Introduction to Soil Chemistry

42 soil basics ii
possible coatings are quite varied. However, three of the most important coatings
with which one should be familiar to understand the chemistry of soil are
water, iron oxides, and organic compounds. All soil surfaces will be “contaminated”
with a combination of these three compounds plus others, such as
manganese oxides and elemental carbon, if they are present. Thus the orbitals,
bonding, energy, and other characteristics of these surface coatings will also
come into play when considering their reactivity and extraction of components
from them [17–19].
2.11. CONCLUSIONS
Soil inorganic solids are composed of particles of decreasing size from sand to
clay. The clay fraction is further divided into principally 1:1 and 2:1 clays. The
1:1 clays are typified by kaolinite, which, compared to other clays, exhibits
lower activity. The 2:1 clays are typified by fine grained micas that are not
expanding and smectites that are expanding. Clays have high sorptive capacity
and are one source of cation exchange in soils. Bonding of soil components
to each other and to surfaces involves all the standard types of bonding,
namely, ionic, polar covalent, covalent, hydrogen, polar–polar, and Van der
Waals. Reactions thus involve s, p, d, and sp orbital overlap and ionic and
partial charges such as those involved in hydrogen bonding. These bonding
considerations also involve the common surface features such as surface
oxygens and hydroxy groups along with exposed aluminum and silicon.
Extraction of components from soil therefore involves all types of bonding
along with energy, reaction path, steric, and rate factors. Also involved will be
micelle formation along with coatings on all soil surfaces.
PROBLEMS
2.1. Identify the three primary particles in soil and describe the chemical differences
between them.
2.2. Identify the three major types of clays in soils and explain how they
differ chemically.
2.3. Describe the different types of bonding and their primary occurrences
in soil.
2.4. Describe the surface features, with particular reference to orbital availability,
involved in surface binding of components in soil.
2.5. In terms of bonding energy, the bonds formed in an exothermic reactions
must be lower than those in the reactants. How is this known?
2.6. Explain how both reaction mechanisms and energy considerations
contribute to the abundance of the species of compounds found in
soils.