Soil Erodibility

The early studies of soil erodibility included the ratio of colloid amount to moisture equivalent, the erosion ratio, the silica sesquioxide ratio, the suspension percentage, and dispersion ratio - none was related closely enough to the erosion rate to be useful for predicting soil's susceptibility to erosion (Middleton, 1930, Middleton et al., 1934, Peele, 1937). Adams et al. (1958) stated that soil properties which influence soil erosion may be divided into two types:
    1. those that affect the infiltration rate
    2. those that resist dispersion and erosion during rainfall and fun off.
Erosion occurs when the kinetic energy of surface runoff which is converted into a shearing force exceeds the intergranular shearing resistance of soil particles by which they resist the removing forces.  In the investigation of the effect of soil strength, the strength of the soil was measured in situ with a shear vane. The shear strength parameters (cohesion and angle of interior resistance) of the soil have been lumped together, and are represented by the shear strength with unit kilonewtons per squar meter [kN/m2]. However, it was apparent that the more significant shear strength parameter of the soil in the context of soil erodibility is cohesion.  The plot of the effect of shear strength on the degree of erosion was drawn, where the degree of erosion was expressed as change in cross-section area is shown in Figure 1.  The degree of erosion is inversely related to soil strength, in which case, the higher the soil strength, the lower the degree of erosion.  A small change in shear strength has a considerable effect on the degree of erosion over the lower range of shear strength.  However, beyond a threshold value of shear strength, the corresponding change in degree of erosion following a change in shear strength, is relatively small.

Cruse and Larson (1977)  tested the hypotheses that during rainfall the soil water matric potential (P) i.e. the negative pore water pressure, the bulk density of the surface layer, and the interparticle soil links influence soil detachment by controlling the strength of the soil surface. To test the hypothesis, they conducted laboratory tests on effect of an impact of a single simulated 4.8 mm raindrop falling on soil surface from a height of 177 cm.  When a raindrop strikes and eroding soil surface, the detachment of soil particles depend on several factors, including intergranular shear, the viscosity of the pore fluid, the rupture energy of liquid and mechanical bonds, and the inertia of the disturbed zone. Of all these factors, Cruse and Larson considered shear strength as the most important in the detachment process the soil mechanical properties.. To test relationship between detachment D and shear strength Tf  Cruse and Larson computed a second degree polynomial regression of (D x 104)1/2 on shear strength (Tf).as shown in  Figure 2.  The correlation coefficient R2 = 0.86 indicates shear strength effectively explained the variability in D, which means that the amount of soil detached by a single raindrop is closely correlated with the shearing strength as measured by a triaxial compression test.  They also found that with increasing soil density (d) the soil detachment (D) decreases (Figure 3).

Yamamoto and Anderson (1973) focused their study of soil erodibility on erodibility indexes. They emphasized that erodibility index cannot be expected to account for all the differences in splash erosion.  Its usefulness lies mainly in identifying from soil characteristics the problem areas that may require more careful land management techniques. Yamamoto and Anderson studied 13 possible erosion indexes from which five seems to have the most significant correlations with splash erosion of some soils in Hawaii measured under simulated intense rainfall. The five indexes that had the highest degree of association with gross splash erosion and maximum splash rate in the presence of other contributing soil and site variables were:

  1. the percent (weight) of water stable aggregates 0.25 to 0.50 mm in size,
  2. the ratio of water stable aggregates less that 0.25 mm in size to the mean weight diameter of water stable aggregates,
  3. the surface area of aggregates larger than 0.25 mm in size,
  4. Anderson's surface-aggregation ratio, and
  5. the ratio of suspension percent to mean weight diameter of water stable aggregates.
The statistical weight of the five erodibility indexes according to Yamamoto and Anderson is shown in descending order in Table 1. 

The lack of close correlation between eroded soil and a single soil property led Wischmeier and Mannering (1969) to use multiple regression analysis for relating eroded sediment with several soil properties. Because the multiple correlation coefficient was high, their regression equation has been used for determining the soil factor value for use in the Universal Soil Loss Equation.

For better understanding of how and to what extent each of the various properties of a soil affects its erodibility Wischmeier studied fifty-five soils by field, laboratory and statistical methods. Wischmeier and Mannering have found that particle size distribution and organic matter content are the two most important indicators of erodibility.

Bruce-Okine and Lal (1975) conducted their studies on erodibility of soil on two tropical soil types from western Nigeria. The soils were tested by raindrop technique to determine their erodibility indexes.  They tested aggregate size, initial soil moisture potential and raindrop temperature for their effect on structural stability of the soil.  High soil moisture potential (more negative) significantly increased the erodibility of a clayey soil containing expanding lattice clay minerals.  The erodibility index of a sandy clay loam soil containing kaolinitic clay minerals and amorphous iron and aluminium oxides was slightly decreased at high moisture potential. The increase in water temperature increased the erodibility of both soils.  Erodibility was found to vary directly with sand and inversely with clay content.

Bruce-Okine and Lal found significant effect of soil texture on the structural stability of soil.  The energy requirement for disruption was directly proportional to clay and inversely proportional to sand content. Similar observations were done by Boyoucos ( 1930) whereby sand to silt + clay ratio was found to have a significant effect on structural stability. The lesser stability of the subsoil compared to that of surface soils may be attributed to low organic matter content.

Bruce-Okine and Lal hve also found different influence of moisture content on erodibility of soil with different clay content.  Whereas the high soil moisture potential significantly increased the erodibility of soil with high content of clay minerals, the erodibility of soil with low clay content was slightly decreased.  However, the different erodibility of soils with different clay mineral content may not be related only to the clay content but also to the nature of the clay minerals present.