permaculture

[Robyn Francis <erda@nor.com.au>]

Mulches with a hard cellulose structure such as sawdust, wood chips,
eucalyptus leaves and pine needles have high N demands to break down. Some N
will be drawn from air but can also use up N from the top soil layer. Hence
these kind of mulches are best used when partially broken down rather than
fresh - adding a nitrogen-rich liquid (e.g. Urine) to stockpile heaps prior
to mulching can alleviate this problem.
This is not a problem with fine carbon mulches like straw and hay.
Ciao
Robyn

On 26/10/07 6:30 PM, "Dieter Brand" <diebrand@yahoo.com> wrote:

> 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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