Agriculture Reference
In-Depth Information
A vineyard soil with montmorillonite clay that shows surface cracking when dry.
Photograph by the author.
Figure 2.9
Aluminum oxides
have a greyish-white color that is easily masked in soils, ex-
cept where large concentrations occur, as in bauxite ore bodies. In acid soils, pre-
cipitates of Al(OH)
3
form in the interlayers of chlorites, vermiculites, and smec-
tites, and generally as surface coatings on clay minerals. This poorly ordered
material slowly crystallizes to
gibbsite
(
-Al(OH)
3
), the principal aluminum oxide
mineral in soil.
Several oxides of manganese are found in the soil, ranging from manganous
oxide (MnO) to manganese dioxide (MnO
2
), according to the valency state of the
Mn. The most stable form of MnO
2
is
pyrolusite
, but the most common Mn ox-
ides in the soil are in the
birnessite
group, in which Mn
2
, Mn
3
, and Mn
4
ions
are bonded to O
2
and OH
. Small black deposits (1-2-mm diameter) of man-
ganese oxides are common in soils that experience alternating aerobic and anaer-
obic conditions. The surfaces of these minerals have a high affinity for heavy met-
als, especially cobalt (Co) and lead (Pb). The availability of Co to plants is often
controlled by the solubility of Mn oxides (section 5.6.2).
The Fe and Al oxides have the general formula (Fe, Al)
2
O
3
.nH
2
O. Because
the mole ratio of oxygen to metal is 3:2, the generic term for these oxides is the
sesquioxides
. The oxides have a variable surface charge, which is usually positive at
acid to neutral pHs. Thus they form very stable coatings on negatively charged
clay minerals. The positive surface charge also means that they adsorb negatively
charged organic and inorganic ions from solution, which confers an
anion exchange
capacity
(
AEC
) on the soil. The influence of the sesquioxides on soil properties is
discussed in chapters 3 and 4.
