Wind Erosion in Western Queensland Australia

Chapter 2 – Land Erodibility Controlsportion of silt and clays in a soil is to increase binding and aggregation of particles, resultingin an increase in the shear stress required to mobilise grains on the surface. The sub-roundedshapes typical of quartz sand grains are not conducive to the formation of inter-particlebonds. This results in a loose and erodible surface. Clays are effective binding agents as theirplate-like structures provide large inter-particle contact areas. Clay particles often carry somecharge, which allows for electrostatic bonding and moisture retention, which facilitate grainbinding (Breuninger et al., 1989). Under extreme drought conditions and disturbance (e.g. bylivestock), these inter-particle bonds may be broken down, resulting in cracking andgranulation of a clay soil surface (Gillette, 1978). Self-mulching clays are prone to thiscondition (Leys et al., 1996). Granulation of clay soils may lead to a surface structure withaggregates of a similar size to sand grains, and therefore elevated erodibility. Figure 2.4illustrates how significant changes in soil erodibility may occur under surface disturbance,i.e. soils with the same textures may have a range of erodibility values.2.2.4 Soil AggregationSoil Aggregate Formation and BreakdownSoil aggregation is driven by soil texture (sand, silt, clay content), organic matter content,calcium carbonate and salt content, and climate (moisture availability). Perhaps the mostimportant of these is soil texture, with the particle size and mineralogy of soil controlling theability of soil inter-particle bonds to form and resist destruction. This ability to form bonds isdriven by the nature of inter-particle contacts, with a fundamental difference between particlecontacts in sandy soil to those in a soil with high clay content (Smalley, 1970). As sandy soilsare least susceptible to aggregate formation, they are also the most consistently erodible.Gillette (1978) reported a model to illustrate the dependence of soils on drought to becomesusceptible to wind erosion (Figure 2.5).The forces controlling the breakdown of soil aggregates are fundamental in controlling theerodibility of any soil with a non-sandy texture (Breuninger et al., 1989). Mechanisms drivingaggregate breakdown include: mechanical disturbance (by animals or machinery), photodegradationduring long periods of exposure (e.g. during drought), clay lattice expansioncontractionduring wet-dry cycles, freeze-thaw cycles, direct abrasion, and salt efflorescence(Harris et al., 1966; Merrill et al., 1999; Sarah, 2005).41