A proposed new reagent for the measurement of cation exchange properties of carbonate soils

A solution of choline chloride (1 mol L-1) in aqueous ethanol (65% by weight) is proposed as a reagent for extracting double-layer exchangeable cations from soils containing calcium carbonate, instead of the previously used solution of NH4Cl (1 mol L-1) at pH 8.5 in aqueous ethanol. This choline chloride reagent gives better estimates of double-layer exchangeable cations and of double-layer charge (exchange capacity) because it dissolves less CaCO3, and reacts less with specifically adsorbed forms of Ca, Mg and K than the NH4Cl reagent does. Because plants can use both adsorbed and exchangeable cations, the new reagent is not suitable for measuring nutrient cation availabilities. Cation exchange values for a range of Australian carbonate soils are presented to show the differences to be expected between analyses by the choline chloride and the NH4Cl reagents.