Its because of planting crops at many years. thanks for letting me answer this questions! 

Effects of Soil ErosionSoil erosion impacts the agricultural industry as well as the natural environment. The effects of soil erosion can be felt both on-site, meaning at the site of soil disruption, or off-site, meaning the location where the eroded soil deposits. Let's start by building our understanding of the impact of soil erosion on-site. When the topsoil is eroded from an area, that area loses its most nutrient-rich layer, and therefore soil quality is reduced. 
The Agents of Erosion Are Water, Wind, Ice, and Waves
cause of soil erosionSoil compaction, low organic matter, loss of soil structure, poor internal drainage, salinisation and soil acidity problems are other serious soil degradation conditions that can accelerate the soil erosion process. This Factsheet looks at the causes and effects of water, wind and tillage erosion on agricultural land.