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The Significance of the Suspension Effect in the Uptake of Cations by Plants from Soil-Water Systems¹

ABSTRACT

The significance of the suspension effect (greater cation concentration or activity in the soil suspension than in the equilibrium dialyzate) in determining the uptake of cations by plant roots was evaluated by comparing the rate of uptake of Rb⁺ and Ca⁺⁺ by excised roots of barley and mung beans from a suspension of clay or cation-exchange resin with that from the corresponding equilibrium dialyzate. Although the cation concentration of the clay or resin suspension greatly exceeded that of the corresponding equilibrium dialyzate, the rate of uptake of Rb⁺ or Ca⁺⁺ by the roots was found to be exactly the same from both the suspension and its equilibrium dialyzate.

The results of this study are at variance with the prediction of the contact-exchange theory but are in agreement with the deductions based on theoretical considerations of the consequences of the root surface or volume charge distribution. The significant conclusion that may be drawn, however, is that the composition of the soil solution or the equilibrium dialyzate should completely characterize the ionic environment of plant roots in soil-water systems.

¹ Agronomy Paper No. 464, Department of Agronomy, Cornell University, Ithaca, N. Y. The work reported in Experiment II was carried out by the senior author in 1955 in the Soils and Plant Relationships Section, Soil and Water Conservation Research Branch, ARS, USDA, Beltsville, Md. Presented before Div. II, Soil Science Society of America, at Cincinnati, Ohio, Nov. 17, 1959.

² Graduate Assistant and Professor of Soil Science, respectively. The senior author is now Assistant Professor, Department of Chemistry, Montana State College, Bozeman.

Received for publication November 12, 1959. Accepted for publication February 27, 1960.

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