CHANGCHUN, Sept. 21 (Xinhua) -- When agricultural scientist Leng Jiantian arrived in the hilly area at the source of the Liaohe River in northeast China's Jilin Province about 10 years ago, he found that local farmers planted only corn year after year. Pests and diseases were frequent, pesticides and fertilizers were overused, and the soil and river were seriously polluted. Through investigation and research, Len