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Interactions of pH-Dependent and Permanent Charges of Clays: III. Calcium and Rb Activities versus Soybean Root Uptake from Bentonite and Illite Suspensions¹

ABSTRACT

Panther Creek bentonite and Fithian illite suspensions were reciprocally saturated with Ca and Rb. ⁴⁵Ca and ⁸⁶Rb were added as tracers for Ca and Rb, respectively. Increments of H-clay were added to aliquots of the Ca-Rb saturated clays to progressively decrease the base saturation of the clays. After equilibration, the activities of Ca and Rb were measured in the clay suspensions with the Orion divalent and the Beckman monovalent electrodes. Uptake of Ca and Rb by excised soybean roots was determined from the radioactivity of each tracer.

Decreased Ca saturation (increased Rb) shifted Ca ions from pH-dependent to permanent charges causing increased bonding and thus decreased Ca activities, fractions active, and root uptake. Except where specific ion bonding effects interfered, increased Rb saturation increased Rb activities and uptake—even though Rb fractions active were decreased, due to greater permanent charge bonding. With the same exception, additions of H-clays equilvalent to pH-dependent cation exchange capacity minimized Ca and Rb uptake as well as their activities and fractions active. Thus activity data generally correlated closely with uptake data, except where there were good reasons for this not to occur.

¹ Published with the permission of the Director of the Ohio Agricultural Research & Development Center as Journal Article no. 31-68. Presented before Div. S-2, Soil Science Society of America, Washington, D.C. Nov. 7, 1967. The results are contained in a dissertation submitted by the first junior author in partial fulfillment of the requirements for the Ph.D. degree, The Ohio State University, 1967. Supported in part by grant no. GB1101, the National Science Foundation.

² Professor; former Research Assistant, now Assistant Professor, Everglades Exp. Sta., Belle Glade, Fla.; and Associate Professor of Agronomy, The Ohio State University and Ohio Agr. Res. & Devel. Center.

Received for publication December 2, 1968. Accepted for publication January 16, 1969.