permaculture

[Dieter Brand <diebrand@yahoo.com>]

Composting does occur at a range of C/N ratios, but, according
  to what I understand, optimal composting conditions are near
  a ratio of 25/1. Using the compost heap as example, which is
  easier to understand, I have understood this to mean, that, if
  the ratio is for example 100/1 then all of the N will be used to
  compost part of the C, going through successive cycles until 
  the ideal ratio is reached, at which time full blown composting
  will take place. Hence, the higher the C contents the more
  time needed for the composting.
   
  Anyway, I didn't want to insist on any particular ratio. What 
  I meant to say was that the carbon in the mulch needs N to 
  decompose. Perhaps some of it comes from the air, but it
  would be surprising if a substantial amount didn't come from
  the soil. How does it move up? I don't know. But I think
  a static image showing the soil on one side and the mulch 
  on the other with a clear-cut interface in between doesn't 
  reflect the reality of a biologically active soil very well. The 
  moment the mulch touches the ground intermingling starts 
  to take place. There are not just earthworms that pull the
  mulch underground, there are numerous small beetles and
  other animals that constantly move between the soil and
  the mulch. After all we are not talking about plastic sheeting
  that will sit on the soil indefinitely, we are talking about 
  organic mulch that will disintegrate into the soil, it is this
  disintegration that sequesters some of the N for a time.
   
  When talking about nutrients like N, one often imagines 
  chemical substances, but If we feed the soil by organic 
  means, soil nutrients are often in biological form which 
  provides for a lot more mobility in the upward direction.
   
  Dieter Brand
  Portugal
  
Paul Cereghino <paul.cereghino@comcast.net> wrote:
  
>Is it not the case that roughly 25 parts of carbon need 1 part of nitrogen
> for composting to occur
>
I have seen this 25:1 ratio as a recommendation for optimizing the rate 
of bacterial decomposition (hot aerobic pile) without loss of surplus N 
as ammonia gas (old urine smell). Compost will occur under a range of 
conditions just smelly or slowly.

>I thought I had 
> observed that a thick layer of very fine carbon rich material like saw dust 
> does inhibit plant growth.
>
I don't think I am contradicting Robyn... that the right mulch on the 
right soil could soak up available nitrogen from surface soils. I have 
also seen thick wet sawdust make soil underneath anaerobic, and ethylene 
gas produced from anaerobic decomposition can directly hurt plant 
roots. I think we are back in one of those 'it depends' situations.

Paul Cereghino
Olympia, WA
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