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 fertilizer.
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
any circumstances.
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
process)
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,
Ronald Nigh
Dana, A.C.
Mexico, D.F. & San Cristóbal de Las Casas, Chiapas
Tel. y FAX 525-666-73-66 (DF)
529-678-72-15 (Chiapas)
danamex@mail.internet.com.mx
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