2016年6月英語四級閱讀練習(xí)：Agriculture

Agriculture is the number one industry in the United States and agricultural products are the country's leading export. American farmers manage to feed not only the total population of the United States, but also millions of other people throughout the rest of the world. Corn and soybean exports alone account for approximately 75 percent of the amount sold in world markets.

This productivity, however, has its price. Intensive cultivation exposes the earth to the damaging forces of nature. Every year wind and water remove tons of rich soil from the nation's croplands.

Each field is covered by a limited amount of topsoil, the upper layer of earth which is richest in the nutrients and minerals necessary for growing crops. Ever since the first farmers arrived in the Midwest almost 200 years ago，cultivation and, consequently, erosion have been decreasing the supply of topsoil. In the 1830s, nearly two feet of rich, black top soil covered the Midwest. Today the average depth is only eight inches, and every decade another inch is blown or washed away. This erosion is steadily decreasing the productivity of valuable cropland. A United States Agricultural Department survey states that if erosion continues at its present rate, corn and soybean yields in the Midwest may drop as much as 30 percent over the next 50 years.

So far, farmers have been able to compensate for the loss of fertile topsoil by applying more chemical fertilizers to their fields; however, while this practice has increased crop yields, it has been devastating for ecology. Agriculture has become one of the biggest polluters of the nation's precious water supply. Rivers, lakes, and underground reserves of water are being filled in and poisoned by soil and chemicals carried by drainage from eroding fields. Furthermore, fertilizers only replenish the soils they do not prevent its loss.