Michel Cavigelli wrote:
> I am in Extension with one of my duties being to discuss supplying N to
>crops via organic sources, and am always interested in additional pieces of
>this puzzle. As you mention, there is a lot we do not know about
>decomposition, N availability following decomposition and soil microbial
>ecology. You mention a few specific examples that are interesting, though.
>I was wondering if you have any citations from the scientific literature
>that address the Azotobacter aspect of this issue and the statement that
>synthetic fertilizers harm Azotobacter. Thanks.
I don't know if "harm" is the word. N, especially NH4, has a well known
effect of suppressing N-fixation in both free-living and symbiotic species,
primarily by inhibiting the production of nitrogenase, the magical enzyme
that catalyzes the N-fixation process (truely one of the most remarkable
biochemical processes in the living world and one upon which all life
depends). References on this can be found in any good text on the
biochemistry of N-fixation. I can recommend:
Stacey, G., R.H. Burris and H.J. Evans (1992) Biological Nitrogen Fixation.
New York: Chapman & Hall.
Azotobacter in particular is also highly sensitive to acid conditions and
so this can be another inhibiting effect of N fertilizer. For general
information on N-fixing organisms of all kinds, including Azotobacter try:
Sprent, J.I. and P. Sprent (1990) Nitrogen Fixing Organisms: Pure and
Applied Aspects. London: Chapman & Hall.
Of course the people with the most direct experience of the use of
Azotobacter in agricultural production are the Cubans, who produce it by
the hundreds of tons to subsititue for the chemicals formerly supplied by
the Soviet Union. They use beer yeast factories that are idle during most
of the year (since the national demand for beer yeast is produced in only 5
months) and adapt them to produce Azotobacter innoculate in volume. They
have also developed a small-scale production unit for on-farm production.
They offer both the innnoculate and the production technology as well as
technical assistance commercially.
I think the Cuban research on this is pretty much unknown in the US. It is
mostly published in in-country jounals in Spanish but you might find
something searching the international ag and biology data bases. US
biotech has taken a totally different tack on this problem and dreams of
"engineering" a plant with an nif gene so it can fix its own nitrogen. (A
nice dream but if some legume has already evolved this rather obvious
alternative there may be good reasons why it can't be done). Such a
superplant would of course, be imminently "patentable", like Roundup Ready
and Terminator and unlike Azotobacter and other naturally occuring
organisms that have only to be fed and nutured to supply N to crops.
I received another inquiry regarding Azotobacter from another SANET reader,
sent privately. I would like to share one of her comments however as it is
something I have heard from organic growers often and it raises a very
>I've gardened organically in the desert for something like 30
>years with great results without adding any where near the amount of
>nitrogen our Horticultural County Agent insists is required. When I took
>his Master Gardener class, I was shocked at the quantities of manure he
>insisted are required if I don't use chemical fertilizer. Could it be
>that I have been benefiting from the action of azotobacter? Or does the
>horticultural establishment simply "require" excessive amounts of
>nitrogen in their effort to maximize yield? --Edna Weigel
Your garden has definitely benefited from the work of biological nitrogen
fixation. This 'mystery nitrogen' often appears in the nutrient budgets of
organic farms. Conventional methods result in the inhibition of this
supply and so it is rarely taken into account, especially from free-living
sources such as Azotobacter, and so must be "made up" by adding more
fertilizer. Even so-called "organic" techniques involving massive doses of
N in the form of manure, compost or bloodmeal, for example, inhibit natural
N fixation and are therefore counterproductive (e.g. what we used to call
'chicken-shit' farming back in California.) In other words, many organic
farmers are still thinking of soil as a chemical system and even have their
compost analyzed for NPK levels and then apply the same formulas to come up
with application rates. This is how your, probably well-meaning, extension
agent came up with those exaggerated estimates of compost needed for garden
Most estimates of free-living N fixers contribution are on the order of 2
to 5 kg/ha. I recall some work from the tropics about 20 years ago
(reviewed by Pedro Sanchez as I recall) criticized this as an underestimate
and talked about quatities on the order of 40kg/ha under proper conditions.
Molina, the agronomist who did the recontruction work in Brazil I
mentioned earlier, claims even higher levels can be achieved. Contribution
of organisms on leaf surfaces, also a tropical phenomenon, has been ignored
In thinking about these levels we should recall that some estimates of
wasteage in the application of chemical N are as high as 50-80% (or
higher), resulting in the volitization and runoff pollution by N which have
become global ecological problems, (as recently noted in Science).
Nitrogen, fixed biologically in situ in the field, may be far more
efficiently used by crops and therefore much lower levels would produce an
equivalent result in yields. As far as I know, no studies have been done
comparing organic and conventional techniques in terms of N-fixation and
clearly even many organic growers are not fully aware of the situation. It
seems clear, though, that the actual and potential contribution of
free-living organisms such as Azotobacter have been largely underestimated.
Obviously a new research orientation is urgent for organic farmers. It
would be wrong to focus only on N-fixation and bacteria in thinking about
organic soil fertility, however. Many other organisms--eg mycorrhiza--play
equally important roles and a holistic view is necessary.
Best regards to all,
Mexico, D.F. & San Cristóbal de Las Casas, Chiapas
Tel. y FAX 525-666-73-66 (DF)
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