Page 61 - Soil Degradation, Conservation and Remediation

2.2 Surface Sealing, Crusting, Hardsetting, and Compaction (Pc) 47

change in water flow patterns or the fragmentation of habitats. Current studies
suggest that soil sealing is nearly irreversible.
Soil hydrology may be severely impacted by surface sealing. For example, a
surface seal will greatly reduce water movement into soil (Ahuja 1983 ). Seal forma-
tion increases soil density and reduces porosity, pore size, and pore continuity.
McIntyre ( 1958 ) found that a seal consist of two parts: (1) an upper skin zone about
0.1 mm attributed to compaction by raindrop impact and (2) a washed-in zone of
about 1.5 mm of decreased porosity attributed to the movement of particles into the
soil with water. Agassi et al. ( 1981 ) suggested that there are two complementary
mechanisms of a seal formation: (1) a physical breakdown of soil aggregates, caused
by wetting and raindrop impact, and (2) physicochemical dispersion of clay

particles, which move into the soil with the infiltrating water, that clog pores to form
a washed-in layer of low permeability. Decreased total porosity increases bulk den-
sity (Assouline 2006 ; Eynard et al. 2004 ) and slows solute transport (Assouline
2006 ; Huang and Bradford 1993 ) and root growth (Lynch and Bragg 1985 ).
Macropores (pore diameters >1,000 μm) increase pathways for water that often

increase infiltration and reduce runoff. But surface sealing reduces the number of
macropores.
In agricultural soils, prolonged and repeated cultivation of surface soils during
cropping regimes destroys natural aggregation. Soil aggregates may collapse due to
plow pressure and by the load of heavy farm machineries and hoof-strokes of farm

or grazing animals. As natural aggregates are destroyed, finer particles are released
and, when exposed, disperse under both raindrop and irrigation action. Dispersed
particles reorientate and fill in larger pores. The soils most sensitive to aggregate

deterioration tend to be sandy loams, sandy clay loams, and sodic soils in the dry
climates and coastal regions. Sodic soils susceptible to slaking and dispersion are
particularly at risk following cultivation. Low organic matter containing soils cannot
develop stable peds that could resist slacking.
In a study, Heil et al. ( 1997 ) reported that many Sahelian Alfisols are prone to

sealing because of low soil organic matter content and exposure of fi ner-textured
subsoil attributable to erosion. Their study sites were located on six soil series of
the Hamdallaye watershed (500 ha), with soil textures ranging from sandy loam to

sand, classified as Psammentic Kandiustalf and Petroferric Kanhaplustult. All seals
sampled in the watershed were structural seals and were morphologically similar,
with a 0.1–1.0 mm thick continuous plasmic clay layer within 4 mm of the surface.
Organic C contents of sealed sites were very low (0.1–0.2 %) at 0–50 mm depth
and slightly higher at unsealed sites. Aggregation was too weak to withstand rain-
drop impact. Extractable Fe and Al contents of the six soil series were related to
clay content, which was likely the controlling factor of seal formation. With simu-
−1
lated rainfall of 90 mm h , the same six soils formed a seal during the initial
30-min rainfall event in most cases, with no change in layer thickness thereafter.
Soil with more than 15 % (silt + clay) content formed a 2-layer structural seal,
whereas coarser textured soils developed 4-layer structural seals. The physical pro-
cesses of soil slaking and sealing are the result of the kinetic impact of raindrops
on the soil surface and the translocation of soil particles by fl owing water. When