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2. Association of American Plant Food Control Officials Laboratory Services Committee (2004) Guidelines for Preparing Fertilizer Laboratory and Analytical Samples. Visit website http://www.aapfco.org to obtain a copy.
3. Comparison and review of three available P2O5 methods for diammonium phosphate and triple superphosphate;Falls;Fertilizer Research,1991
4. Direct potentiometric titration of urea with urease;Falls;Journal of Association of Official Analytical Chemists,1976
5. Falls H, Siegel S, Longest J, and Boggs B (2001) Comparative methods for metals in fertilizers and ‘some’ pitfalls to avoid. Paper presented to the Phosphate Conference, 19 October, Lakeland, Florida. Visit website http://afpc.net to obtain a free copy.