book4unit2课文关键词填写.ppt

* ______the past half century, using chemical fertilizers has become very common in farming. Many farmers welcomed them ______ 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 ______ to the land and, even more dangerous, to peoples health. What are some of the problems caused by chemical fertilizers? First, they damage the land by killing the helpful bacteria and pests the ______ harmful ones. Chemicals also stay in the ground and underground water for a long time. This affects crops and, therefore, animals and Over as damage as well as humans, ______ chemicals get inside the crops and cannot just be washed off. These chemicals ______ in the food supply in peoples bodies over time. Many of these chemicals can ______ 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 ______ organic farming. Organic farming is simply farming without using any chemicals. They ______ keeping their soil rich and free of disease. since build up lead to turn to focus on 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 in minerals and so more fertile. This also keeps the air, soil, water and crops ______ chemicals. Organic farmers also use many other methods to keep the soil fertile. They often change the kind of crop in each field ______ 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 richer free from ev