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Chemical Factors in Cation Exchange Between Root Surfaces and Nutrient Media¹

ABSTRACT

Experiments were designed to throw light on two main factors in cation uptake; the cationic activity of the nutrient medium and the mean free energy change of the reaction in which nutrient ions are exchanged for hydrogen ions. The third factor—the activity of the hydrogen ions, was not fully investigated, but has been reported on by other workers.

The theoretical basis for the determination of free energy changes is first examined. For roots growing in colloidal substrates, three possible procedures follow. (1) By adding algebraically the individual free bonding energies of the two cations to the two colloidal surfaces, the free energy change for the reaction can be calculated. This involves individual activity determinations by potentiometric methods. (2) By small exchanges, ratios of activities can be obtained which can be used directly in the equations. (3) By applying Donnan conditions to the equations, simplified equations involving ratios of concentrations are obtained. From these, the free energy changes can be calculated from determinations of total cationic composition of the two exchangers.

The composition of soybeans (roots and tops) grown in different true solutions—chlorides and bicarbonates of K and Ca, and in K and Ca Amberlite, K and Ca Putnam clay, and K and Ca bentonite— was examined. Corresponding to the 2,500–3,000 calorie advantage of bicarbonates over chlorides, greater uptake of K and of Ca from the former was observed. Where the three dominant factors were the same for two media (acidic KCl and acidic K-Amberlite) growth and uptake were closely similar. Differences of 1,500–2,000 calories in the free energy change for different substrates markedly affected uptake.

¹ Contribution from the Department of Soils, Missouri Agricultural Experiment Station, Columbia, Mo. Journal Series No. 1335. Presented in symposium before Division II, Soil Science Society of America, Cincinnati, Ohio, Nov. 19, 1952.

² Professor of Soils and Graduate Assistant in Soils, respectively.

Received for publication December 15, 1952.