Do Wood Chips really take Nitrogen from the Soil?

All over the Internet ,when you go looking for information about using wood or wood chips in gardening, you read about how certain items will suck nitrogen out of the soil. When you start looking for factual evidence though, it seems very hard to find any scientific study that backs this up. Why is that?

Wood, when it comes in contact with soil, activates mechanisms in certain bacteria which then consume nitrogen as food so they can decompose the wood. If this wood is sitting on top of the soil (fallen tree, dropped tree branch and so on) then these mighty bacteria will become active, but only to a depth of around 5mm. This means that the answer to the nitrogen "robbing" wood is only involved in a minute area under and around the piece of wood. Bury that piece of wood and you will be able to measure nitrogen depletion about 10mm from the wood (in all directions since the wood is under the soil). This brings the "nitrogen loss" caused by wood into the magical realm of "Urban Myth" since such quantities of nitrogen are small (minute really) when put into proper comparison of ava. This can be shown simply by looking at the soil in a forest floor, dead wood fall, leaves, dead undergrowth are all present and yet new growth is always present, even next to dead wood you will find nitrogen needy ferns growing. If the myth about wood was true, these plants should not be there, or at least not thriving as they do.

Most of us use wood chips for paths and mulching around trees, areas where the small amounts of nitrogen being used by the bacteria are truly negligible in significance to the growth rate of the plants near by. Keep in mind that this is because in nature, almost all the available N is in compound forms and slow release is the norm. Trees feeder roots are normally located from half the distance from the trunk to the outer edge of the canopy drip line (some species go out past this drip line further than others). What does this mean to us as growers? It means we don't have to worry so much about nitrogen loss when it comes to bushes and trees, unless we want to cover an area from the magical 6" diameter away from the trunk all the way out, 3 or 4 feet past the outer edge of the canopy drip line. Tree roots (the feeder ones we are most interested in) live from half the distance trunk to drip line out to around 3 or 4 feet past the drip line. These "important" (and they really are) roots are found at a point of 1-2 cm below the soil surface down to around 30-40 cm deep. The holding roots (tap root and large spreading main roots) go deeper since their job is to hold the tree in place under the stresses of wind events. How deep these roots penetrate is dependent on how far down bed rock is found. That is why my state (Arkansas) has such a large number of downed trees in heavy storm events, the bed rock is close to the soil surface, no deep roots means the trees aren't well anchored. On my farm we are lucky, there is up to five feet of soil depth and the bedrock is highly fractured, so tree roots can anchor really well (and these roots keep adding to the fracturing of the bedrock).

The gardens, where we should be worried about nitrogen being bound up by wood (especially chips) are the perennial and annual vegetable and herb gardens, these plants don't have roots (for the most part) that go deep and widely spread, usually their roots are very near the surface and within a 1 meter diameter circle from the main stem.
Squash and other vining plants put roots out all along their vine leaf nodes, but these are still shallow roots, so they are vulnerable to nitrogen binding by any wood chip mulch we might put down.

Nitrogen, the kinds we plant growers are most interested in, as I mentioned earlier, is a slow release nutrient when provided in natural forms. For faster access to plant roots, synthetic forms are needed, not what we want to hear!
Natural Nitrogen comes to plants in large chain molecules, Nitrites, Nitrates, Ammonia salts are the normal, natural forms we can put into soils via composts, manures, urine and teas made from mixtures of these along with greenery. Compost is a very long term additive, it actually takes five years for it to give up all the soil and plant goodness it contains. Which makes it very much an ideal additive in gardens.

Now that you know more about nitrogen forms found in nature, it should be easier to go about using woodchips for a mulch. They really don't cause any problems by "robbing Nitrogen" from your plants, especially if you use compost around or over them. The nitrogen from the compost will leach through those wood chips and still find the soil beneath your mulch chips. If you build a hugel and use greens and or compost as part of your filler ,then most likely you have made up for any Nitrogen loss that buried wood might cause. And if you top dress with compost or do a chop and drop of cover crops, like most people do, then you have added more slow release nitrogen than the wood might take up. Mother Nature loves to use wood to build soil, so there really isn't any reason we should not follow her lead and do the same. We just don't need beavers to make chips out of trees like she does.

Redhawk

 

Bill, straw is less dense, more surface area, breaks down faster.

Bryant, thanks for the topic, i think any gardener who tries to
keep things natural and have to keep up with the heavy feeders
has a pretty good idea of how wrong this "folk wisdom" is, but
it is also possible to make mistakes, like mixing too much wood
chips into an area.

here is a pic showing contrast of nitrogen robbing and not
via wood chips mixed (you can see the pea plants on the left are
not as large as the ones on the right -- later in the season they
catch up):



overall, if you do not mix wood chips in with your garden soil then
there is little problem. like you say, top mulch, add compost or
any N you want to balance any temporary loss of N, but know that
eventually the N does come back from the wood being broken
down.

oh, and p.s. it's not only about bacteria, also about fungi, they play
a big role in wood decomposition and the more arid the climate the
longer it will take for wood to be broken down.

when reading several books on gardening they mention to use
partially rotted wood chips to help keep the N impact lower. what
we do here is use the wood chips as mulch in many areas and
then after several years they've been broken down and turned into
nice humus which is then used as an amendment to the veggie
gardens. the clay here does well with this added to it and any
supplemental feeding for the heavy feeders is done via worm
compost (worms and worm castings) and higher N green manure
crops (chopped alfalfa or birdsfoot trefoil).

it is a system which has worked very well for me the past six years.
each garden i amend holds up well and keeps improving even if i
don't amend it again for several years. with good crop rotation
and me adding other scrounged organic materials things looks
pretty good compared to what the soil looked like when i started
(this is an example of the contrast):

 

hau Bill and Songbird,

First the straw is not such an impact as wood chips for the reason(s) that Songbird mentioned.
Wood chips, when you have access to them, are super for use as mulch.
If you can acquire wood chips with urine and manure mixed in (stable bedding, etc.) that is wonderful. Mixing that into the soil, instead of laying it on top, allows faster breakdown via bacterium and fungi interactions to develop faster, which reduces any N lag. It also serves as a carbon sink, helping the active organisms retain much more carbon than would be naturally released (CO2) by surface decomposition of the cellulose.

My research shows that bacterium hit wood first and they exude compounds that attract (actually activate) the fungi spores that are omnipresent (just like the bacteria) in soils. I did a little study way back in 1970 that showed that both bacterium and fungi are present in death valley and other western deserts, they just don't remain active once the moisture has evaporated, they go dormant until the next rainfall event and then bloom just like the desert plants. Most cacti have fungi attached and inside their roots, just waiting for moisture so they can get back to work. The symbiosis in soil can boggle the mind. I have not found any organism in soil samples that doesn't have a symbiotic relationship with at least four other organisms. It seems that they can even attract missing organisms to a certain extent. Some fungi can exude compounds to stake a claim on territory, keeping other, perhaps competing fungi and bacteria far enough away to monopolize their space, just as plants can.

 

Hi guys, late comer.

I'm looking for info on this topic as I've just taken over a nursery and the everyday potting mix they use contains aged bark mixed in.

Recipe is
1 part aged bark chips
1 part sterilised soil
1 part green waste
OSMO PRO organic fertilser as per instructions per/l
met 52 for vine weevil

If the bark is mixed in like this will it have a detrimental effect, or will the addition of fertiliser as well as green waste make up for any losses?

 

This is an informative review article that may help understand the wood chip mulch issue.
I must point out, Bryan, that you criticize articles for not including references to source material then omit any source references from your own comments. I encourage everyone to incorporate source material references in their comments.
This is a shorter article by the same author.

 

Callum, it will depend upon many factors.

is it aged before being used is just one of them.

BahaJohn, i usually write from personal experience. i'm not
doing funded research.

 

Thanks for yours and Babajohn's information. I haven't used wood chips on my garden for nitrogen ever. What I prefer to do is using fish meal, blood meal for a quick supply and worm casting for the long run.

 

I've used wood chips successfully in my gardens for years. As a mulch. I never mix them into the soil. The soil critters do that as the wood chips break down. I also use hay, straw, and chopped weeds as mulches, both for my gardens and swale systems. Basically any bio matter I can get my gardening little hands on.
Some plants need a bit of a help, with good topsoil in their plant starts, and I plant starts directly into the woodchips. The roots find their way down to the soil, and then the plants seems to explode with growth. I do add rock dust to my garden beds, for well trace minerals that our depleted soils seem to lack. Again, I add these on top of the soil, and let the soil critters do their thing.
Weeds, woodchips, hay, straw, and anything else does great on top of the soil. No digging needed (except to harvest root veggies).

 

worms are so amazing. i've never used fish meal or blood meal
other than to use what Mom bought many years ago to get rid of it.
my worms are the main source of nutrients and they are used to
compost all vegetables, meat and any other organic materials
that come to us (paper, junk mail, cardboard, ...). it's a nice system
and very simple in that i don't have any larger animals on site
other than what nature provides (rabbits, deer, raccoons, ...).

the gardens are gradually improving.

the wood chip humus is so nice and dark. i put some of that in
with the clay when i restart my worm buckets each spring.

out and around the place i also sometimes have leaves, bark,
sawdust, chunks of rotting wood and it all is appreciated. once
in a while road-kill too (better in a garden than smelling up things).

 

it's nice to have anything organic available. i'm lucky to have
friends who will bring me things and once in a while they get
back jam, bread, veggies.