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Exchangeable Potassium and its Selectivity by Soils as Quantity-Intensity Parameters for Soil Potassium¹

ABSTRACT

Significance of K selectivity by soils, as revealed by the Gapon exchange selectivity coefficient, in improving exchangeable K as an availability index was tested on 12 UAR alluvial and desert soils under the high intensity cropping of a Neubauer experiment. Supply of K to plants decreased as selectivity increased and the selectivity coefficient, k, was used to correct exchangeable K, , for differences in selectivity among soils. The ratio /k, proved superior to exchangeable K alone for assessing K supply and accounted for as much as 95.3% of the variation in K uptake by barley plant during 24 days of growth. At variance with exchangeable K and the selectivity coefficient, their ratio /k was found independent of the method of extraction and may prove to be a characteristic soil property defining the quantity-intensity aspects of soil K supply. For soil testing purposes, the product of potassium-adsorption ratio and cation-exchange capacity may offer a simple substitute for the /k ratio in soils having Ca and Mg as the dominant exchangeable cations.

¹ Contribution from the Soils Department, University of Alexandria, U.A.R.

² Graduate student and Professor of Soils, College of Agriculture, Alexandria, U.A.R.

Received for publication March 7, 1969. Accepted for publication July 2, 1969.