from using wood chip mulch,
and I wouldn’t use extra nitrogen fertilizer when using them.
They help control weeds, retain water, moderate soil temperatures,
and over time add nutrients, including nitrogen, to the soil.
Well, that's all for now.
Thank you very much for watching,
and until next time remember
you can change the world one yard at a time.