Agriculture Reference
In-Depth Information
of these effects must be expected to alter between locations. The soils developed on older
geological surfaces may be expected to have clay mineral suites that reflect past rather
than present climates. Further, when extreme parent materials such as volcanic ash and
serpentine predominate, soil clay mineral assemblages are unlikely to reflect current
climatic influences but will reflect parent material properties.
The cation exchange capacity of clays
Soil colloids (principally phyllosilicate clays and organic matter) have a mixture of
positively and negatively charged sites on their surfaces to which are attracted a range
of ions, both organic and inorganic. In most soils, the negative charges predominate
to give a net negative charge to a soil or horizon; less commonly, a net positive charge
may pertain in certain soils, particularly at low pH,
The permanent net negative charge on the phyllosilicate clays is of considerable
importance in plant nutrition. It is the most important mechanism through which plant
nutrient and other cations are held at the surfaces of the soil colloids. The major basic
cations and (which raise the soil pH) and the acidic cations and
(which lower it) are retained at negatively-charged sites on both organic and inorganic
colloids within soils. The capacity to retain these positively charged ions is called the
cation exchange capacity and is defined as the product of the specific surface area and
the surface charge density.
An analogous anion exchange capacity, usually of lesser magnitude, also exists in soils
and is responsible for the retention of such anions as and at colloid surfaces.
It is associated with such positively-charged sites as the surfaces of variable charge
oxide minerals, principally those of iron and aluminium. These elements are normally
present at low concentrations in most soils but, because of their small particle sizes
(Table I.5), have very large specific surface areas and thus reactivities.
A further source of charge, variable in nature, occurs on the surfaces and particularly at
the edges of phyllosilicate clay mineral sheets due to the dissociation of hydroxyl groups.
As considered in a later section, organic matter has a high CEC because of the dissocia-
tion of and groups. Its substantial net negative charge at most soil pH
levels is entirely variable in nature. As will be seen, soil organic matter may be the only
colloid able to retain appreciable quantities of the nutritionally-important cations in
acid soils. This is particularly relevant in such highly-weathered soils as the tropical
oxisols and ultisols (Uehara, 1982).
In soils with variable charge components, both the pH and ionic strength of the soil
solution influence the net charge of the soil as illustrated in Figure I.10. This figure
shows the relationships of these effects on the net charges of soils from two horizons of
an iron- and aluminium-rich oxisol (van Raij and Peech, 1972). The net charge becomes
positive at pH values below the point of zero net charge (normally in the range 3.5-4.5).
Such values are significantly lower than the points of zero net charge for the component
soil oxides (pH 7-9) which will have net positive charges in most soils. In comparison,
the points of zero net charge of many clay minerals lie in the pH range 4.5-6.0.
From the substantial differences illustrated, it is clear that such agricultural practices
as liming and fertilising will alter the net surface charge of the soil through their
influence on both pH and soil solution properties.
