Market Farming

["Bill Shoemaker" <wshoemak AT inil.com>]

>> Sojourner wrote:
>>
>> > The belief that sawdust will "tie up the nitrogen" in your soil is a
>> > myth based on studies done with sawdust that had been "worked in" to
the soil - e.g., was decomposing (or rather not) in anaerobic conditions.
>> >
>> > Spreading sawdust on TOP of your soil will not cause a nitrogen
deficiency.  Aerobic decomposition, taking place in the presence of oxygen
(which is what goes on when you surface spread) is a whole  'nother animal
than anaerobic.

    Spreading sawdust on top of the soil CAN cause a localized nitrogen
deficiency because the microrganisms that feed on that layer of sawdust
laying on the ground need nitrogen too, whether they are aerobic or
anaerobic bacteria. They cannot get nitrogen from the air. They can only
draw upon nitrogen reserves in the soil or in the mulch because of the form
that nitrogen takes when in the soil. Since sawdust has virtually zero
nitrogen in it the nitrogen is withdrawn from the top layer of soil.
HOWEVER, if the sawdust has manure mixed in or is well composted, it will
not need soil nitrogren as it will supply the microorganisms itself.

        Mixing it in the soil is a problem for the above reason, but also
because the soil may not be aerobic enough itself, for instance, heavy
clays. If the soil has already been built up organically and can breathe to
a reasonable depth, the oxygen levels in the soil will be adequate to
support a reasonable level of aerobic decomposition. But that is never as
good as composting the sawdust or other organic material first. Composting
provides an initial breakdown of carbon bonds in the organic material that
relieves soil organisms from the burden, and "expense", of doing it.

Bill Shoemaker
Sr Research Specialist, Food Crops
Univ of Illinois - St Charles Hort Research Center