普通高中课程标准实验教科书 英语必修4 Unit 2 Working the land（在线收听）

 Over the past half century, using chemical fertilizers has become very common in farming. Many farmers welcomed them as a great way to stop crop disease and increase production. Recently, however, scientists have been finding that long-term use of these fertilizers can cause damage to the land and, even more dangerous, to people's health.
        What are some of the problems caused by chemical fertilizers? First, they damage the land by killing the helpful bacteria and pests as well as the harmful ones. Chemicals also stay in the ground and underground water for a long time. This affects crops and, therefore, animals and humans, since chemicals get inside the crops and cannot just be washed off. These chemicals in the food supply build up in people's bodies over time. Many of these chemicals can lead to cancer or other illnesses. In addition, fruit, vegetables and other food grown with chemical fertilizers usually grow too fast to be full of much nutrition. They may look beautiful, but inside there is usually more water than vitamins and minerals.
       With these discoveries, some farmers and many customers are beginning to turn to organic farming. Organic farming is simply farming without using any chemicals. They focus on keeping their soil rich and free of disease. A healthy soil reduces disease and helps crops grow strong and healthy. Organic farmers, therefore, often prefer using natural waste from animals as fertilizer. They feel that this makes the soil in their fields richer in minerals and so more fertile. This also keeps the air, soil, water and crops free from chemicals.
       Organic farmers also use many other methods to keep the soil fertile. They often change the kind of crop in each field every few years, for example, growing corn or wheat and then the next year peas or soybeans. Crops such as peas or soybeans put important minerals back into the soil, making it ready for crops such as wheat or corn that need rich and fertile soil. Organic farmers also plant crops to use different levels of soil, for example, planting peanuts that use the ground's surface followed by vegetables that put down deep roots. Some organic farmers prefer planting grass between crops to prevent wind or water from carrying away the soil, and then leaving it in the ground to become a natural fertilizer for the next year's crop. These many different organic farming methods have the same goal: to grow good food and avoid damaging the environment or people's health.
  原文地址：http://www.tingroom.com/lesson/gzynjyy/81631.html