Soil has a structure that can be quite delicate. That delicate structure can easily be damaged beyond repair by compression, the pressure and forces of water and wind; by deforestation, poor grazing and crop growing practices; and even by being walked upon or rained on.
Soil is an aggregation of sand, silt and clay particles, which can be broken apart when rain drops infiltrate the soil. Raindrops carry power and can impact the ground at 20 mph. As the particles are broken apart, the water can then infiltrate the soil. If there is enough rain, the soil becomes so saturated that no more water can infiltrate. The water flows over the soil to the lowest point.
This water flow occurs with power that can carry away the soil, creating three types of erosion: Gully erosion, Rill erosion and Sheet erosion. There are also wind erosions, saltation, suspension, surface creep, and the ultimate of soil erosion: desertification.
The first level of erosion is sheet erosion where the water flows over the saturated soil in sheets. In Rill erosion, water overflow changes from a sheet form to a channel form that creates rills.Then the water erodes the bottom and sides of the rill.
Gully erosion increases the size of the rills until a channel forms that is too large to allow control and retention of the soil. The force of the water increases once it is contained within a channel.
With wind erosion, the force of the wind creates a process where medium and fine grains are lifted into the air, then cause more breakdown when they impact the rest of the soil. This process is called saltation. In a process called suspension, the finest particles are lifted into the air after saltation. Those particles stay for extended periods and can travel for long distances, essentially depleting the soil of the finest particles.
Surface creep occurs when the larger particles of the deconstructed soil have been loosened by saltation, and can now move along the surface.
Desertification is a complex process that generally involves human and environmental interactions. Overgrazing, where the plants are decimated and there is not enough decaying plant material to create humus will make the soil thin, while the soil is so compacted by animal hooves that seeds cannot gestate and create root structures.
Even excessive human or vehicle traffic can permanently compromise the soil structure, as evidenced in wheel and foot tracks that can still be seen after hundreds of years. Underground life forms cannot move through the compacted layers to keep the soil friable enough to sustain root formation and plant growth.
Poor crop rotation and management can leach the nutrients from the soil and cause crop failure. After crop failure the conditions are ripe for exposed topsoil crusting from rain and sun, setting up the perfect conditions for rain erosion and wind erosion. Over drying of the soil eventually leads to sand collection and sand dune encroachment.
Deforestation leads to the decimation of protective tree canopies and of ground vegetation, leading to exposure of the soil to the forces that cause erosion.
CITATION
http://www.soils.umn.edu/academics/classes/soil2125/doc/s10chap3.htm
