- Ruts formed by wheels are an example of shearing.The trace of a tyre in the sand image by astoria from Fotolia.com
Soil is damaged directly by mechanical impact from surface traffic. According to the U.S. Forest Service, shearing -- one component of soil degradation -- is the destructive transfer of force through the soil. A common example of shearing "is when the passage of a wheeled vehicle forms ruts," reports the Forest Service. "The downward force of the wheel shears--or displaces--the soil beneath it, forcing the soil to bulge upward beside the wheel." - Compaction is used in road engineering.Road roller image by JCVStock from Fotolia.com
Shearing causes soil to become dense and compacted, reducing the soil's air and water content, as reported by the Encyclopedia of Soil Science. The soil gets compressed, making it less able to provide oxygen or water for plant roots. An extreme example of compaction is a dried mud clod or an adobe brick. Compaction also makes the soil more susceptible to additional destructive forces because the shearing breaks bonds between the soil particles. This means the surface soil will be vulnerable to displacement and erosion from wind and water and will be less able to support vegetation. - Healthy soil has many different sized particles.Pebble pattern on soil image by Vytenis from Fotolia.com
Though soil is normally composed of aggregates of different sizes, the effect of shearing is a structureless soil that becomes homogeneous, with too many same-sized particles. Similar to the action of grinding herbs or seeds with a mortar and pestle, soil particles are mashed by the shearing forces of wheel, animal or human foot traffic. Shearing is most likely to occur on finely textured soils under moist to saturated conditions, where the surface is easily degraded into muddy tracks with deep muck holes, reports the Forest Service. Shearing is uncommon in coarse soils. - Soil needs space for air and water.baby plant. ficus image by joanna wnuk from Fotolia.com
Soil structure is more than just particles -- it includes the space around the particles. A well-structured soil contains both air and water. During shearing, the spaces -- or voids -- are collapsed when the particles are crushed and compressed. Additionally, if a soil is saturated with water, the shearing force will push water through the voids but will deposit soil particles in the voids once the force is released. According to the Forest Service, "the force of this rapid water flow erodes internal soil structure and clogs soil voids with displaced sediment."
