1. Blakemore LC, Searle PL, Daly BK 1987: Extractable Iron, Aluminium and Silicon. Methods for Chemical Analysis of Soils, NZ Soil Bureau Scientific Report 80, Ed. Blakemore LC, Searle PL, Daly BK, pp. 71–76. NZ Soil Bureau Department of Scientific and Industrial Research, Lower Hutt, New Zealand.
2. Bowen HJM 1979: Environmental Chemistry of the Elements, pp. 71–141. Academic press Inc. London, UK.
3. Potassium Cycling
4. Plant-Available Potassium Assessment with a Modified Sodium Tetraphenylboron Method
5. Cultivated soil classification committee 1995: Classification of Cultivated Soils in Japan (3rd Approximation), p. 17. Miscellaneous publication of the National Institute of Agro-Environmental Sciences, Tsukuba, Japan.