Agriculture Reference
In-Depth Information
Faunal modification of soil texture
Soil texture may be altered by those soil animals that are most pedogenetically active,
particularly termites, ants and earthworms (Chapter IV). Animals of these groups usually
select the finer soil particles in building their surface constructions. These particles may
be transported from a range of depths in the profile and used to build their surface
mounds and temporary surface structures, or simply cast on or near the surface. Over time,
erosion reduces the mounds and accumulated casts and the soil is re-distributed over
the surface. On sloping sites, the finer particles may be transported away from the site
by water, leaving the larger particles to accumulate as a coarser-textured surface
horizon. This may result in a surface horizon of distinctly coarser texture overlying one
of finer texture (see, for example, Williams, 1968; Wielemaker, 1984). Elsewhere, an
accumulation of finer-textured surface soil results with an upper limit of particle size
related to the capacity of the fauna to transport them.
1.1.1.2 Clays and clay minerals
The inorganic soil particles less than 2 in diameter are collectively termed clay.
They are varied in nature, ubiquitous in soils and play a critical part, either directly or
indirectly, in almost all soil processes and interactions. Many of the materials in this size
range that are found in soils may be classed as minerals. That is, they are naturally-
occurring homogeneous solids possessing a definite but not fixed chemical composition
and an ordered atomic arrangement (Schulze, 1989). The occurrence of these minerals
in soils results from three mechanisms, namely, inheritance from parent materials,
formation in situ by crystallisation from solution or through the alteration of existing
minerals into new forms (transformation) (Allen and Hajek, 1989) (see Chapter II).
These minerals are most usefully classified by their dominant anion or anionic group
(Schulze, 1989) and the following are the groups of most importance in soils:
1.
2.
3.
4.
5.
6.
7.
Sulphides
Oxides, oxyhydroxides and hydroxides
Halides
Carbonates
Phosphates
Sulphates
Silicates
Not all of these mineral groups are present in all soils and some only occur under
particular environmental conditions. The simple mineral halite (NaCl) is one such
example and is found naturally in arid areas or as an evaporite in certain tropical
coastal saline soils. Other common simple minerals include calcite
gypsum
and dolomite In contrast, the silicate minerals are
relatively stable, ubiquitous in soils and the types and proportions present are of
the greatest importance in determining soil properties and behaviours in the face of
agricultural and engineering usages.
Many soils retain a range of coarse fragmentation products derived from their parent
rocks and therefore still possess some of the primary minerals derived from the parent
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