Agriculture Reference
In-Depth Information
trients Fe
3
, Cu
2
, Mn
2
, and Zn
2
and the “heavy metals” Cd
2
, Ni
2
, and
Pb
2
, are also adsorbed. But chelation with organic compounds and precipitation
in insoluble salts play a more prominent part in the retention of the last two groups
of cations. Ions such as Cl
, SO
4
2
, and NO
3
are held as exchangeable anions
in soils that are highly weathered and contain appreciable sesquioxides (positively
charged). This is more common in the subsoil where the influence of organic mat-
ter is negligible.
The capacity of a surface to hold exchangeable cations depends on its charge
per unit area. The most convenient surrogate for this variable is the moles of charge
per unit mass (see box 2.5), which is a measure of the
cation exchange capacity
(
CEC
). Consider the reversible exchange reaction
(NH
4
)
(Ca-clay)
(NH
4
-clay)
1
/
2
(Ca
2
) (4.11)
In this reaction, one mole of NH
4
charge in solution exchanges for one mole of
Ca
2
charge on the clay. The terms (NH
4
) and (Ca
2
) represent the molar ac-
tivities of the ions in solution, and (Ca-clay) and (NH
4
-clay) represent the activ-
ities of the adsorbed ions. In most cases, we may substitute concentrations for ac-
tivities (box 4.4). According to the Law of Mass Action, the equilibrium will be
displaced to the right (favoring displacement of Ca
2
ions into the solution) by
increasing the concentration of free NH
4
ions or by removing Ca
2
ions from
the site of the reaction. This principle is the basis of
CEC
measurements, as out-
lined in box 4.5.
Reversible reactions of the form of equation 4.11 can be written for several
cation exchange equilibria in soil, for example
(Na
)
(Ca-clay)
(Na-clay)
1
/
2
(Ca
2
)
(4.12)
(Na
)
(Mg-clay)
(Na-clay)
1
/
2
(Mg
2
)
(4.13)
These reactions are discussed in chapter 7 in the context of soil sodicity.
Box 4.5
Methods of Measuring the CEC of Clay Minerals and Soils
Methods of measuring
CEC
fall into two groups (Rayment and Higginson 1992):
1. Exchangeable cations are displaced at a fixed pH by a concentrated salt
solution, such as M NH
4
Cl or M NH
4
OOCH
3
at pH 7. Displacement is
achieved by the high concentration of the “index” cation (NH
4
). The index
cation is then displaced by another cation (e.g., Na
) and the amount of
NH
4
adsorbed (cmol (
)/kg) is measured.
2. Exchangeable cations are displaced by an unbuffered salt solution at no fixed
pH, and the amount of index cation that has been adsorbed (e.g., Ba
2
from
0.1M BaCl
2
/0.1M NH
4
Cl solution) is measured. Displacement is achieved
because of the high affinity of Ba
2
for the surface.
If the concentration of exchangeable cations is to be measured in the extracts,
a prewash with aqueous ethanol is used to remove any soluble salts.
CEC
values
determined by methods of the first group are widely reported in soil survey data,
and are used in some soil classifications. However, in soils with variably charged
(continued)
