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Potassium Exchange Behavior in a Weathering Sequence of Volcanic Ash Soils

IRFA/CIRAD, Univ. Louvain, CIFA Unit; Place Croix du Sud 2

Joseph E. Dufey^*

Univ. Louvain, Soil Science Unit, Place Croix du Sud 2

Léon Vielvoye

Section de Physico-chimie Minérale, M.R.C.A., Place Croix du Sud 1, B-1348, Louvain-la-Neuve (, Belgium)

Adrien J. Herbillon

Centre de Pédologie Biologique, C.N.R.S., B.P. 5, F-54501 Vandoeuvre-les-Nancy (, France)

*Corresponding author.

ABSTRACT

Tropical volcanic ash soils are commonly associated within chronosequences and they usually sustain intensive agricultural use. In the Mungo plain, Western Cameroon (Africa), such soils are associated with the following weathering sequence: basaltic ash allophane halloysite kaolinite. This paper was aimed at studying the cation exchange properties of six soils representative of such sequences and looking into their agronomic implications in intensive banana (Musa acuminata Colla) cropping systems. Calcium-potassium exchange equilibria were performed at pH 4 and 0.01 mol L^–1 chloride concentration on A and B horizons of the chosen soils. Their K/Ca exchange selectivity was interpreted through the fitting of the isotherm curves according to a two-site model and the calculation of the Vanselow and Gapon selectivity coefficients. For some acid soil horizons, K selectivity was measured in different conditions of Al saturation and pH. The least weathered allophanic soils exhibit the lowest measured K selectivity. The halloysitic soils show a strong selective adsorption of K ions, which is associated with the presence of halloysite-smectite mixed-layer clays. Potassium selectivity further decreases in the kaolinitic soil. High Al saturation increases K selectivity in acid soil horizons while increasing pH promotes the adsorption of Ca ions. As K retention is low in young allophanic soils, banana plants frequently suffer from K deficiencies in rainy periods. On the contrary, K excess-induced Mg deficiencies are general on acid kaolinitic soils and are related related to two factors: low Mg reserves, and a high retention of K relative to Mg promoted by high Al saturation.

Received for publication December 3, 1988.

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