Introduction to Soil Chemistry

speciation 197
vivid example of this is thalidomide, which exists in two optical isomers, one
relieving nausea the other causing birth defects.
Inorganic and metallorganic compounds also have conformations and
isomers, including optical isomers. It is expected that these various conformers
and isomers will also have varying biological activities.
There are many other reasons why speciation might be important. Among
these are plant nutrient availability, biological contamination by metals, human
or animal toxicity, movement of species in the environment, biological accumulation
or amplification, obtaining data needed for models, understanding a
species’ role in the reactions and fates of various environment components,
and development of a basic understanding of a species’ chemistry. Any one of
these by itself is an important reason to understand speciation, and often
several of these reasons will be important at the same time.
Why might one be specifically interested in the species of components in
soil? From a positive perspective it is desirable to provide plants with nutrients
in forms that are available and yet are not present in concentrations high
enough to cause environmental harm. From a negative aspect, some species
of certain components are more and some less toxic. Some species are biologically
available; some are not. Some species may accumulate in biological
tissue; others will not. When a species is detected in the environment, it is
important to know which of the categories described above it falls into.
A first step in deciding on an analytical procedure or a species to look for
is to understand that the species of interest may be in one of, four “compartments”
in soil (see Figure 10.3): the solid (both inorganic and organic), the
liquid (soil solution), the gaseous (soil air), or the biological (living cells) compartment.
It is important to remember that species are constantly moving both
between compartments and between species.
Once the compartment is decided on, then the species, as illustrated in
Figure 10.1, to be analyzed for can be decided on. In this case it is important
Solid
Gaseous
Biological
Figure 10.3. Soil compartments.
Solution