Precision of Soil Particle Size Analysis using Laser Diffractometry

Precision, particularly in terms of repeatability in particle size analysis (PSA), has recently resurfaced as an issue due to the increased use of laser PSA for PSA. Because laser diffractometry produces much more detailed data than does traditional pipette analysis, and because a much smaller sample is used in the analysis, precision or repeatability of laser-produced PSA data is a legitimate concern. For example, of the 1485 soil samples analyzed in our study, most of which are silty or loamy and each of which was analyzed at least twice, 11.5% changed texture class, from the first to the second PSA measurement. Statistical analysis of these paired, subsample data was therefore used to establish a standard for normal variance among the subsamples, as a test of precision. Subsample pairs with an absolute cumulative bin difference (CBD) of <1 SD above the population's mean CBD were determined to have acceptable precision. This approach provides both a simple method for assessing the variation in PSA data sets and establishes a comparable standard for determining when additional measurements are needed to find a more precise result. Researchers may be tempted to simply use mean PSA data for samples that have been run multiple times. However, we found that using the per bin mean for only the two best matching subsamples was the optimal approach. Our analysis also indicates that, as particle size gets coarser (at least within silty/loamy samples), the precision of laser-generated particle size data generally decreases.