Introduction to Soil Chemistry

microorganisms 55
microaerophilic, while all other areas will be either aerobic or anaerobic.
Because of these environmentally different areas, a well-drained soil will have
both oxidized and reduced forms of all components present at the same time.
Carbon dioxide (CO2), oxidized and methane (CH 4), reduced carbon, sulfate
(SO 4 2- ), oxidized and hydrogen sulfide (H2S), reduced sulfur, ferrous (Fe 2+ )
reduced, and ferric (Fe 3+ ) oxidized iron can be found in soil at the same time.
In continually submerged soils there is no oxygen, and so the entire environment
is anaerobic and reducing. Under these conditions there will be a
predominance of the reduced forms mentioned above, namely, methane,
hydrogen sulfide, and ferrous iron.
Soil microorganisms play an extremely important role in cycling of environmental
elements such as carbon, nitrogen, and sulfur. The cycling of these
elements and others is often represented as their respective cycle (carbon cycle,
nitrogen cycle, sulfur cycle, etc.) [11]. Of these the two most important are the
carbon and nitrogen cycles. Organisms chiefly responsible for the carbon cycle,
animals, plants, and microorganisms change carbon from carbon dioxide to
plant and animal tissue and eventually back into carbon dioxide.A critical step
in this process is the decomposition of organic matter by microorganisms, and
if the organic matter is in contact with soil, these will be soil, microorganisms.
During this process other elements important to life are either taken up or
released to be used again. One of these other elements, nitrogen, and its cycle
have many critical steps carried out by soil microorganisms.
There are many other cycles such as the phosphorus, potassium, halogen,
and sulfur cycles; the latter is illustrated in Figure 3.8. All the transformations
illustrated are carried out by soil microorganisms. It is interesting that sulfur
is converted from its elemental form to either fully oxidized or fully reduced
forms by various microorganisms in soil. The starting point in the cycle may
be either a reduction or an oxidation depending on the electron status of the
starting and ending compounds and the environment where the reactions are
occurring. It should also be noted that there are a broad range of organic sulfur
compounds, linear, branched, cyclic, and aromatic, which, although not shown,
can also occur as part of the sulfur cycle [11].
Microorganisms are important in soil chemistry, soil analysis, and instrumental
methods for two extremely important reasons: (1) between taking soil
samples and their analysis, these organisms can cause extensive changes in the
chemistry and chemical composition of the soil sample, completely changing
the species and amount of components found; and (2) almost all extraction
procedures will cause the destruction of any cells, animal, plant, and microbial,
found in soil with the subsequential release of cellular constituents into the
soil solution. Some constituents will be degraded by the extraction process,
some enzymes may continue to function, and some cellular components will
complex or form chelates with metallic components in the soil. These reactions
will lead to a more complex mixture of components than one might
expect to be the case. All of these eventualities can affect or change the analytical
or instrumental results obtained [11–14].