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!

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

Sciency Answers: Fertilizers: Organic, natural, conventional or... what?

Dave sent me a question -- asking about my thoughts on some specific natural fertilizers that had been recommended to him. So, today's sciency answer is a quick run down on different types of fertilizers.

There are tons of differents kinds of fertilizers -- conventional liquids and powders, and a seemingly endless array of different organic options.

The first thing to say is:
Your plants don't care.
A plant's roots absorb certain specific compounds from the soil. And the phosphorus they get from an organic fertilizer is identical in every respect to the phosphorus they get from a synthetic fertilizer.

Which isn't to say what you choose doesn't matter. How a fertilizer is produced has an environmental impact. Synthetic nitrogen fertilizer is produced by the Haver process, an effective but energy intensive method to convert the nitrogen gas in the atmosphere into forms of nitrogen plants can use. Phosphorus for conventional fertilizers are actually mined -- and minable phosphorous in the earth is a non-renewable resource, and at our current usage rates, we're predicted to run out in about 30 years (A nice article on the topic from Slate: http://www.slate.com/id/2258112/entry/2258053/). In contrast, organic fertilizers like compost or manure let us recycle the nutrients we have. Though it is worth mentioning that shipping some sort of special organic fertilizers from across the country isn't going to be exactly carbon neutral either. Like very thing else, local is the best here.

The other big difference between different sorts of fertilizer is how concentrated and how "fast acting" they are. Highly concentrated fertilizers, like the genetic bag of 20-20-20 synthetic fertilizer OR the highly purified equivalent organic fertilizer bags, usually break down quickly in the soil releasing lots of nutrients in a rapid burst. This gives a satisfyingly rapid response from your plants, but also makes it much more likely you'll over fertilizer, potentially harming beneficial soil biology, and much increasing the risk of polluting fertilizer run off into ground water and wet lands. Most less concentrated fertilizers, like compost, don't release nutrients right away. Rather, soil microbes have to further decompose the compost to actually break down the nutrients in it to the forms plants can uptake. The result is a much slower response from the plant, with fertility that stays more constant, and much less chances for overfertilizing and polluting run-off.

The final benefit of using things like compost as a source of fertility is that they provides food for a whole host of soil organisms, creating a healthier, richer soil that retains water better, promotes healthy root growth, and allows plants to better utilize the nutrients that are already there.

So, in short: Highly concentrated, fast-acting fertilizers (organic OR synthetic) can give you impressive, quick results, but don't do anything for your soil, and can easily lead to polluting run-off. Less concentrated, slow-release fertilizers, like compost, take time to show results, but produce healthier soils that can grow great plants consistently over time. So in general, I am skeptical of any special fertilizer. Organic or not, the basic chemicals of fertilizers are all the same, and anything that will show results over night is probably not great for your garden in the long term. In the end, the basic standards, just local compost and mulch, are the simpliest, easiest, and best.

Sciency Answer: Too much compost? (Plus! Fun with lime!)

Leslie, who blogs over at Mountain Plover blogs.icta.net/plover, send me a great question.

My question is about adding compost... can you add too much? And if so, what harm does it

In my master gardener training, we were told to always recommend adding 4 inches of organic matter, dug in 8 - 12 inches. Colorado soils are notorious for their lack of humus. However, my brother-in-law (who lives near Denver) had a soil test (from an independent lab) come back telling him he had too much compost! (He has been growing veggies for a number of years, adding compost and shredded leaves to the soil every fall.) Apparently, this was throwing his other minerals out of balance. They actually recommended adding lime and gypsum, two things CSU told us to avoid. Now I'm totally confused.

"The compost and leaves have narrowed the calcium to phosphorous and the calcium to potassium ratio. Both should be 18 to 1. The reason a small amount of limestone is added is because calcium and potassium compete for space on the clay colloid. When limestone is applied it will push potassium off the clay colloid and then be taken up by plants or will leach below the root zone. This is a little trick to help reduce excessive potassium faster. The gypsum is applied because it will help correct a serious situation in your soil: excess sodium. Once applied, the calcium and sulfate in gypsum will disassociate. The sulfates combine with sodium to become sodium sulfate and is leached in the presence of adequate moisture. This will help reduce your sodium level. Having sodium at 166ppm is much worse by far than having calcium at 8000 lbs. per acre or even higher. High calcium levels are not that much of an issue. Calcium nitrate is applied to bump up the nitrates in the soil for quick, early growth in the spring. The amount of accumulated calcium will be negligible."

Whew! What a lot of great questions! And a lot of excessively technical language from the soil lab. I'll try and break this down in bits and see if we can make sense of it all.

Fertilizer Vrs. Soil Amendment

When thinking about adding stuff to soil, I like to mentally break things down in to two categories: Soil amendments and fertilizer. I think of things like peat moss, charcoal, sand, as soil amendments. Their primary purpose is to change the texture and structure of your soil so it is a better space for plant roots and beneficial microorganisms to grow in. Fertilizers, on the other hand, are plant vitamin pills, providing specific chemicals they need to supplement their main diet of carbohydrates they make from water, sunlight, and air. It is pretty hard to overdose on most soil amendments. You can't really have too much organic matter in your soil – after all, peat or coir based potting mixes are essentially 100% organic matter. Fertilizers, on the other hand, are usually damaging to plants at high levels, just as you can overdose on almost any vitamin.

Compost is, of course, both a fertilizer AND a soil amendment. Compost has a lot of organic matter which improves soil texture, and it also has varying amounts of nitrogen, phosphorous, etc. The exact amounts will vary widely depending on that your compost is made from, but certainly you can add too much fertilizer by adding too much compost. The most common compost over-dose, in my experience, is adding way too much nitrogen when using composted manure, but you can overdo other elements as well.

To lime or not to lime?

The rest of Leslie's question is interesting because it make, once again, the point that all gardening is local. The soil testing lab recommended adding lime and gypsum – but her Master Gardening training at Colorado State said to avoid both of those! What gives?

The usual reason to add lime or gypsum to soil is to correct a calcium deficiency, and/or to make a soil less acidic. Where I live, we get around 30-40 inches of precipitation a year and all the water leaches out the alkaline chemicals, usually producing an acidic, calcium deficient soil. Around here, adding lime is pretty much standard practice. Leslie gardens with only 16 inches of precipitation a year, and that lack of water lets calcium build up in the soil, so adding more is just making a problem worse. There are exceptions to the rainfall-soil pH connection of course... lucky people like Gail who, thanks to limestone bedrock, have alkaline soil despite a wet climate. I gardened briefly on limestone when I was at Ohio State... it was a pain. Suddenly rhododendrons went from over-planted and boring to difficult and desirable. Because wet, alkaline soils are rare, it is hard to find plants adapted to those conditions.

All of which is a bit beside the point in this case. The soil lab is recommending lime and gypsum here for a different reason. Leslie's brother has too much potassium and sodium in his soil, and adding lime and gypsum is a kind of chemistry trick to let those chemicals leach out of the soil faster. It will increase the amount of calcium in his soil, which isn't a good thing, but having lots of sodium is a much WORSE thing, so is the best option for the healthiest soil.

Get Tested

The bottom line here is: get your soil tested periodically by a qualified lab in your area. Soils vary radically depending on your climate, they type of bed rock you have, and how you manage it. Compost is a catch-all term, and composted cow manure is going to be very different than composted yard waste or composted coffee grounds. Find out what your soil is actually like now, so you can make the right choice on what to add to it in the future.

Have a question? Get a sciency answer! Just e-mail me (engeizuki at gmail dot com) or ask one in the comments!