一、引言公元前129年,张骞出使西域引入紫花苜蓿之后,就以其特异的饲料价值和培肥改土的效应,而介入农田,形成与粮、油、棉轮作的草田轮作制度。这一制度始于陕西、河南及甘肃的陇东南地区,到了唐代已蔚然成风。尔今在西北、华北地区形成了饲、肥兼用的苜蓿带。明清以后,南方种紫云英、苕子、金花菜作绿肥已经很普遍。 I. INTRODUCTION In 129 BC, when Zhang Qian came to China to introduce alfalfa in the western region, he intervened in farmland to form a rotation system of grasslands with grain, oil and cotton with its special value of feed and the effect of soil improvement and fertilization. The system began in Shaanxi, Henan and Gansu Longdongnan area, to the Tang Dynasty has become common practice. Now in the northwest, North China formed a feed, fertilizer and use of alfalfa zone. After the Ming and Qing Dynasties, the southern species of milk vetch, prickly heat, ginseng for green manure has been very common.