1. AOAC (1995).Official Methods of Analysis.Volume 1. 16th Edition. AOAC International, Arlington, VA, USA. 5–6.
2. IAEA (1983).A Guide to the Use of Nitrogen-15 and Radioisotopes in Studies of Plant Nutrition: Calculations and Interpretation of Data.IAEA, Vienna, Austria. 1–65.
3. Keeney, D. R. and Nelson, D. W. (1982). Nitrogen-Inorganic Forms. In:Methods of Soil Analysis. Part 2: Chemical and Microbiological Properties.(Page, A. L., Ed.). American Society of Agronomy and Soil Science Society of America, Madison, WI, USA. 643–698.
4. Knowles, R. and Blackburn, T. H. (1993).Nitrogen Isotope Techniques.1st Edition. Academic Press Inc., San Diego, CA, USA. 311 pp.
5. Olsen, S. R. and Sommers, L. E. (1982). Phosphorus. In:Methods of Soil Analysis. Part 2: Chemical and Microbiological Properties.(Page, A. L., Ed.). American Society of Agronomy and Soil Science Society of America, Madison, WI, USA. 416–418.