Hydroxyl interaction with an alkaline clay soil during column leaching

Abstract

A study was made of the diffusion of hydroxide ions, added as leachant, from the inter-aggregate pores of a red-brown earth subsoil into the microporous fabric of the aggregates themselves, and then out again when the columns were leached with water. Electrical conductivity and pH were monitored within the column, in the leachate, and in extracts of the soil. The effects of flow rate, aggregate size, neutral salt content, change of cation, and the state and concentration of the hydroxide were considered. In systems having calcium as the dominant cation, the hydroxyls were able to freely enter the fabric and to react with edges of clay particles. In systems with sodium dominant both of these processes were actively inhibited. Strong anion exclusion, arising in the double layers at clay surfaces, operates in sodium but not in calcium systems. Leaching with water, which by hydrolysis reverses the edge adsorption of hydroxyl, revealed a low reservoir of hydroxide ions in sodium systems but a large one, resulting in strong buffering at high pH, in calcium. It is contended that other anions, although less mobile and less reactive than hydroxyl, will also respond to double layers in their approach and their entry to clay soil fabric. Consequently, any ionic solutes, introduced in the leaching stream or produced by root processes, will sometimes be impeded in penetrating soil fabric, just as such solutes are impeded in leaving it.