Several Mechanisms of Erosion

Wind (aeolian) erosion
Water erosion (splash, sheet, rill, gully)
Rill erosion moves the most topsoil, followed by sheet and splash erosion
Predicting losses by erosion:
Universal Soil Loss Equation (USLE)
Wind Erosion Prediction Equation  

Humans are the primary cause of erosion
19 billion hectares of croplands worldwide suffer from erosion
Kazakhstan lost tends of millions of hectares to wind erosion
Soil degradation over the next 40 years in Africa could reduce crop yields by half
The on-farm cost of agricultural land degradation in Canada is $670 million per year

Over-application of fertilizers, even organic ones
Nitrogen and phosphorous runoff can lead to algal blooms
Nitrates in soil can contaminate ground water and pose human health risks
Pesticide use
Improper disposal and storage of industrial waste