1. Alexandrova, L. N., Use of sodium pyrophosphate for extraction of free humic substances and their organo-mineral compounds. Pochvovedenie2, 90 (1960).
2. Drozdova, T. V., Use of chromatography for studying fulvic acids. Pochvovedenie, 1 (1955).
3. Duffield, R. and Calvin, M. J., Am. Chem. Soc.68, 557 (1946).
4. Fokin, A. D., Isotopic exchange methods in soil Science. Izvestia Timiryazev Acad. of Agr. Sciences2, 94–106 (1969).
5. Fokin, A. D. and Karpukhin, A. I., Isotopic exchange method in investigations of soil irion organic compounds. Izvestia, Timiryazev Acad. Agr. Sci.2, 128–133 (1967).