Rapid Soil Particle Size Analyses Using Laser Diffraction

Abstract

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