Procedure for Determining Sodium-Calcium Selectivity in Calcareous and Gypsiferous Soils

A procedure to determine the exchanger-phase composition (and thus the Na/Ca selectivity) of calcareous and gypsiferous soils was developed and tested on three soils and three phyllosilicate minerals. The procedure corrects for anion exclusion and mineral weathering by measuring the Cl, SO,,, and HCO3 in the soil solution and in the extracting solution. Anion exclusion was found to be significant in the montmorillonite system, but generally less than 10% of the cation-exchange capacity in the soil, illite, and vermiculite systems. Mineral weathering contributed significantly to the total extracted Ca. Thus, exchangeable-Ca values were overestimated by 30 to 500% in the gypsiferous soils and 3 to 20% in the calcareous soils when weathering was neglected. The proposed method allows the determination of cation-exchange capacity, exchangeable-cation composition, and anion exclusion at any solution composition, concentra-