Experimental evidence of laser diffraction accuracy for particle size analysis

Laser diffraction analysis is a fast, reliable and automated method that provides detailed and highly resolved soil and sediment particle size distribution. In recent studies, the methods were compared against independent methods based on direct observation of particles by digital imaging. The data showed that laser diffraction results were in better agreement with the digital imaging independent method than with sedimentation-based methods. However, analysis was performed over a limited number of samples. In this study, 47 soil samples with a wide range of textural properties were analyzed with Laser Diffraction, Pipette, Sieving, Sedigraph and Digital Imaging methods. Detailed statistical analysis using Altman plots and Honest Significant Difference tests demonstrated (at 95% significance) that the five methods do not show statistically significant differences for grain sizes above 100 µm. However, in the lower end of the size range, i.e. less or equal to 50 µm, Laser Diffraction showed much better agreement with the reference method selected for comparison, which was Digital Imaging. New regression equations were derived with slope coefficients for linear regressions between Pipette and Laser of 0.2952 (R2 = 0.8625) for clay, 1.4261 (R2 = 0.5746) for silt and 1.031 (R2 = 0.6586) for sand, classified with the International Soil Science Society (ISSS) system. For the United States Department of Agriculture (USDA) classification system, the slopes were: 0.261 (R2 = 0.8625) for clay, 1.3493 (R2 = 0.8179) for silt and 1.063 (R2 = 0.888) for sand. These data were consistent with previous studies. Based on regression and equivalent diameters, Laser Diffraction data were represented on textural triangles for classification, allowing for employing Laser Diffraction for soil texture classification. Two alternative for representing the Laser Diffraction data in textural triangles were employed: (1) using regression equations to convert data to be represented on the standard triangles and (2) modify the upper limit for the clay range, from 2 to 8 μm. Finally, based on the additional evidence presented in this research, demonstrating that the Laser Diffraction was in better agreement with the optical method with respect to traditional sedimentation methods, it is suggested that the standards for particle size analysis be changed from sedimentation to Laser Diffraction methodologies.