Introduction to Soil Chemistry

198 speciation
to consider all possible species that might be present in the compartment. It
is also important to consider the changes that a species is likely to undergo
during the sampling and analytical procedures to which it will be subjected.
This topic is discussed further in Section 10.5 [1,2].
10.1. CATIONS
Any positive chemical species can be considered a cation or as being in a
cationic form. Some will be present as relatively simple, single-oxidation-state
cations, while others may be more complex in that they may have several oxidation
states and the cations may have oxygens or hydroxy groups associated
with them.
In all cases cations will associate with water molecules. Thus it is common
to find them expressed as hydrated or hydroxide species. Such a representation
more closely resembles the actual condition of cations in soil, particularly
if the discussion relates to cations in the soil solution. This brings up a troublesome
question or problem. When a cation is removed (extracted) from a
solid matrix into an extracting solution, a species change most likely occurs.
In the solid matrix the cation may not be associated with water molecules,
while in solution it most certainly will be. For hydrated species the amount of
associated water and the activity will also change. This raises the issue of the
actual species present in the solid matrix. When analyzing for a specific cation
species or discussing cation speciation in soil, it is important to keep this issue
and problem in mind.
Molybdenum should be discussed in this section because it is a metal;
however, it is always present in soil as an oxyanion and so will be discussed in
the next section [3,4].
10.1.1. Simple Cations in Soil
Simple cations are those that exist in only one oxidation state in soil and are
associated with water, although they may also be chelated and form other associations
with inorganic and organic components.
The alkali and alkaline-earth metals are examples of relatively simple
cations that occur in only one oxidation state and are surrounded by waters
of hydration in the soil solution. The most common in soil, in order of decreasing
abundance, are calcium (Ca 2+ ), magnesium (Mg 2+ ), potassium (K + ), and
sodium (Na + ). Sodium is typically present in small amounts in high-rainfall
areas, whereas it may be relatively high in low-rainfall areas.
Although simple, these cations can occur as a number of different species.
They will be present as exchangeable or in solution as hydrated cations. They
can be part of the inorganic component structure, for example, as in isomorphous
substitution, or be trapped between clay layers. They will also be associated
with organic matter and with colloidal inorganic and organic matter,