Geology Reference
In-Depth Information
A
n-
M
n+
M
n+
n+
M
n+
M
M
n+
M
n+
A
n-
M
n+
A
n-
M
n+
n+
n+
A
n-
M
n+
M
M
n+
M
n+
M
n+
A
n-
A
n-
M
n+
M
n+
n+
M
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M
n+
Negative
Stern
Diffuse layer
Free solution
Surface
layer
cations > anions
cations = anions
Figure 13 The distribution of ions at a negatively charged surface of a soil component
The exchange is between equivalents of charge. Strictly, ionic activity,
not concentration, should be considered, but as ionic concentration in
soil solution is so low, it is commonly used. This is known as the Gapon
equation. This approach stresses the importance of the ion activity
ratio, which tends to remain constant. As a result, monovalent ions
are lost from a soil by leaching in preference to divalent and tri-
valent ions. A consequence is the acidifcation of soil by leaching (see
Section 5.4).
5.5.2.1 Cation-Exchange Capacity (See Also Chapter 4). The CEC of
a soil is a measure of its ability to hold cations at negative sites by
coulombic bonds. Because the exchange is by equivalents of charge, the
units used for CEC are centimoles of monovalent charge per kilogram of
soil (cmol
c
kg
1
). (Previously the units of milliequivalents per 100 g
soil were used (meq 100 g
1
); the numerical value of CEC is the
same whether expressed as cmol
c
kg
1
or meq 100 g
1
.) Typical values
of CEC for the aluminosilicate clays given in Section 5.2 are shown
in Table 6. These values for clay minerals are accounted for mainly by
the permanent negative charge formed owing to isomorphous substitu-
tion, with only a minor component coming from the variable, pH-
dependent charge on the clay edge. Humified organic matter has a
CEC typically between 150 and 300 as cmol
c
kg
1
, all of which is
pH dependent; the hydrous oxides also have a pH-dependent charge,
