permaculture

[Paul Cereghino <paul.cereghino@comcast.net>]

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