PREDICTING CATION-TRANSPORT IN SMECTITIC SOILS

The distribution of a competitively adsorbed cation between solution a nd sorbed phases strongly influences the mobility of that ion species in soil. Selectivity coefficients used to describe this distribution t ypically vary from soil to soil. In certain instances, however, there is evidence that this variability may be accounted for on the basis of exchange selectivities of predominant mineral exchangers. This study was undertaken to determine whether cation transport in soils in which either hydroxy-interlayered smectite or interstratified smectite-verm iculite is the predominant exchanger could be adequately described usi ng exchange data for montmorillonite. Experimental data for binary (Ca -Mg) or ternary (Ca-Mg-Na) miscible displacement through columns of Ol ivier series (fine-silty, mixed, thermic Aquic Fragiudalf) or Yolo ser ies (fine-silty, mixed, nonacid, thermic Typic Xerorthent) soils were adequately described when cation-exchange selectivities for montmorill onite were used.