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.
From: Ronald Nigh <email@example.com>
To: firstname.lastname@example.org <email@example.com>
Date: Tuesday, November 03, 1998 11:56 AM
Subject: Sawdust, nitrogen and the organic mythos
>This thread is a little stale now-I was unable to respond to it earlier-yet
>I think it has brought up a number of very important issues that were only
>left half resolved.
>The low N availability in the presence of sawdust is predicted roughly by
>current models of soil organic matter, though the fit is not so good as to
>convince us that we really know everything we would like to know about the
>soil ecology involved. We think of N as 'limiting factor' on the
>process-thus recommendations such as were made to add synthetic N salts to
>the sawdust manure mixture. But this is a cop out. It is like having a
>broken wind mill and 'fixing it' by hooking the pump up to an electric
>motor-it solves the immediate problem of pumping water but it doesn't
>achieve the real goal which is to pump water cheaply from wind energy.
>Spaying N fertilizer on your compost supplies the temporarily scarce N to
>the plants, but it doesn't reach the real goal which is to tune the soil
>microbiology so that sufficient N is built up naturally. In fact, the
>synthetic N actually disrupts that microbiology and is, therefore
>counterproductive, not to mention expensive and environmentally unsound.
>This is the real organic mythos; not slavish rejection of chemicals just
>because they 'aren't organic' as one poster suggested, but a commitment to
>farming with natural sources of fertility. It can be done, as has been
>demonstrated time and again, despite the dire predictions of those who see
>the soil only as a chemical system. The key is to create the conditions
>for the microorganisms who perform the functions we want. Organic farmers
>have a lot of practical experience with this but our scientific knowledge
>is still rudimentary. One key organism seems to be Azotobacter-this
>aerobic, free-living bacteria does almost everything we want. It fixes
>nitrogen, using as an energy source recalcitrant subtrates such as
>lignins-celulose mixture in wood and straw and produces extracellular slime
>that binds soil particles into crumb structure. It probably does not do
>this alone but in symbiosis with fungi and protozoans, etc and it likes to
>have sufficient Ca and P around for maximum activity. It is also more
>effective in well-structured soils where abundant anerobic microsites
>provide reduced fermentation products as energy sources. On the other
>hand, its activity is immediately suppressed by the addition of N
>Azotobacter is just one that we know a little bit about-clearly many other
>organisms are involved in the natural cycles of fertility in a productive
>organic field and the use of chemicals probably always plays havoc with
>these systems. That is why chemicals are not used in organic farming under
>I might add that in raw sawdust, as one poster pointed out, toxic natural
>organic substances-tanins, terpines etc.-- can be present that inhibit
>microbial activity and therefore could affect the availability of N or
>other nutrients in the early stages of decomposition.
>In practical terms what this means for the use of sawdust bedding as an
>organic fertilizer is: compost the mixture so that the decomposition of
>the wood fibers is largely achieved before being applied to the soil. This
>would involve mixing with a Ca source, ashes and probably more organic
>matter since I would suspect that the micture of manure, urine and sawdust
>still starts out with a fairly low C:N (and high P) which is not usually
>desirable for composting. (This depends on a number of factors including
>the time frame and the kind of compost you actually want at the end of the
>Designing a simple system of earth worm composting is an excellent
>alternative, resulting in a superior compost with ideal effects on soil
>microbiology in many cases (not always, nothing works everywhere) Another
>alternative would be to use this substrate for mushroom production,
>producing an excellent compost and a valuable cash crop. These two
>approaches can be combined.
>Another possible solution is sheet composting (i.e. applying directly to
>the field and waiting for decompositon to occur before planting) which was
>suggested by the original poster. This might work, but would depend on a
>number factors such as soil type and climate, timing and other things. Its
>principal advantage is that it is easy. It would not be the preferred
>approach in most cases.
>Sawdust animal bedding is a valuable ingredient for organic fertilization.
>It is important to experiment in your situation to find the use to which it
>can best be put.
>Regards to all,
>Mexico, D.F. & San Cristóbal de Las Casas, Chiapas
>Tel. y FAX 525-666-73-66 (DF)
> 529-678-72-15 (Chiapas)
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