1. Official Methods of Analysis (1984) 14th Ed., Association of Official Analytical Chemists, Arlington, VA, sec 2.015
2. Methods Used and Adopted by the Association of Florida Phosphate Chemists (1980) 6th Ed, Bartow, FL, Section XI, 2C
3. Ardaman & Associates, IMC Fertilizer, Inc. File Number 81-018A/C, sec 7.3.1