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Cationic Activities in Systems of Plant Roots¹

ABSTRACT

The activities and mean free bonding energies of Na, K, and Ca on alfalfa, soybean, red top, and Reed canary grass roots were determined by the use of the clay membrane electrode technique. Samples of electrodialyzed roots were saturated with each cation by addition as the hydroxide after the exchange capacity of the roots had been previously determined.

Examination of the techniques employed showed that reproducible determinations of the activities could be made after a five minute contact of the clay membrane electrode with the root system. Identical readings from both small and large electrodes indicated that there was no problem in obtaining a representative cross section of the root systems for measurement. There was no appreciable liquid junction potential at the point of contact of KCl bridge with the root system. The cations added were found to be practically all recoverable in a 0.1N HCl extraction without serious contamination from ions within the roots.

It was possible to determine a value for the cation exchange capacity of the roots by titrating the electrodialyzed roots with base, stirring, and maintaining the system at pH 7 for ten minutes. Bonding energy determinations on these systems indicate that K is bonded stronger than Na on alfalfa roots, but Na is bonded stronger than K on soybean roots. The legume roots held all of the cations with greater energy than did those of the grasses; however, there was a shift in the relative values obtained for the grass root systems when compared to those of the legumes.

¹ Research paper No. 1083, Journal Series, University of Arkansas, Fayetteville, Arkansas. Published with the permission of the Director of the Arkansas Agricultural Experiment Station. Presented before Division II, Soil Science Society of America, Cincinnati, Ohio, November 19, 1952. A major part of this work was submitted by the junior author in a thesis for a partial fulfillment of the requirements for the M.S. Degree in the Graduate School, University of Arkansas. This investigation was largely supported by the Mathieson Chemical Company, Inc., and this support is gratefully acknowledged.

² Associate Professor and Graduate Assistant, respectively.

Received for publication November 5, 1952.