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The Energies of Replacement of Calcium by Potassium in Soils^{1, 2,}

ABSTRACT

Energy of exchange is a measure of the intensity factor in the delivery of a balanced supply of nutrient cations from the exchange complex of the soil to the growing plant. Energies of exchange for the replacement of calcium by potassium in a soil may be computed from the cationic composition of water extracts from the soil. Cationic compositions of displaced soil solutions varied with the concentrations of soluble anions that in turn depended upon the moisture contents of the soils. But the variations were such that they reflected constant energies of exchange for each of the soils at different moisture contents. Energies of exchange for the replacement of calcium with potassium ranging from –3,500 to –4,000 calories were associated with potassium deficiencies in plants. Energies of exchange from –2,500 to –3,000 calories represented suitable balances between potassium and calcium. Energies of exchange of –2,000 calories or less were associated with excessive amounts of potassium in relation to the amounts of calcium that were present. The relationships between plant nutrition and energy of exchange appear to be universal in scope and applicable to all soils.

¹ Energies of replacement, preferably of exchange, as used in this paper, are based on thermodynamic considerations which guided the thinking throughout this investigation. This method of approaching cationic exchange differs from the conventional one, but its results expressed as cationic ratios agree in the end with those deduced through the principles of Donnan. The uncertainties surrounding the soluble anions in the clay system, of necessity considered in the Donnan concept, are avoided and the cation exchange is treated by strict thermodynamic principles. This justifies the use of energies rather than ratios for expressing the results. Such treatment reduces cationic exchange to a simpler reasoning more readily comprehended.

² Contribution from the Department of Soils, Missouri Agr. Exp. Sta., Columbia, Mo. Journal Series No. 1466. Presented before Div. II, Soil Science Society of America, St. Paul, Minn., Nov. 11, 1954.

³ Associate Professor of Soils.