9Magleby, R. , C. Sandretto, W. Crosswhite, and C. T. Osborn. Soil Erosion and Conservation in the United States: An Overview [ J ] . AIB-718. U,S. Dept, Agr. Econ. Res, Serv., 1995: 29.
10Crosswhite, W. and C. Sandretto. Trends in Resource Protection Policies in Agriculture. Agricultural Resources Situation and Outlook Report AR-23 [M] . U.S. Dept. Agr. Econ. Res. Serv. Sept. 1991: 42 - 49.