The self‐diffusion of sodium in soil: factors affecting the surface mobility

SUMMARY A study is presented of the self‐diffusion of sodium in a sub‐soil. Impedance factors ( f s )associated with the surface phase have been derived, representing the mobility of sorbed sodium ions relative to mobility in an ideal solution. Effects of drying, of cationic com‐position and of enrichment with either native clay fraction or ‘pure’ clay minerals were investigated. The liquid phase impedance factor ( f L ) without added clay was 0.29: clay enrichment affected soil structure and hence f L . Values of f s were in the range 0.19–0.05, and were about one‐third of f L . The value off, decreased as clay content increased, being halved when 50% native clay or 10% clay mineral was added. In a sodium saturated soil f s was 0.06, less than half that in a calcium saturated soil.