Agriculture Reference
In-Depth Information
Even moderate Al interlayering greatly affects clay properties. Such materials
are important in submerged soils that have become ferrolysed (Section 7.1).
Amorphous Aluminosilicates
. These occur in soils influenced by volcanic activ-
ity and are associated with very high moisture retention and anion fixation, and
low to very high pH-dependent CEC. They may also bind organic matter tightly,
protecting it against decomposition. Examples are allophane and imogolite.
Oxides
Being widespread in the lithosphere and insoluble in the usual range of soil
pH, oxides and hydroxides of Al, Fe and Mn are common in soil clays. Red
or yellow coloration of soils is apparent at Fe oxide contents of only 0.1% or
less, especially if the Fe is amorphous and coats other minerals. The most visible
change occurring when soils are submerged is the conversion of the red and
yellow compounds of Fe(III) to the bluish-grey compounds of Fe(II).
Metal oxides and hydroxides have little or no structural charge but develop
pH-dependent charge as the hydroxyl groups at the lattice edges gain or lose
protons. The surface charge is a function of both pH and the concentration of
salts in the solution, as these affect the dissociation of the -OH groups. How-
ever the pH at which the surface negative charge is equal to the surface positive
charge—the point of zero charge (pzc)—is independent of the salt concentration
if the salt does not react with the surface. The point of zero charge is an impor-
tant characteristic of the surface. Table 3.10 gives pzc values for common soil
materials. Values are large for metal oxides and hydroxides but small for silica
and soil organic matter.
In real soils where oxides, layer silicates, organic matter and other materials
are present in intimate mixtures, with the oxides and organic matter often coating
the surfaces of the other materials, the different functional groups interact with
Table 3.10
Points of zero charge (pzc)
of oxides and aluminosilicates
Material
pzc
α
-Al
(
OH
)
3
5.0
γ
-AlOOH
8.2
α
-FeOOH
7.8
γ
-Fe
2
O
3
6.7
Amorphous Fe
(
OH
)
3
8.5
MgO
12.4
δ
-MnO
2
7.2
SiO
2
2.0
Feldspars
2-2.4
Kaolinite
4.6
Montmorillonite
2.5
