Agriculture Reference
In-Depth Information
The crystalline oxides of iron and aluminium.
Although other metallic elements
(Ti, Mn, etc.) occur in soils and are sometimes important, iron and aluminium are
among the most abundant metallic elements in the earth's crust. Due to their limited
solubilities, both are particularly abundant in highly-weathered soils (notably of the
tropics), the spodosols and the andisols where they may occur as discrete particles or
form coatings on other materials. Their presence in significant quantity may result from
the particular pedogenetic processes occurring within the soils involved, or the higher
iron or aluminium contents of their parent materials. For example, soils derived from
basaltic parent materials are usually richer in iron which, in reduced form, moves
readily in soils and may become concentrated through reduction - transport - recrys-
tallisation processes.
The oxides, hydroxides and oxy-hydroxides of both iron and aluminium occur in
a range of chemical forms in soils, depending on pH, environmental conditions and
such factors as organic matter concentration (Huang and Violante, 1986; Schwertmann
et al
., 1986). Aluminium also routinely substitutes for iron in some iron oxide minerals
commonly found in soils (Huang and Violante, 1986; Schwertmann and Herbillon, 1992).
The most common of the iron oxides in soil are goethite and haematite
although others may be prominent in specific environments. One such
example is lepidocrocite which is characteristic of such poorly drained
environments as swamps. Goethite may be the sole pedogenic iron oxide present
in soils of the cool and temperate zones. In many of the warmer parts of the world,
it occurs in association with haematite which imparts the vivid red colouring charac-
teristic of many tropical soils (Schwertmann and Taylor, 1989).
Ferrihydrite is a poorly crystalline form of iron present in many soils;
it is somewhat unstable and converts to goethite in cool temperate environments and
haematite in warmer areas. Maghemite is another iron oxide, probably largely
formed by the conversion of other iron oxides through firing (Schwertmann, 1988).
Gibbsite is common in highly-weathered soils and in laterites. It occurs
in highly crystalline form as small hexagonal plates or rods (Table I.5).
In contrast to the largely permanent charge of the phyllosilicate clay minerals, a
variable charge is associated with the oxide minerals. Both the magnitudes and
the signs of the net charges of the different oxide minerals vary with pH and, depending
on the oxide, the sign may be negative, neutral or positive at normal soil levels.
Both depend on the pH at which the net charge of the oxide is zero,
i.e
., its point of
zero net charge.
Net charge is negative above this point and positive below it.
The points of zero net charge of iron oxides occur between pH 6.5 and 8.0 and for
aluminium oxides between pH 7.5 and 9.5. This has considerable importance in
determining the retention of cations and anions (both inorganic and organic) in soils
where these oxides are present in appreciable quantities. The implications for plant
growth are important since nutrient cations may be readily lost from such soils, particu-
larly in agricultural situations where organic matter is present in low concentrations.
Exchangeable forms of aluminium in soils are mostly monomeric (Huang and
Violante, 1986).
