permaculture

[Dieter Brand <diebrand@yahoo.com>]

Paul,
   
  Is it not the case that roughly 25 parts of carbon need 1 part of nitrogen
  for composting to occur no matter whether the carbon is in the compost
  heap, the soil or the mulch?
   
  I don't of course have any scientific evidence, but I thought I had 
  observed that a thick layer of very fine carbon rich material like saw dust 
  does inhibit plant growth. In most cases this is not so dramatic as to be
  visible because the concentration of carbon is not that high or because
  the material is so coarse, like coarsely shredded branches, that 
  decomposition is spread over a longer period.
   
  In his research on Ramified Wood Chips in agriculture, the Canadian 
  scientist Pierre Lemieux, for examples, notes that an N-source, like
  manure, needs to be used to offset the effect of the wood chip mulch.
  But he was experimenting with very thick layers of wood chips most
  of us wouldn't use.
   
  Dieter Brand
  Portugal

Paul Cereghino <paul.cereghino@comcast.net> wrote:
  why (apparently) mulch materials don't cause N-starvation for plants 
in contrast to just laying other types of organic matter just on top 
of the soil?


I haven't found any evidence in the science world that top-dressing 
carbon rich material causes N in the soil to migrate upward and get 
bound-up in a decomposition cycles (temporarily unavailable in a 
particular location). Free N available in the 'soil solution' as NH4 or 
NO3 is dissolved in water or weakly bound to organics or clay. Water 
mostly goes downward. More likely is that nitrogen sloughing out of a 
complex mulch migrates into the root zone. 

Plant roots go foraging for Nitrogen and will go where the N is, taking 
advantage of variation in the soil, trading with fungi, following an 
earthworm burrow, curling around the husk of a beetle. I think the 
'nitrogen starvation' idea has taken on a life of its own without a lot 
of solid evidence. I think the concept started with the problem of 
evenly tilling in woody material in annual agriculture as a soil 
amendment across a whole root zone causing a temporary deficit of free N 
while the woody stuff is broken down.

Paul Cereghino
Olympia, WA
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