18 April 2011

Sciency Answers: Wood chips and nitrogen

Nancy has a question:

Hi, Joseph!  Looking for a Sciency Answer.
I keep reading that you can't use wood chips as mulch in gardens as it robs the garden of nitrogen.  Is that so?  How does that work?  What can I do about it?  Just add more fertilizer?  Wood chips are such cheap and easy mulch.

The all importance of nitrogen
Wood chip mulches, and other high carbon, low nitrogen mulches, can suck up some of the available nitrogen in your soil. This happens because mineral nitrogen is essentially not just for plants, but for all life. Nitrogen is a key ingredient in proteins, and are a fundamental part of how life on earth works. The genes in our DNA are simply blueprints for making proteins, proteins which go on to build our entire bodies. No nitrogen, no protein, and without protein, no life. Which applies to all the soil microorganisms that want to decompose your woodchip mulch. To build their bodies and make their cool wood-digesting enzymes, they require nitrogen, specifically, mineral nitrogen, the form they can use. But mineral nitrogen is often in short supply in the soil. Everything wants it, and yet it is very easily leached away by rain water, and in the right conditions, can be converted into nitrogen gas which simply floats away in the air (forming, indeed, 78% of the air we breath). Only a few organisms, most famously the rhyzobium which form symbiotic partnerships with the roots of legumes like beans, can convert the gaseous nitrogen back into the mineral form other organisms can use.
So nitrogen is a key building block of life, and often scarce.

Carbon = food
Carbon compounds, on the other hand, from sugars to starch to wood, are primarily food for soil life. When a leaf or branch falls to the ground, all sorts of bacteria and fungi quickly begin munching away on it, breaking down the carbon structure to release the energy in it to power their life. When you, the gardener, take a whole bunch of carbon, in the form of wood chip mulch, and put it on the soil, the microorganisms rejoice and start reproducing like crazy. "So much food!" they say, "Let's all have a million babies to eat it all up!" But remember, each of those little baby bacteria requires a bit of nitrogen to live. If there is a lot of nitrogen in the soil to match the amount of carbon food sources, the microrganism population skyrockets and rapidly gobble down the organic matter, releasing all sorts of nutrients and making a lovely rich soil in the process. It is this balance of nitrogen and carbon that people aim for in their compost piles to achieve extremely rapid decomposition.
 When nitrogen runs low
If, however, there is a shortage of nitrogen, and lots of carbon, the bacteria are limited. Without enough nitrogen, they can't reproduce to match the food supply, so any bit of loose nitrogen they find floating around gets quickly snatch up and used to make more bacterium and fungi babies. With microorganisms scavenging up any loose nitrogen they can find, it doesn't leave much for plants to use to build their proteins and chlorophyll. The nitrogen has been immobilized. It is still there, just microorganisms are busy using most of it. Over time, however, as the microorganisms finish eating up all the carbon, they run out of food, and begin to die, releasing the nitrogen in their bodies back into the soil where plants can take it up again, re-mobilizing the nitrogen.
The wood chip nitrogen sponge
So when you add a layer of high carbon, low nitrogen mulch like wood chips to your garden, they act like a sponge, soaking up some of the nitrogen from your soil, and then gradually released it again. If you already have a shortage of nitrogen in your soil, this can cause a shortage for your plants , but you can solve that by simply adding a high nitrogen fertilizer, whether it be synthetic or organic fertilizer like manure, to make up the difference. And actually, adding nitrogen with carbon is better than just putting down the nitrogen fertilizer by itself. If you add concentrated nitrogen along, it will be instantly highly available to your plants, but it will also quickly leach away into the ground water, away from your plants that need it, and polluting streams and wetlands, forcing you to keep fertilizing to keep your plants growing happily. Combine that nitrogen with a lot of carbon, like wood chips, however, will keep the nitrogen around, releasing it slowly and stably over time, keeping your plants happy and minimizing polluting run-off.
The bottom line
So yes, wood chips can soak up nitrogen, but that is actually kind of a good thing. And the reality is, the effect is pretty small, and you don't need to worry about it in most cases. In my vegetable beds, where I want my plants to grow very rapidly and lushly in a single season, I add a higher nitrogen layer of compost annually along with new layer of wood chip mulch to keep the nitrogen abundant. In my ornamental perennial and shrub beds, however, I pretty much just mulch with wood chips, only adding compost if the soil is particularly poor and plants aren't growing well. Most plants don't really need high fertility levels. Extra nitrogen will help they grow bigger and faster, but for many flowers, that actually just means they are more likely to fall over and require staking, and sometimes even produce more leaves at the expense of flowers. I give a few greedy flowers, like my lilies, a big dose of compost every year to really push them to decadent proportions, but other wise, wood chip mulch produces healthy, happy plants for me.

In short, say yes to wood chip mulch!

Have a question? Get a sciency answer! Just e-mail me: engeizuki at gmail dot com


Liz said...

Thanks for this post, I heard about this earlier and wan't quite sure on everything. Now I am.

Rachelle said...

You could also thinly layer in untreated grass clippings and be sure to mist-water your garden. Misting will gently bring the nitrogen in the air down to the ground so it is acessible to all the plantand microbial goings-on versus torrential rain which can leach nitrogen away from plants roots. I have seen some really great gardens planted in wood chip mulch. Also if the "wood chip" mulch is from brush chippings from fall, it will include a higher proportion of leaves, which also will give the wood chips a bit more nitrogen content.

Yeah, what he said.

Anonymous said...

There is no evidence of this. See "Wood chip mulch: Landscape boon or bane?"

Anonymous said...

"Misting will gently bring the nitrogen in the air down to the ground ..." Huh??? Where did you hear that? If it were true there would be no Nitorgen in the atmosphere ans all of the rain in the last few million years would have washed it out. Also the only plants that use atmospheric Nitroger are legumes.

Susan in the Pink hat said...

I always sprinkle bloodmeal about to supply nitrogen to the wee planties. I also use pine straw as it doesn't break down as quickly as woodchips and maybe, possibly helps acidfy my alkaline soil. Even if it doesn't it's free and is much nicer than chopping up a whole tree for mulch.

Joseph said...

Glad you enjoyed it!

Grass clippings are indeed high in nitrogen, and gentle watering can minimize leaching in the soil, but it won't bring nitrogen down out of the air. Air is 78% nitrogen, so access to nitrogen gas is not the limiting factor in nitrogen fixation, rather it is the special conditions nitrogen fixing microbes require.

I agree 100% with Linda in the article you link to. Yes, wood chip mulches can "soak up" some nitrogen, but no, that is not a problem, except potentially for a few crops like vegetables and annuals.

I love pine straw mulch... it is so beautiful! But I would have to buy it because I don't have any pines, and it is surprisingly expensive. The wood chip mulch I use is made from waste wood and scraps, not whole trees being ground up for that purpose.

Christopher C. NC said...

Joseph "wood chip mulch in the vegetable garden' or some variation of it is the current #1 search that brings folks to my blog. I've done several posts on it and use tree trimmers wood chips in the vegetable garden adding extra nitrogen with chicken and cow poo and have excellent results.

Did you miss the recent big wood chip dust that started in Chicago?

Maybe it was just me but there was a very strange scrolling time warp thing on this post that made sections of text disappear into another dimension and come back out the other side.

Joseph said...

I had no idea wood chip mulch was such a hot topic! And no, I don't know what the chicago wood chip issue is! Do tell. And I wonder what was going wrong with the post when you looked at it... It looks fine for me, but maybe you use a different browser?

Rachelle said...

Joseph, I will put out the idea that gardeners need to be keen observers. Seattle and Britain being good examples of areas that have gentle rains. If torrential rains can leach nitrogen away from the roots of plants, then atmospheric nitrogen can be cleansed from the air with gentle rains (see http://www.eurekalert.org/pub_releases/2008-05/uov-at051208.php). I don't think it is my imagination how the air smells fresher after a gentle rain. I have also seen the leaching of available nitrogen from a cornfield leaving corn (a big nitrogen user) yellowish green.

Christopher C. NC said...

The time warp seems to be gone now must have been some weird anomaly.

The big wood chip and more so dog poo dust up in Chicago started with Michele Ownens doing Mike Nowak's radio show promoting her new book. This link should get you started if you are so inclined.


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Dale said...
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heat treatment for bed bugs said...

Nitrogen is harmful, though. I wouldn't suggest using it in any circumstances. The risks are just too high.