There are nine kinds of farmers' works in Hanshu Yiwenzhi, among which Shennong and Yelao are works of the Warring States period, and none of them have been preserved. However, the Spring and Autumn Annals written by Lu in 239 BC has four articles: Going to Agriculture, Man and Land, On Soil and On Time. Shangnong, the earliest group of agricultural papers in China, focuses on agricultural policy and the other three articles focus on agricultural technology.