Impact of clay on soil erodibility through crust crushing energy
Clay was used to amend the original four soils in terms of increasing crust crushing energy. After an erodible soil surface is created by tilling a crusted surface, soil loss can influence temporal wind erosion factors such as random roughness, aggregate GMD as well as aggregate crushing energy and density within the erodible soil surface environment.
No statistical relationships were found between crust crushing energy and soil water content, random roughness, and aggregate density for the four soil types according to the p-value (>0.1) (Figure 6) except the aggregate geometric mean diameter (GMD) and crushing energy. Crust crushing energy appeared to increase logarithmically with GMD for all soil types (Figure 7a). As the disruptive force is imparted to a crust surface by tillage, the crust fractures into aggregates. Crusts with low stability may fracture into smaller sizes due to their limited cohesive forces. Aggregate size distributions are highly correlated with aggregate stability, which is also called aggregate crushing energy (Kemper and Rosenau, 1986). Colazo and Buschiazzo (2010) and Hevia et al. (2007) used aggregate size distributions to determine aggregate stability. Aggregate stability increased with aggregate size for aggregates > 0.84 mm. In this study, we assumed that crust crushing energy was equal to aggregate crushing energy because both are controlled by the magnitude of cohesive forces between soil particles (Amézketa,1999, Saleh, 1993; USDA, 2016). This therefore is not surprising that aggregate GMD increased with increasing crust crushing energy as well as aggregate crushing energy. These results indicated that crust crushing energy also was an important factor affecting ASD for a disturbed crust surface. As clay amendment increased, the crust surface was more difficult to disturb even though the crust could be broken into greater aggregate sizes. However, as the crust crushing energy exceeds a certain value, the crust will not break down into aggregate but rather into clods (often called secondary aggregates, Zobeck, 1991a) due to their considerable cohesive forces. Not only does tillage practices affect dry ASD (Hevia et al., 2007), but aggregate crushing energy also appears to impact ASD.
Gillette et al. (2001) found crust hardness correlated with rough crusts, the latter of which may influence microrelief. Large aggregates associated with high crushing energy may be more prone to result in high random roughness. However, we found no statistical relationships between crust crushing energy and random roughness in this study (Table 2). This was not surprising because random roughness usually associates with tillage intensity (Hagen, 1991), rainfall amount and kinetic energy, and runoff (Zobeck and Onstad, 1987). These parameters was assumed not to change due to the application of standard tillage and rainfall simulator. Similarly, no consistent increase or decrease in soil water content was found with increasing crust crushing energy. In addition, soil water content varied little and was much less than the wilting point water content, thus water content and random roughness were expected to have little effect on soil loss in this study.