Introduction to Soil Chemistry

surface features 35
Because different components are held to soil components by different
types of bonding and attractions, the interaction can be relatively strong or
weak. Thus extraction procedures must be capable of extracting the desired
component when it is held by different forms of bonding to different
components.
2.2.7. Other Ways of Investigating Bonding
Bonding and other interactions between the components in soil (i.e., the clays)
and organic components can be investigated by conducting adsorption experiments.
An organic molecule is added to a suspension of clay and the amount
adsorbed after a fixed amount of time is determined. The amount adsorbed is
plotted against the amount added to produce adsorption isotherms. The shape
of the graph is then used to indicate the type of interaction between the molecule
and the clay. With this type of investigation, various types of adsorption
phenomona can be distinguished.
Two of the most common ways of handling such data is to try to fit the data
to either a Langmuir or a Freundlich type of equation, or alternatively to
simply determine which of these two equations best describes the data
obtained. Although some useful information can be obtained about the interactions
between the components being studied, neither provides specific information
about the type of bonding in terms of orbitals, or interactions such as
those discussed in the previous sections. Spectroscopy, as discussed in Chapter
7, is typically the method used to determine bonding details [11,12].
SOIL COMPONENTS IN COMBINATION
2.3. SURFACE FEATURES
Both sand and silt surfaces are dominated by oxygen and its lone pairs of electrons
in p orbitals. In some instances broken surfaces may also have siliconhybridized
sp 3 orbitals 4 available for bonding. Comparison of sand, silt, and
clay reveals the surface area of sand and silt to be low and the interaction
between surface bonding orbitals and components in the surrounding medium
relatively weak.
As a first approximation, the surfaces of the clays can be grouped into three
types: (1) surfaces consisting exclusively of oxygens with their lone pairs or
electrons, in p orbitals, extending at an angle away from the surface into the
surrounding medium; (2) surfaces containing —OH groups with the partially
positive hydrogens extending into the surrounding medium—because of the
4 This would be a hybridized orbital formed by the hybridization of one s and three p orbitals as
opposed to 2s,2p hybridization in carbon.