Over the past few decades, it has been well established that a variety of soils may develop a layer of reduced permeability at the surface when exposed to rainfall, after tillage practices have been applied and no cover has been provided. This process is referred to as surface crusting [59, 60]. The process has been extensively studied for well-aggregated soils [61-67] and, to a lesser extent also for poorly aggregated, coarse-textured soils [68-71].

Falling raindrops are the most effective agent of both soil consolidation and soil dispersion. Their action is understood easily by considering the momentum of a single raindrop falling on the soil surface. Such momentum is partly reflected and partly transferred, each component depending on the slope of the soil. The transfer of momentum to soil particles has two effects. First, it provides a consolidating force compacting the soil and, second, it produces a disruptive force as the water rapidly disperses away from the point of impact and falls again to the surface.

The consolidation effect is seen in the formation of a surface crust, usually only a few millimeters thick, which results from clogging of the pores by soil compaction. This is associated with the dispersal of fine particles from soil aggregates and/or clods, which then fill in the pores between soil aggregates [72]. The most recent studies have pointed out that crusts have a dense surface skin or seal about 0.1-mm thick formed of well-oriented clay particles. Beneath this skin, there is a layer 1- to 3-mm thick, where the larger pore spaces are filled by the finer detached and washed-in material. Whereas Hillel [73] has tried to explain crusting as the consequence of collapsing of the saturated soil aggregates, Farres [74] has demonstrated that raindrop impact is the most important agent of crust development.

A distinction often is made between a crust and a seal. In fact, sealing refers especially to the reorganization of the surface soil layer during a rainstorm, whereas crusting refers to the hardening of the surface seals when the soil dries out. Over a number of rainstorms, a structural crust initially develops in situ following the destruction of aggregates and the

Figure 4.11. Kinetic energy required to detach sediment. Source: [77].

clogging of pores by finer material. This is followed by the formation of a depositional crust due to the deposition of the finer laminated sediments transported by the overland flow and deposited in soil surface depressions where puddles form during storms [75]. Generally speaking, larger areas of depositional crusts may form downslope where the gradient generally decreases and the amount of sediment input is increased by the deposition of sediment transported by rills.

The most important consequence of the formation of a surface crust is the reduction of the infiltration capacity and, consequently, an increase in surface runoff. In loamy soils, a reduction of the infiltration capacity from about 45 mm/h on an uncrusted soil to about 6 mm/h on the same soil with a structural crust has been observed [76].

In general terms, the intensity of the crusting process decreases with an increase in the clay and organic-matter content. It follows that loam and sandy-loam soils are the most vulnerable to crust formation.

Studies carried out by Poesen [77] on the kinetic energy required to detach 1 kg of sediment by raindrop impact have shown that a minimum energy is required for particles of 0.125 mm, and particles ranging from 0.063 to 0.250 mm are most vulnerable to detachment (Fig. 4.11). Coarser particles are more resistant to detachment because of their weight, whereas the finer clay particles are resistant to detachment because the raindrop energy has to overcome the adhesive and/or chemical-bonding forces linking the minerals of the clay particles [78]. This means that the soil with high percentage of particles within the most vulnerable range (i.e., silty-loams, loams, fine sands, and sandy-loams) will be the most susceptible to detachment and crusting.

The actual response of a soil to a given rainfall depends upon its moisture content and structural state on the one side and on the intensity of the rainfall on the other. Le Bissonnais [79] describes three possible responses:

1. When the soil is dry and the rainfall intensity is high, the soil aggregates dissolve quickly by slaking due to the breakdown of bonds by air compression ahead of the wetting front. The infiltration capacity decreases rapidly as a consequence of the sealing of pores.

2. When the surface aggregates initially are wetted partially or the rainfall intensity is low, then microcracking occurs, and the larger aggregates break into smaller ones. Surface roughness thus decreases and the infiltration capacity remains high because the pore spaces within the microaggregates are not sealed.

3. When the aggregates are initially saturated, the infiltration capacity depends on the saturated hydraulic conductivity of the soil, and a large quantity of rain is required to seal the surface.

As well as inducing a decrease in infiltration and an increase in runoff, soil crusting and sealing also represent an impedence to seedling emergence as a consequence of the dense surface skin or seal of oriented clay particles, which occurs when the surface dries up. Moreover, the orientation of soil particles produces a continuity in the direction of macropores, favoring an increase in surface evaporation and, consequently, a greater loss of water from the rhizosphere.

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