Effect of moisture content and soil type on self diffusion of ⁸⁶Rb in soils

Self-diffusion coefficients ( D ) for Rb+ have been measured in a heteroionic Lower Greensand soil over a wide range of moisture contents at three different compactions. The moisture content is shown to have a considerable effect on the value of the self-diffusion coefficient, a rapid rise in the value of D taking place between 5 and 10% moisture content. It is shown that this effect is not simply due to the increased volume of water-filled pore space at higher moisture contents, and other possible factors which could influence the shape of the diffusion coefficient/moisture content curve are discussed. Diffusion coefficients for Rb+ in eight other soils at pF 2 are also reported, but no simple correlation with any of several soil properties measured could be found. A value of 1·0 × 10 −7 cm 2 /sec. would seem to be a reasonable generalization for the self-diffusion coefficient of Rb+ in moist calcium-dominated soils.