Plowing a field exposes the soil to erosion. Bare earth is rare in nature at elevations below timberline. Think about the grasslands and forests you may have walked in your life. How much bare soil do you see? Not much, if any. Outside of arid environments, plants typically blanket the ground surface, their leaves intercepting rainfall and their roots binding the soil. Now think about the brown color of the water running off a freshly plowed field or the exposed soil of a construction site during a sudden downpour. Leaving the soil bare for some part of the year makes it vulnerable to erosion, whether by wind or rain, resulting in a rate of soil loss tens to hundreds of times faster than nature makes it.
The above is quoted from the book, Dirt: The Erosion of Civilizations, a book that traces the fall of almost every civilization to its root cause: Soil erosion.