Agriculture Reference
In-Depth Information
rock and formed by high temperature or high pressure igneous and metamorphic
processes (Chesworth, 1992). Quartz (a tectosilicate) is a major and common component
of soils; it may form the major part of the sand and silt fractions and frequently also the
coarser clay-sized components (Drees
et al
., 1989). It may be either primary or secondary,
that is, synthesised from Si within the soil rather than inherited from the parent material.
Soils also contain a range of other secondary products notably clay minerals that have
been re-synthesised from the weathering products of the parent rock and other materials,
or inherited from the breakdown of sedimentary rocks. Inorganic soil particles of clay
size therefore include a diversity of crystalline and non-crystalline materials that, in most
soils, form associations with a variety of other substances, both inorganic and organic.
Because of their colloidal properties, the dominant phyllosilicate clay and other
minerals must remain flocculated if the soils within which they occur are to have
acceptable levels of stability. As colloids, they have large surface areas per unit mass or
volume (specific surface areas) (Table I
.5
) and many of their properties derive from this
(Petersen
et al
., 1996). These include their capacities to retain both water and a range
of plant nutrient and other elements at their surfaces. The large surface areas influence
the rates and pathways of many soil chemical reactions and directly dictate the amount
of water retained in the soil. Also, the amounts and types of the phyllosilicate clay
minerals present influence the strengths and dispersabilities of the soils within which
they occur. Clays are usually associated with other materials, both organic and inorganic,
to form structural aggregates of all sizes that impart stability to the soil and control such
important processes as water movement and aeration. Clays also play important roles in
controlling the activities of micro-organisms and in reducing enzyme activity within
soils (Burns, 1986) (see Chapter IV, section 4.1.3.2).
Classification of soil clay and other minerals
The principal materials of clay size that occur in soils are the crystalline phyllosilicate
and oxide minerals, the 'amorphous' minerals often derived from materials of volcanic
origin and the gels or metallo-hydrates. The crystalline phyllosilicate clays are the larger
common clay minerals; as explained later in this chapter, the more reactive paracrys-
talline and amorphous materials are substantially smaller (Figure I.5).
The phyllosilicate clay minerals.
The silicate minerals are based on two primary
structures, the silicon tetrahedron and the aluminium octahedron. The terms tetrahedron
and octahedron refer to the regular structures associated with co-ordination of the central
cations; silicon is bonded to four ions arranged in a tetrahedron while the central
aluminium ion of the octahedron is bonded to six ions to form a regular octahedron
(Figure I.6) (Millot, 1979). By sharing certain of their oxygen ions with adjacent octahedra
and tetrahedra, these structures may be arranged to form compound structures including
chains, rings, three dimensional frameworks and sheets. Of these, the most widespread and
important in soils are those that occur as sheets, the phyllosilicate clay minerals.
The phyllosilicate clay minerals are ubiquitous in soils and, except in the most
coarsely-textured soils, may make up much of their volume. Their abundance in soils
is a consequence of the dominance of their major constituents (Si, Al and O) in
the earth's crust. These are also the best known of the clay minerals since a number have
