Page 62 - Soil Degradation, Conservation and Remediation

48 2 Physical Deterioration of Soil

the drop impact forces exceed the internal cohesion of the impacted soil aggregates,
they break down into primary mineral particles. These particles are transported by
surface runoff or washed into the soil surface layer (Schmidt 2010 ). When depos-
ited the translocated particles could clog soil pores and form superfi cial layers
characterized by higher bulk density and lower saturated hydraulic conductivity
than the soil beneath (Betzalel et al. 1995 ). Due to the loss of soil water storage and

infiltration capacities, soil erosion and the risk of flooding are substantially
increased.

2.2.2 Surface Crusting

On further consolidation and drying, surface sealing gives rise to surface crusts.
Sealing formation and crust are very common phenomena in many soils worldwide,
especially in arid and semiarid soils. Rainfall causes a series of interactions between
water and soils: disintegration, detachment, entrainment, deposition, and compres-
sion. These actions result in the formation of seal and, subsequently, the crust of
soils. Crust is a thin layer at the soil surface characterized by a greater density,
higher shear strength, and lower hydraulic conductivity than the underlying soil
(Zejun et al. 2002 ). The mechanism of crust formation involves two main comple-
mentary processes: (1) physical action including disintegration of soil aggregates
and soil particles compaction caused by impaction of raindrop and (2) physical-
chemical action including dispersion of aggregates, movement of soil particles that
clog the conducting pores and form a less permeable layer at topsoil region (Cai
et al. 1998 ). The formation of seal and crust depends on many factors, including the
texture and stability of soil, intensity and energy of rainfall, gradients and length of
slope, and electrolyte concentration of the soil solution and rainwater (Remley and
Bradford 1989 ).
There are two types of surface crusts: structural crust and depositional crust.
A structural crust is a surface layer of the soil, a few millimeters to a few centime-
ters thick, more compact than the material beneath. The import of external materi-
als is not involved in the formation of the structural crust. Structural crusts are
developed also due to trampling by livestock or through traffic by agricultural

machinery. Structural crusts may be hardsetting crusts and traffic crusts. Hardsetting

is a process of compaction of soil along with increased bulk density occurring
without the application of an external load. The term hardsetting was first used by

Northcote ( 1960 ) in his soil classification system of Australia. Hardsetting is a
characteristic of soil horizons, usually cultivated seedbeds, which contain unstable
soil aggregates. Soil aggregates collapse and the seedbed slumps when the soil is
wet, and a hard, structureless mass of soil results upon drying. Hardsetting, how-
ever, involves a much greater thickness of material, which commonly includes not
only the A1 or Ap horizon but also the E horizon (Greene 2005 ).
Hardsetting is also a surface crust which involves the collapse of some or all
of the aggregates during and after wetting of previously loosened topsoil. The