Introduction to Soil Chemistry

82 soil basics iv
ionized will be determined by the pK a of the acid or phenol or in the case of
amines, their pK b. The solubility of organic molecules that contain ionizable
groups is greatly increased once they become ionized.
Once ionized, the acid molecule carries a negative charge and can thus
attract cations and participate in the cation exchange capacity of soil. The contribution
of this source of negative charge will depend on the pH of the soil
solution and will change as the pH changes. This is thus a variable or pH
dependent cation exchange capacity (CEC).
Because of the variability of this component of soil, the CEC determined
at two different pHs should expectedly result in different CEC values; this can
also happen if two different laboratories determine the CEC. For this reason
it is essential that CEC always be measured in the same way, taking care to
define the pH of the solutions used in its determination.
4.9. SOIL pH
In any aqueous solution the pH is a measure of the hydrogen ion or proton
activity. However, in many if not most cases, pH is treated as the concentration
of protons in solution rather that their activity. The soil solution is no different
except that the measurement is much more complex. The complexity
arises from two sources:
1. An electrical potential develops at all interfaces. In soil, there are interfaces
between solids and solution, solution and suspension, suspension
and the electrode surface, and the reference electrode and all these
interfaces.
2. The concept of activity is extremely important in soil. Protons or hydronium
ions attracted to exchange sites or other components in the system
will not be measured as part of the solution composition.
For these reasons, a standard method of measuring soil pH is chosen and all
phenomena related to pH or involving pH is related to this “standardized” pH
measurement. The most common method is to use a 1:1 ratio of soil to water,
typically 10mL of distilled water and 10g of soil. In this method, the soil and
water are mixed and allowed to stand for 10min and the pH determined using
a pH meter.
This method does not measure exchangeable protons attached to cation
exchange sites; therefore, it is also common to use a salt solution (either KCl
or CaCl2) instead of distilled water in determining soil pH. The K + or Ca 2+ in
the solution exchanges with exchangeable hydronium, thus bringing it into
solution where it can be measured. These procedures therefore usually give a
pH that is less, more acidic, than that obtained using distilled water. The justification
for this approach is that it is thought to more closely relate to the
pH experienced by plant roots.