Active and exchangeable cations in soils

Abstract

The amounts of cations, Ca, Mg, K and Na, that could be extracted from soils by salt solutions varied with the cations, anions, acidity or alkalinity, and solvent of the extracting reagent. The variations were largest for soils that contained organic matter as the main source of those cations, and smallest in clay soils with little organic content. Calcium was the cation most affected and sodium the least affected. It appeared that the extractants removed all diffuse double-layer exchangeable cations, and variable portions of the other active cations including inner-sphere cations, specifically adsorbed cations, and those chelated by organic materials. A moderate, non-specific extractant containing a quaternary ammonium salt, choline chloride, is recommended for the displacement of exchangeable cations with a minimum contribution from other active cations. For an estimate of all forms of active cations, e.g. for soil nutrient assessment, a solution of ammonium sulfate is suggested.