> >Sure, organisms like Azotobacter and Azospirillium are
> >desirable, but they don't add enough N for economically
> >sustainable crop production, and in the big picture are
> >not the central issue.
>
> This is the official line. If you think about it a little you
> will realize that it is just not true.
I've been trying to dig up information about this. This may be one of those
things that works well in the greenhouse but less well in the field. The
abstracts I have turned up on this say things like:
"Agricultural plants slowly use the biological N, so the application of N
fertilizers is necessary to obtain high yields."
"Particularly for dryland systems, these associations appear to be very
unreliable as a means of raising the nitrogen status of plants."
"Maximum benefit can be realized only through analysis and resolution of
major constraints to BNF performance in the field..."
"Azotobacter culture increased the yield by 5% and Azospirillum by 3%."
"Seeds inoculated with non-symbiotic fixing bacteria could save about 25 kg
N without much affecting the grain yield."
It looks like the N contributions are minor compared to things like plowing
down a stand of alflafa, application of manure, or modest fertilizer
application. If you have information about the contribution of free-living
nitrogen fixers, please send it, I would like to know.
> >I respect the "organic mythos" as an esthetic value, and
> >consider "organic" gardening to be an interesting challenge.
> >But I can't see any biological reason why modest additions
> >of fertilizer are detrimental to long-term soil health. It
> >is drainage and tillage that burns up the soil,
> >not fertilizer.
>
> It would be silly to argue about this. What is a "modest
> addition"?
I'm not arguing about this, just point out the available evidence. It looks
like
1. Free-living N fixers are small players in the N-budget of most annual
crops
2. Addition of fertilizer N increases soil organic matter and biological
activity, all else being equal.
Of course there may be a lot we don't know!
> If you really respect the "organic mythos as an esthetic
> value" then you would understand why "a modest addition"
> of chemical fertilizer can't be permitted in certified
> organic products.
I'm more interested in sustainability than organic gardening (which is fine
if you have an upscale market).
> But the problem is not "modest additions". Modest amounts
> of N are easily available from natural sources (eg.
> azotobacter)--so why not use them?
Because to use azotobacter one probably must create an N-deficient situation
in many crops, such as corn. The reward you get is low yield. In practice,
it is more practical to use legume rotations and cover crops if you must
stay away from N fertilizer for esthetic or market reasons.
> Modest additions are not the problem. The problem is
> hundreds of kilos of N per hectare being applied in
> American agriculture,
I agree. We need better recommendations and perhaps more incentives for
farmers to avoid excessive N application rates.
Dale
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