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Potassium and Rubidium Adsorption and Diffusion in Soil¹

ABSTRACT

Uptake of cations by plant roots growing in soil is believed to be influenced by the relative adsorption strengths of cations by soil exchange sites. The adsorption of K and Rb by four soils was investigated to determine the differences in adsorption of K and Rb which may occur and how they are affected by soil properties with the view of using this information to evaluate the uptake mechanisms for K and Rb by plant roots growing in soil.

Rubidium was adsorbed preferentially to K by all four soils. Selectivity coefficient k_Rb/K, indicating the size of the effect ranged from 1.9 to 4.4. Adding K decreased k_Rb/K, increased the amount of nonexchangeable Rb, and decreased exchangeable Rb. There was a negative correlation (r² = 0.78) between exchangeable K in the soil and k_Rb/K. The level of K in the soil had a greater effect on k_Rb/K than varying types of exchange sites present due to differences in clay and amounts of organic matter in the four soils investigated. The size of the selectivity coefficient indicated that the difference between K/Rb ratios in the solution and exchange phases of these soils used and differences between soils made them suitable for use in plant uptake studies of the mechanisms of K and Rb uptake.

¹ Journal Paper No. 6777. Purdue Univ. Agric. Exp. Stn., West Lafayette, IN 47907. Contribution from the Dep. of Agronomy. This research was supported in part by the U.S. Energy Research and Development Administration under contract AT (11-1)-1495.

² Post Doctoral Research Associate and Professor of Agronomy, respectively. The senior author is now Professional Associate, Dep. of Land Resource Sci., Univ. of Guelph, Ont., Canada.

Received for publication July 11, 1977. Accepted for publication November 22, 1977.