Page 63 - Soil Degradation, Conservation and Remediation

2.2 Surface Sealing, Crusting, Hardsetting, and Compaction (Pc) 49

hardsetting processes can be divided into two physically distinct processes:
slumping and uniaxial shrinkage. Slumping occurs during and after the wetting
of a soil containing water-unstable aggregates. The aggregates soften and swell

simultaneously, and some or all of the finer particles (silt and clay) become sus-
pended. Some of the clay fractions disperse. Aggregates disintegrate because

they have insufficient strength to withstand the stresses set up by rapid water
uptake, caused by rapid release of heat on wetting, trapped air, the mechanical
action of rapidly moving water (Collis- George and Greene 1979 ), or by differen-
tial swelling. Uniaxial shrinkage is important because the closer proximity of
particles increase the strength upon drying hardsetting soils. Laboratory experi-
ments on the behavior of aggregate beds of a hardsetting soil, wetted under
tension or at zero potential, show that at least during the early stages of drying,
uniaxial shrinkage occurs. Mullins et al. ( 1987 ) have proposed the following
explanation for the development and increase in strength observed in hardsetting
soils, starting with a cultivated bed consisting of dry aggregates: wetting of the
system mobilizes some or all of the silt and clay. This may occur through slaking
and/or dispersion. During the early stages of drying, the mobilized material is
carried behind the retreating water meniscus to occupy concavities on the surface
of sand grains and any remaining aggregates, forming annular bridges between
them (Kemper et al. 1987 ). Traffi c crusts are formed by the external pressure of
farm machinery and animals which can cause a serious reduction in penetrating
water and seedling emergence. Overgrazing can induce crust formation by two
mechanisms: surface compaction of wet or moist soils and mechanical destruc-
tion of the surface soil aggregates. Some soils are naturally and genetically hard-
setting. They are abundant in tropical areas (Fabiola et al. 2003 ). Naturally,
hardsetting soils are unable to develop water-stable aggregates. Hardsetting con-
dition can occur in soils with high exchangeable sodium percentage. Some
hardsetting soils are impervious, compacted as well as cemented. Amorphous
silica and imogolite-like aluminosilicates may act as cementing agents (Chartres
et al. 1989 ).
On the other hand, a depositional crust develops when soil particles, suspended

in water, are deposited on the soil surface as the water infiltrates or evaporates.
Externally derived materials are always involved in the construction of depositional
crusts. Depositional crusts develop by deposition of suspended particles on the soil
surface. These are found in some cultivated and noncultivated soils. The main
sources of fi ne eroded soil particles are fl ood and furrow irrigation water, raindrop

impact splash of loose soil particles, overflow and floods from rivers and runoff, and
sheet erosion. The clay and silt particles in turbid suspension can either disperse or

fl occulate. They flocculate when the electrolyte concentration in the suspension
exceeds the flocculation threshold of the clays (Oster et al. 1980 ). Depositional

crusts formed from flocculated particles have an open structure and high permeabil-
ity. Conversely, when the suspension electrolyte concentration is below the fl occu-
lation threshold, dispersed particles settle to form the depositional crust, the
hydraulic conductivity of which is several orders of magnitude lower than that of
the parent soil.