2William R. Raun, John B. Solie, Gordon V. Improving Nitrogen Use Efficiency in Cereal Grain Production with Optical Sensing and Vari- able Rate Application. AGRONOMY JOURNAL,2002 ( 8 ) : 815-820.
3Shujie Yao,Zinan Liu. Determinants of Grain Production and Technical Efficiency in China. Journal of Agricultural Economics, 1998 (4) :171-184.