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permaculture - Re: [permaculture] N-starvation and mulch

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  • From: Dieter Brand <diebrand@yahoo.com>
  • To: permaculture <permaculture@lists.ibiblio.org>
  • Subject: Re: [permaculture] N-starvation and mulch
  • Date: Sat, 27 Oct 2007 01:33:28 -0700 (PDT)

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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