Agriculture Reference
In-Depth Information
■
Cations in the Stern layer are held more strongly because of specific forces, in ad-
dition to the simple electrostatic force of attraction. Within the monovalent cation
series, the overall strength of adsorption increases in the order Li
Na
K
Rb
Cs
. Similarly, the overall strength of adsorption of the divalent
cations increases in the order Mg
2
Ca
2
Sr
2
Ba
2
.
■
Sesquioxide surfaces and the edge faces of kaolinite crystals develop pH-dependent
charges as a result of the association, or dissociation, of H
ions. The attraction
of cations and anions is modified because the net surface charge changes with pH.
The pH at which the diffuse layer charge is zero defines the
point of zero charge
(
PZC
) of the surface.
■
The
cation exchange capacity
(
CEC
) is measured by replacement of the resident
exchangeable cations with an “index” cation. One common method uses a strong
salt solution, for example, M NH
4
Cl at pH 7. Others use an index cation of high
affinity for the surface at a lower concentration (e.g., 0.1M BaCl
2
) at the soil pH.
The former method overestimates
CEC
in acid soils with a pH-dependent charge;
the latter methods are preferred for such soils because they measure the
effective
CEC
(
ECEC
).
CEC
values range from 3-20 cmol charge (
)/kg for kaolinites to
100-150 cmols charge (
)/kg for vermiculites.
■
The difference between the
CEC
and the sum of exchangeable cations (
Ca, Mg,
K, Na) defines the
exchangeable acidity
(Al
3
and H
). Al
3
is an acidic cation
because it readily hydrolyzes in water to release H
ions. Because of slow clay
mineral dissolution, acid clays are primarily Al
3
-clays. Neutralization of the hy-
drolysis products of Al
3
(hydroxy-Al ions and polymers) accounts for much of
a soil's
titratable acidity
(or total acidity) over the pH range 4-7.5. Titrating a clay
or soil with an alkaline solution is a way of measuring its
pH buffering capacity
.
■
Ions move to plant roots by
mass flow
and
diffusion
. The former process is im-
portant for ions in relatively high concentration in the soil solution (e.g., Ca
2
and NO
3
). The latter is the main process for the movement of adsorbed ions
such as K
and H
2
PO
4
, normally present at a low concentration in solution.
Many plants, including grapevines, have evolved a fungus-root symbiotic associ-
ation, or
vesicular-arbuscular mycorrhiza
, that enhances the uptake of P, Cu, and
Zn from soils deficient in these elements. Insoluble P compounds may also be dis-
solved by organic acids secreted by roots and rhizosphere microorganisms.
