Rapid Soil Particle Size Analyses Using Laser Diffraction

Soil particle size analyses (PSA) are needed to relate soil texture to soil performance or behavior. Standardanalyses of dry soils usually include dispersion of the soils followed by particle size determination by a variety oftime-consuming methods. Clay- and silt-sized particles are usually measured by sedimentation using a hydrometer or pipettemethod. Sands are then measured by sieving. Recent advances in laser diffraction technology have led to the production ofdevices specifically designed to rapidly measure the particle distribution of dispersed particles. This study compares the PSAsof 43 soil samples collected from the Southern High Plains of Texas measured by the pipette method and sieving with resultsobtained using a laser diffraction particle size analyzer (LDPSA). No pretreatment to remove organic matter or salts was used.The LDPSA required about 300-mg soil sample and overnight dispersion while shaking in a sodium hexametaphosphatesolution. Each sample was analyzed in about 10 min, including device clean up. The correlation of the laser analyses withpipette analyses varied by particle size and mineralogy. Better correlations were obtained when non-calcareous soils wereseparated from calcareous soils. Regression analyses relating laser with pipette methods for non-calcareous soils yieldedcoefficients of determinations of 0.97, 0.99, and 0.99 for the <2-, <50-, and <100-.m fractions, respectively. Use of the laserparticle size analyzer greatly reduced the time and labor required for soil PSAs. Since a relatively small sample size isrequired, care must be taken to ensure a representative sample is selected for analysis.