Dear Floyd
Well now you gotten more advice of all kinds than you'll be able to figure
out what to do with. I know nothing about farming where you are and so
don't really have any advice, but I have been interested in the question
you raise about microbial decomposition and have seen some of the answers
play out in farms and gardens in Mexico. These are in tropical lowlands and
tropical mountains, so conditions are really very different from yours.
It seems that leaving residue on the surface, all things being equal, has a
more beneficial effect on soil structure than incorporating residues into
the soil. Note that I'm talking about soil structure, or crumb structure,
not organic matter content. Soil structure is more directly important to
plant (especially root) growth than is OM content. The reason for this is
that soil structure is maintained primarily by colloids (mucous-like
organic substances) that are the byproduct of aerobic cellulose
decomposition by fungi and bacteria (both). One bacteria, Spirocheta, is
especially important in this, but several organisms are actually involved.
So breakdown on the surface is basically aerobic and causes this microbial
gel to rain down into the soil, sticking the particles of clay and organic
matter together making a nice crumby structure where roots grow freely and
water and air reach down several inches into the soil. There are some other
details, but that is basically how it works. This would suggest that your
goal should be to have a better, deeper structure rather than to have a
certain % of OM.
The important thing to get here is that crumb structure is not a physical
or chemical *property* of soil, but a *process* that depends on microbial
action and a source of raw OM as food. As soon as the OM disappears, the
microbial action stops, the crumbs dissolve and the soil becomes compacted,
even if you don't run a tractor over it. (Think of a forest continually
raining leaves onto the soil). All you need to keep soil nice is a constant
supply of raw OM and the proper conditions for its breakdown, maybe some
earthworms and such..you don't usually need plows, or rototillers or
choppers or any of that stuff.
In a soil with poor structure, i.e. compacted, mulching like this seems to
work better to improve structure. But in soils with good structure, root
decompostion works better for maintaining a good structure. Since the roots
have access to air in a crumby soil, it is not anerobic decomposition that
is taking place, so the colloids are formed to maintain structure. In
either case, it should not be necessary to incorporate residues into the
soil by cultivation. ( Some people like to disk the residues in 3 o 4
inches so they don't get in the way of planting etc.--but it should be no
deeper than that.)
A soil with crumb structure is actually a two-phase, aerobic-anerobic
system. The surface of the particles are exposed to air, while inside
there are increasingly anerobic microniches. This allows products to be
passed from one phase to another, fermtenation products such as fatty
acids, ethylene, etc., can form in anerobic environments and then get
oxidized by aerobic organisms. This is the sign of a healthy, productive
soil.
I have see farmers put this into practice in the tropics, where leaving
residues on the soil surface--just mowing the cover crop at the proper
time--gives us the additional advantage of protecting the soil against
torrential rains which we get frequently. Even fertile soils in the
tropics rarely build up much OM (1-or 2% maximum) because of the hot, humid
conditions; microbial breakdown is just too fast for much humus to form.
But organic grain farmers in Nebraska have also told me that they feel
leaving residues on the surface gives better results.
(One more point: Elaine Ingham talks about a rough balance between fungi
and bacteria in agricultural soils, while fungi dominate in forests and
bacteria in pastures. Remember this is just an overall rule of thumb--both
kinds of organisms are always present and are important.)
I'll be interested in hearing how it all works on your place.
Regards.
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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