Geoscience Reference
In-Depth Information
two to three. The opposite process is reduction when the trivalent iron oxides are
changed to bivalent iron oxides. Analogously, compounds containing tetravalent
manganese are transformed by reduction to those containing trivalent manganese in
anaerobic conditions that exist in waterlogged soils.
The type of transformation of soil organic compounds depends upon the activity
of families of bacteria related either to electron donation (oxidation) or consump-
tion (reduction). Aerobic bacteria assimilate up to 10 % of the carbon, while anaero-
bic bacteria assimilate only about 0.5 % or less, leaving behind many waste carbon
compounds. Such assimilation has many practical consequences, e.g., up to one-
third of the fertilizer applied to a farmer's fi eld to provide nitrate for his crop can be
removed and lost from the soil by microbial respiration. Or, when aromatic com-
pounds are applied to soils to wipe out insects, they also serve as an instantaneous
source of organic carbon that stimulates the growth of bacterial families. As a result,
the insecticide is quickly demolished instead of the insects. This unwanted oxida-
tion occurs when phenol and benzoic acid are used as insecticides.
Another example is the existence of sulfate-reducing bacteria living in the anaer-
obic soil of a rice fi eld soil on the roots of the farmer's crop. The action mentioned
here of another variety of soil bacteria capable of reducing the structural trivalent
iron locked inside of clay minerals is discussed in more detail in Sect.
5.2.2
. There
are many, many more examples of bacteria transforming never-ending numbers of
mineral and organic components of soils.
In order to make at least one of our examples more transparent, we compare the
trivalent iron in crystal lattice of clay minerals to a prisoner with life sentence.
Dressed in a red coat, he has been confi ned to an isolated island in a prison sur-
rounded by concrete walls and a fence of electrically charged wires. All guards
believe that his escape is impossible. In addition to prisoners wearing red coats who
have many restrictions, there are prisoners wearing blue coats living with no restric-
tions. They are bivalent iron ions. Once, a couple of boats paddle noiselessly to the
prison by a bacterial crew trained how to smuggle a blue coat into the prison. They
succeed and the life-sentenced trivalent iron receives and immediately puts on the
blue coat. He is thus changed to a prisoner without restrictions. The mode of smug-
gling is the secret of reduction bacteria; they inherited it after millions of years of
attempts.
The number of bacteria living in the soil is far beyond our imagination. About
one hundred trillion bacteria live in a column of soil having a surface area of 1 m
2
.
That number of bacteria could remarkably increase by a hundred times. Since they
are very small, it is instructive to say that if we were able to sieve all bacteria from
1 g of soil, we would count up to one hundred million bacteria. In other words, in a
handful of soil there exists the same number of bacteria as the number of people
inhabiting our planet Earth.
A very important group of soil bacteria are
actinomycetes
. In many species when
observed under a microscope their shape resembles that of fungi - their elongated
cell is extended by very fi ne fi laments. Although the majority of actinomycetes are
aerobic, a few are anaerobic. They get their energy from the decomposition of
organic materials. The process leads to the release of plant nutrients from the decay-
