Reconciling Particle Size Distributions Obtained By Laser Diffraction and Sedimentation.

Reconciling particle size distributions obtained by laser diffraction and sedimentation

-Ayush Joshi Gyawali, Dr. Ryan Stewart

Department of crop and soil environmental science, Virginia Tech.

Soil Particle size distribution (PSD) is an important soil property that is used to determine soil texture and to estimate soil parameters like cation exchange capacity and the soil water characteristic curve. While methods based on sedimentation (e.g., pipette, hydrometer) have traditionally been used to measure particle size distribution, laser scattering/diffraction technologies are becoming more common. Laser diffraction enables quantification of a full range of particle sizes (e.g., from 0.1 microns to 2 millimeters), while requiring only small amounts of sample and short measurement times. However, laser diffraction instruments does not seem to match with the PSD produced by pippete method. Some correction factors have been proposed to relate the clay, silt and sand fractions obtained by laser diffraction with those obtained using the pipette method, but there is not yet a satisfactory approach to adjust for differences in the full PSD curves obtained by the two methods.  To address this gap, we model the PSD using the Weibull distribution and provide correction factors that can be used to correlate the fitting parameters, alpha and beta, between the two methods. The alpha and beta values are found to be correlated well when we break the soil samples into soil type as compared to grouping all soil type in one. These preliminary results suggest that it may be possible to consistently reconcile PSDs from these two methods, thus providing the means to more accurately compare PSD results from different studies.