The Measurement and Mechanism of Ion Diffusion in Soils Iii. The Effect of Moisture Content and Soil‐solution Concentration on the Self‐diffusion of Ions in Soils

Summary The self‐diffusion coefficients of Cl, Na, Sr and phosphate have been measured in Upper Greensand, sandy clay loam (CEC 7.45 me/100 g.) between pF 1.8 and 5.4 and, for Na and phosphate, over a range of soil‐solution concentrations. An attempt has been made to estimate the relative contributions of solution‐ and exchangeable‐ions to diffusion. There is some indication that exchangeable Na may contribute to diffusion at the lowest moisture contents used, but little indication of this at pF 2.1 (15 per cent moisture content) when the proportion of ions on the solid was as high as possible. Exchangeable Sr appears to make no significant contribution to diffusion at any moisture content when the soil‐solution concentration is 0.1M. Exchangeable phosphate appears to make no contribution to diffusion at pF 2.1 between 5 × 10 −6 M and 9 × 10 −4 M.