Agriculture Reference
In-Depth Information
volume changes consequent on freezing:thawing or hydration:dehydration, as in the
mechanical pedoturbation (churning) that occurs in vertisols.
Biological processes may also effect a translocation of soil materials (bioturbation)
and can result from the activities of soil animals (faunal pedoturbation) and through such
processes as windthrow by plants (floral turbation) (Hole, 1961). Bioturbation largely
takes place within the upper 30 to 60 cm of the soil whereas physico-chemical processes
may operate throughout the whole soil profile, and within the underlying regolith
(Brabant, 1991).
One consequence of continued bioturbation has been the widespread formation of
biomantles. These are defined as differentiated zones in the upper parts of soils produced
largely through bioturbation but aided by other subsidiary processes (Johnson, 1990).
Such biomantles may be single to multilayered depending on the combination of agents
that gave rise to them. Over time, continued erosional redistribution of bioturbated mate-
rials may give rise to surface layers characteristically-different from the underlying
materials in particle size distribution and in structural and other properties.
3.3.1
PHYSICAL AND CHEMICAL PROCESSES
Some soils are formed by weathering in place. They have a Bw (w = weathering) horizon
in which the accumulated clay minerals (colloids) result from
in situ
weathering of the
parent material. In other soils, the differentiation of a Bt (translocated) horizon results from
the movement or redistribution of certain components of the weathered materials (par-
ticularly clays) down the soil profile by two important processes: leaching and eluviation.
Leaching has been defined previously as the movement of material through the soil
profile both in solution and suspension. However, it seems desirable to restrict the word
leaching to the transport of materials in solution and to define eluviation (or pervection)
as the transport of solid particles in suspension (Duchaufour, 1982). Leaching occurs
when there is substantial movement of water through the profile and this may occur
either downwards or laterally beneath the surface. It affects the more soluble salts such
as potassium or sodium rather than the less soluble alkaline earth metals, calcium and
magnesium. Under some circumstances, upward movement of salts may occur through
capillaries. As shown in Chapter I (Section I.1.1.1.2), even poorly-soluble compounds
such as silica, particularly where present as phytoliths, may be broken down to silicic
acid and either leached from the system or taken up by plant roots (Lucas
et al.,
1996).
Translocation in soils may occur through the following five major processes differing
in their intensities (Duchaufour, 1997) (Figure II.7).
Salination
Salination typically occurs in arid or irrigated soils when soluble potassium and sodium
salts present in the profile accumulate close to the surface. These salts move to
the surface in capillary water as a consequence of high evaporative demand, or because
of high saline water tables. In soils where the deeper horizons contain appreciable salt,
salination may result from tree clearing since the water not transpired or evaporated can
leach sub-surface salts which may appear as saline surface seeps in lower-lying parts
of the landscape.
