Introduction to Soil Chemistry

200 speciation
many more species than the simple cations. Typically the higher oxidation
states predominate under oxidizing conditions, while the lower oxidation
states predominate under reducing conditions. However, it is common to find
both or all oxidation states existing at the same time in either an aerobic or
an anaerobic soil [7,8].
10.1.2.1 Iron
Iron and its various oxidation state species are very common components of
the environment. In addition to the common simple oxides FeO add Fe 2O 3,it
is found in minerals such as hematite, goethite, and ferrihydrite, and in a
number of hydroxy and oxy compounds. Because of its common occurrence
in the environment in general and in soil in particular, the total iron content
of soil is seldom a useful piece of information.
Most commonly iron is discussed as being in either the ferrous (Fe 2+ ) or
ferric (Fe 3+ ) state. Changes between these two depend on the soil’s pH and Eh
as discussed in Chapter 4. Acid conditions and low Eh values tend to lead to
the production of ferrous ion, while high pH and high Eh values result in predominance
of ferric ion. It should be noted that ferrous ion is more soluble
than ferric ion and thus will be more available to plants.
Iron cations in both the ferrous and ferric states can act as exchangeable
cations; however, ferric ion is generally unsoluble and thus not present on
exchange sites as such while its other species involve other oxidation states,
and compounds of iron, oxygen, and hydroxy groups tend to form other
cationic species which may be exchangeable. There are still other species
involving other ligands and ferrous and ferric ions that are chelated, thus
forming yet other species. Any compound having atoms with electron pairs
that can be shared with positive species will associate with iron cations in soil.
Any iron species may become attached to soil components such as sand, silt,
clay, and inorganic and organic colloids to form still more species.
Because of its common occurrence and biological importance, it is an essential
micronutrient for most organisms, a number of analytical procedures (see
Bibliography) for analysis of iron species have been developed and concentrate
on biologically available species [9].
10.1.2.2 Manganese
Manganese in soil has many characteristics that are similar to those of iron;
for instance, it exists in multiple oxidation states: Mn 2+ ,Mn 3+ , and Mn 4+ .
Although manganese can exist in the laboratory in other oxidations states,
these are the ones most common in soil. Manganese forms various oxide and
hydroxide species and chelates with many components. Its low oxidation state
(i.e., Mn 2+ ) is more soluble and more available than is its high oxidation state
(i.e., Mn 4+ ).