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Rapid Soil Particle Size Analyses Using Laser Diffraction

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan

Citation:  Paper number  032207,  2003 ASAE Annual Meeting . (doi: 10.13031/2013.13799) @2003
Authors:   Ted M. Zobeck
Keywords:   Particles, particle size, laser diffraction, soil physical properties, soil texture

Soil particle size analyses (PSA) are needed to relate soil texture to soil performance or behavior. Standard analyses of dry soils usually include dispersion of the soils followed by particle size determination by a variety of time-consuming methods. Clay and silt-sized particles are usually measured by sedimentation using a hydrometer or pipette method. Sands are then measured by sieving. Recent advances in laser diffraction technology have led to the development of devices specifically designed to rapidly measure the particle distribution of dispersed particles. In this study, I relate PSAs of 32 soil samples measured by the pipette method and sieving to results obtained using a laser diffraction particle size analyzer. No pretreatment to remove organic matter or salts was used in this study. After the approximately 300 mg soil samples were dispersed by overnight shaking in a sodium hexametaphosphate (soap) solution, complete particle size analyses took about fifteen minutes per sample, including device clean-up. The correlation of the laser analyses with pipette analyses varied by particle size and mineralogy. Better results were obtained when calcareous soils were separated from non-calcareous soils. Regressions analyses relating laser and pipette methods of non-calcareous soils yielded coefficients of determinations of 0.97, 0.99, and 0.98 for the <2 m , <50 m, and <100 m fractions, respectively. Use of the laser particle size analyzer greatly reduced the time and labor required for soil PSAs. Since a relatively small sample size is required, care must be taken to ensure a representative sample is selected for analysis.

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