Introduction to Soil Chemistry

onding considerations 33
The two oppositely charged species then attract each other, forming a compound.
Although the seminal characteristic of compounds held together by
ionic bonds is that they dissolve in water, giving a solution containing ions, it
is essential to keep in mind that many ionic compounds are insoluble. The solubility
of these compounds depends on the relative strength of solvation and
bonding energy.
Some ionic compounds contain a combination of bonds. For instance, in
polyatomic ions such as ammonium (NH4 + ), the hydrogens are bonded to the
nitrogen by polar covalent bonds. The ionic bond is thus between this covalently
bonded moiety and another ion.
Ionic bonds are typical of inorganic compounds, and thus the mineral or
inorganic components of soil often contain ionic bonds and are soluble in
water. This means two things: (1) the soil solution should always be expected
to contain salts and their corresponding ions and (2) the inorganic components
of soil should be expected to dissolve in the soil solution, some at a very slow
rate, resulting in their ions being present in low concentrations.
The third aspect of this is that analysis of inorganic or ionic compounds
must take into account not only their solubility in the soil solution but also
the possibility that they may be present as exchangeable ions. Extraction procedures
designed for inorganic components thus need to take these two characteristics
into account [7].
2.2.3. Ion Exchange Interactions
The soil colloids, both inorganic (i.e., clay) and organic (i.e., humus), contain
charges that are balanced by cations and anions associated with the charged
sites. Most soil clays and humus contain a predominance of negative charges
and thus act as cation exchangers. Some clays also have significant numbers
of positive charges and will act as anion exchangers. Thus soil can have both
cation exchange capacity (CEC) and anion exchange capacity (AEC).
When considering cation or anion exchange capacity the pH of the soil
solution is extremely important. There will be competition for binding sites
between H 3O + and other cations in the soil solution. In addition, some surface
atoms may become protonated, thus decreasing the available negative sites on
these surfaces. Therefore, the observed CEC will be less at high proton concentrations,
that is, at low or acid pH levels, and higher at basic pH levels. Thus
the CEC of the soil at the pH being used for extraction is the important value,
not a CEC determined at a higher or lower pH.
Exchangeable cations must be removed from the exchange sites to be
detected and quantified. To accomplish this, the soil sample is extracted with
a solution containing a cation having multiple charges or present at high concentration.
At the same concentration, cations having more positive charges
will replace those on exchange sites having smaller charge. This condition can
be overcome if the less charged cation is in high concentration when even a
cation with one positive charge can replace a cation with multiple charges.