人教版高中英语ppt配套课件：必修4 unit 2 section ⅰ.ppt

有机农业是指按照一定的生产标准，在生产中遵循自然规律和生态学原理，采用一系列可持续发展的农业技术以维持持续稳定的农业生产体系的一种农业生产方式。 Organic farming study Agricultural scientists in Switzerland have demonstrated the long-term effects of organic farming methods.Organic farms use no man-made chemicals to increase crops or control insects and disease.The study showed that organic fields produced fewer crops，on average，than other fields.However，the scientists say the organic methods improved the health of the soil and used half as much energy. Scientists at the Research Institute of Organic Agriculture in Frick，Switzerland，supervised the study.They reported their findings in the Science magazine，the publication of the American Association for the Advancement of Science. It was one of the longest-running studies of its kind.The Swiss scientists compared the effects of organic and traditional farming methods over a period of more than twenty years.They grew crops in four fields of land in Switzerland. Traditional farming methods were used in two fields.Organic farming methods were used in the others.The scientists grew potatoes，barley(大麦)，winter wheat，beets(甜菜)and grass clover(苜蓿)．The crops，planting methods，and soil use were similar in all the fields studied. The study found that the organic fields produced about twenty percent fewer crops than the other fields.However，the farmers used less fertilizer and energy on the organic fields.They found the organic fields were healthier. The scientists say the organic fields had more nutrients in the soil.The soils in the organic fields also had more biological activity than the other fields.The organic fields had more helpful organisms and a larger number of different or-ganisms.The organic fields had almost two times as many insects，including helpful insects.In traditional farming，chemicals designed to kill harmful insects can also kill helpful ones. The researchers say the findings show that organic crop production rates change little over time，and that soil fertility increases.They say