This productivity, however, has its price. Intensive cultivation exposes theearth to the damaging forces ofnature. Every year wind and water remove tons ofrich soil from the nation's croplands, with the result that soil erosion has become anational problem concerning everyone from the farmer to the consumer. Each field is covered by a limited amount of topsoil, the upper layer ofearth which is richest in the nutrient and minerals necessary for growing crops. Inthe 1830s, nearly two feet of rich, black top soil covered the Midwest. Today theaverage depth is only eight inches, and every decade another inch is blown orwashed away. 48.（ A United States Agricultural Department survey states that iferosion continues at its present rate, corn and soybean yields in the Midwest maydrop as much as 30 percent over the next 50_years.）
美国农业部的一份报告表明，如果土壤侵蚀以目前的速度继续下去的话，中西部地区玉米和大豆的产量或许会在未来的50年中下降多达30%。
