The partitioning of exchangeable magnesium, calcium and sodium in relation to their effects on the dispersion of Australian clay subsoils

Abstract

Clay subsoils were analysed by a procedure that uses a choline chloride reagent to distinguish double-layer exchangeable cations from other more strongly adsorbed forms extractable by an ammonium sulfate reagent. In those clay subsoils that contained little organic matter, the exchangeable cations made up nearly all of the cations that reacted with salt solutions. This means that it makes no practical difference to values of Ca/Mg ratios, or of ESP, whether the analytical methods discriminate between those exchangeable cations and other adsorbed cations, or do not, provided the soil samples contain little organic matter. The adoption of the new choline chloride method will not make necessary any revision of the criteria already in use in Australia for sodium- or magnesium-affected subsoils. Some possible mechanisms for the deleterious effect that large amounts of exchangeable magnesium have on the properties of subsoil clays containing illite are discussed. It is shown that the single-charged cations MgCl+ or MgOH+ are not involved. The clays hold exchangeable magnesium slightly less strongly than they hold calcium, and this weaker binding remains the best explanation for the magnesium effect.