Lepper, H. A., Fisher, H. J., & Warren, L. E. (1945). Official and tentative methods of analysis of the Association of official agricultural chemists (6th ed.). Association of Official agricultural chemists.

Lepper, Henry Albert, Harry Johnstone Fisher, and Lewis Eugene Warren. Official and Tentative Methods of Analysis of the Association of Official Agricultural Chemists. 6th ed. Washington, D.C: Association of Official agricultural chemists, 1945.

Lepper, Henry Albert, et al. Official and Tentative Methods of Analysis of the Association of Official Agricultural Chemists. 6th ed. Association of Official agricultural chemists, 1945.