1. 1. Barber S.A. Soil nutrient availability: A mechanistic approach, London, John Wiley, 1995, 384 p.
2. 2. Barber S. A., Humbert R. P. Advances in knowledge of potassium relationships in the soil and plant in fertilizer technology and usage, Symposium Publ. by Soil Sci. Soc. Amer., Madison Wis., 1963.
3. 3. Vazhenin I.G., Karaseva G.I. About potassium forms in soil and potassium nutrition of plants, Pochvovedenie, 1959a, No.3, p.11-21 (in Russian)
4. 4. Vazhenin I.G., Karaseva G.I. About the agrochemical methods to determine labile potassium forms in soil, Pochvovedenie, 1959b, No.8, p.87-91 (in Russian)
5. 5. Vazhenin I.G. Methods to determine potassium in soil. Agrochemical methods of soil analyses, Moscow, Nauka Pbs., 1975, p.191-218 (in Russian)