The self diffusion of sodium in a naturally aggregated soil

Summary The self diffusion coefficient, in a naturally aggregated soil, of a solute which exists in both liquid and sorbed phases, was measured using a pulse labelling technique. This technique could have given evidence of slow equilibration between the two phases, but none was found. The magnitude of the self diffusion coefficient indicated that the solute is mobile in the sorbed phase. The mobility in the sorbed phase is described by a ‘virtual mobile fraction’. Within the limits set by the heterogeneity of natural aggregates, this was independent of both the volumetric moisture content and the bulk density.