Particle Size Analysis with Laser Diffractometry – Comparison to Standard Methods.

The particle size distribution is a basic property that governs many physical, chemical, and microbial processes in soils. Historically, the hydrometer or pipette methods have been the standard for measuring this important parameter. Recent advances in laser diffractometry make this technique a viable alternative that over time will probably replace pipette and hydrometer measurements. The advantages of laser diffractometry include: rapid batch analysis, high repeatability, small sample requirement, and a detailed size distribution applicable to all classification systems. In order to make the transition to laser diffractometry, a comprehensive comparison of methods for a wide range of textures is essential. We used twenty-one reference soils ranging from sand to clay from the University of Arizona, Soil, Water, and Environmental Science (SWES) club to compare laser diffractometry with pipette measurements. The soils were sieved through a 2-mm sieve and homogenized with an array of sample splitters. Subsamples were pretreated for organic matter (OM) removal using bleach at a pH of 9.5, carbonate removal using sodium tri-acetate at pH 5.0, removal of both OM and carbonates, or left untreated. Results from this study correlate modern and classical methods, show the influence of sample pretreatment on laser diffraction results, and provide detailed size distributions for the selected reference soils.