Size distributions and minimum Stokes diameters of soil particles

Abstract

Published data on the settlement of particles in soil suspensions were used to plot the percentage by weight of particles (P) with a Stokes diameter ≤D v. log D. For subsoils, P/log D plots became linear as D approached the limit of measurement of 60 nm. This allowed linear extrapolation to P = 0. The D value of a clay particle was converted to particle radius by assuming particles to be circular plates with a constant ratio of radius to thickness of 2.5. When the linear plot plus its extension covered the whole clay range, calculated surface areas of the clay were then consistent with reported values. The smallest value of D found by extrapolation was 5 nm. It was deduced to correspond to a single sheet of beidellite with 2 water layers on each surface. Plots for samples from a different horizon of the same profile were used to obtain a second estimate of the minimum value of D.Over the silt/fine sand range, plots for subsoils could also be linear, but often were partly sigmoidal due, for example, to the presence of loess. Separate plots for the clay minerals and loess were deduced from the P/log D plot for one sample using the reported mineralogical composition of different size fractions.