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This article in SSSAJ

  1. Vol. 27 No. 6, p. 619-623
    Received: Dec 3, 1962



Ion Sorption by Soil Separates and Exchange Equilibria Involving Sodium, Potassium, and Rubidium1

  1. F. H. le Roux and
  2. N. T. Coleman2



Sodium-potassium exchange reactions were studied with four California soils, utilizing chromatographic and equilibrium procedures. Apparent exchange constants varied with ion saturation and indicated a larger affinity for potassium as its equivalent fraction on exchange sites decreased. This was especially true for soils containing biotite-hydrobiotite-vermiculite in the coarse fractions. Clay and silt-size materials from such a soil were packed into columns, and the sorption-desorption behavior of rubidium was measured using the radioisotope Rb86. Irreversible sorption of rubidium with corresponding reductions in cation-exchange capacity was large for silt, intermediate for coarse clay, and negligible for fine clay. Sodium-rubidium exchange isotherms obtained for various size fractions both before and after a portion of the exchange-capacity was “blocked” with Rb or K indicated a quite specific sorption of Rb on certain sites, with “normal” ion-exchange behavior for the remainder of the CEC. X-ray diffraction studies suggested specific sorption of K and Rb by vermiculite and hydrobiotite, irrespective of the Na/K or Na/Rb ratio in saturating solutions. However, even when sites with specific affinity for Rb or K were eliminated, there was significant variation in the apparent exchange constants obtained for different soils. For soil containing vermiculite in the coarse fractions, the specific sorption of radiorubidium may be a conveniently determined guide to potassium fertilizer needs.

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