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Compost/Soil N question
I have been poking about on the web looking for an answer to some soil questions. So far no answers. Perhaps someone at your organization can help. I recently sheet composted my lawn using newspaper, bunny poop, cardboard, leaves, coffee grounds and then topped it off with wood chips, as I was taught to do. I have since learned that wood chips "use up" available nitrogen. Where does the nitrogen go? Is it transferred into the microbes and is it still available to plants? Or is it transformed into something else. Will my eventual soil need nitrogen amendments and would it be a good idea to put a layer of either high nitrogen compost or coffee grounds over the wood chips to replenish the nitrogen and speed up the decomposition?
Thanks,
N. Pomerance
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Why were you told
Why were you told to top it off with wood chips. Maybe to make it look pretty for the neighbors' sake? Mine gets topped off daily with kitchen scraps and I turn in the top foot or so every couple of days to let the worms have at it. I don't understand the why topping off with wood chips bit.
If just a thin layer for the purpose of good looks, I wouldn't sweat it at all. A few unbroken down wood chips in soil is really good environment for encouraging/sheltering fungi. And fungi promoted by woodchips help plant roots reach out way further than they can on their own. As long as plant roots can get to plenty other soil then the wood chips will give you no N-deficiency problem. But if its a butt load of wood chips, and you are planning to deep dig the wood chips into the soil, like French Intensive method does, then, big concern, by all means get some coffee grounds or high nitrogen compost mixed in to the wood chips now and keep them v moist so that something actually happens before they hit the soil. Of couse doing that defeats the purpose, whatever it was, of topping off with chips, but the soil comes first.
Of course if your pile is going to be used as a compost mulch, no problems. Read Teaming with Microbes: gardeners guide to soil food web by Jeff Lowenfels
Know your C/N
You said you:
learned that wood chips "use up" available nitrogen. Where does the nitrogen go? Is it transferred into the microbes and is it still available to plants?
Although some N is lost to the atmosphere, most of the N "used up" is transferred into the microbes and, as such, it is eventually available to plants.
To figure out what to do here, first estimate the starting C:N ratio of your compost pile.
(see http://www.compostinfo.com/ or search on "compost" and "C/N ratio")
The usual target C/N ratio at the start of the composting process is about 30/1 on a weighted average basis, but this ideal target can be adjusted depending on the bioavailability of the carbon and nitrogen. Wood chips have low bioavailability (compare to fine sawdust) thus you can either adjust higher or discount a portion of the wood chip C source. Fresh grass clipping have high N availability so may need to adjust on that basis also.
As carbon gets converted to CO2 (and assuming minimal nitrogen losses) the C/N ratio decreases during the composting process, with the ratio of finished compost typically close to 10/1. This is also typical C/N for soil. Soil microbes have a C/N of about 6.
Materials with bioavailable C at >10/1 serve to fuel microbial respiration, a process which places microbial demands on available soil nitrogen. The microbial population can out compete plant demand for soil nitrogen if large volumes of high C/N materials are placed in proximity to plant roots. Thus the concern.
High C/N in finished compost can make great mulch, which avoids soil incorporation and the induced soil N deficiency which results from incorporation.