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The sieve-pipette is the standard method for determining soil particle size distribution (PSD) because it is precise and reproducible. However, this method requires considerable time. Light diffraction methods for determining PSD are fast, but there is no standard procedure and often, results do not agree precisely with the pipette. The objective of this study was to develop a simple and fast procedure for sample handling and treatment of light diffraction method. A commercially available laserlight diffraction instrument was used. Soil samples were loaded dry into the instrument for ease and speed. A combination of chemical and physical dispersion within the instrument was found to be convenient and effective. Time required to analyze a sample was at most 15 min. Reproducibility between different operators was good, with S.E. ranging from 0.2% to 3.6%. Furthermore, we attempted to identify optimal values for the real refractive index and imaginary refractive index used in the optical model for light diffraction. Values of 1.42 and 0.001 for real refractive index and imaginary refractive index, respectively, were found to give acceptable results when compared with the pipette method. The light diffraction method was not significantly different from the pipette method for sand (P = 0.084), silt (P = 0.743), and clay (P = 0.052). Correlation between the light diffraction and pipette method for sand, silt, and clay was acceptable (R 2 = 0.88, 0.80, and 0.69, respectively). The light diffraction technique does not have a perfect agreement with the pipette method, but it provides data rapidly and was reproducible. This method can be very valuable when a large number of samples need to be analyzed for relative comparisons between different sites. (Soil Science 2006;171:663-674)

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