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.
Thanks,
Nancy

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!

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