Page 64 - Soil Degradation, Conservation and Remediation

50 2 Physical Deterioration of Soil

Sealing formation and crust can significantly reduce the infiltration of soil and

increase runoff at the surface of soil, which in turn increase the transport capacity
for entraining detached materials from soils (Levy et al. 1994 ). Surface crust and
soil erosion are closely related that many scientists have paid attention to the
formation of sealing and crust. Both soil crust and soil erosion involve particles’
detachment and transport processes. Although most erosion models do not take
account for the affection of crust to erosion, new concepts have been introduced
which involve the explicit delineation between detachment and transport pro-
cesses (Bissonnris et al. 1998 ). So it is believed that the new soil erosion model
should take the basic concept of sealing and crust as one of its supporting
theories.

2.2.2.1 Soil Factors Affecting Crust Formation

Crusting primarily involves the destruction of soil aggregates. Aggregates are
mainly held together by electrochemical forces binding the clay mineral particles
together. Aggregate stability depends on clay mineral types, their electrochemical
characteristics and the electrolyte concentration of the soil solution. The more stable
the aggregate of a soil is, the less susceptible it is to crusting.

Clay Minerals

Kaolinitic soils have usually stable aggregates because of edge-to-edge and edge-
to- face connections between the clusters, since some positive charges exist on the
broken edges, while the broad particle faces expose a negative charge. Kaolinite
is a non-expanding or non-swelling type of clay. So, soil aggregates made up of
dominantly kaolinite clay do not slake easily upon wetting. On the other hand,
montmorillonite is an expanding clay that swells extraordinarily when wetted.
Therefore, soils containing high proportion of montmorillonite form water-unstable
aggregates. Such soils, for example, Vertisols, slake easily when wetted. They
become sticky when wet and very hard when dry.

Exchangeable Cations

The formation of all soil crusts involves aggregate disintegration and dispersion.
The dispersion of soil colloids is affected by nature and distribution of exchange-
able cations held by electrical charges on colloidal surfaces. Exchangeable sodium
+
+
++
(Na ) is a much more dispersive cation than hydrogen (H ), calcium (Ca ), magne-
++
+
sium (Mg ), and other polyvalent cations. As the percent of exchangeable Na
rises, a solution of much higher concentration is needed to cause fl occulation. In
other words, clay with a high exchangeable sodium percentage will disperse quickly
under the dilution effect of rainwater at the soil surface.