Inherent factors limiting the use of laser diffraction for determining particle size distributions of soil and related samples

A number of reports have proposed that recently developed laser diffraction instruments show potential for automating the measurement of particle size distributions (texture) in soil and related materials, but relationships to standard sieve-sedimentation measurements have been poor and inconsistent. Measurements with a commercially available laser diffraction instrument (Horiba LA-920) on five soils having a wide range of textures (silty clay to sand) showed wide variability for proportions of clay, silt and sand when the weight of the sample was varied resulting in unacceptable consistency and conformity with hydrometer measurements. Detailed measurements were subsequently made on systematic combinations of four known silt size fractions (0–5, 38–45, 75–90 and 125–150 μm diameter), to explore the reasons for the variable results. Samples weighing less than 0.10, 0.15 and 0.20 g of the 38–45, 75–90 and 125–150 μm diameter fractions, respectively, were not detected by the instrument. Calculations of the probable numbers of particles in these samples ranged from 0.06 × 106 to 1.0 × 106. There were, however, 2445 × 106 particle in a 0.05 g of 0–5 μm diameter sample, and was detected by the instrument. This showed that there is a threshold of the number of particles required for detection. Alternatively, similar and greater weights of small diameter particles result in potential saturation of the detector. Saturation occurred when laser light transmission was low (< 20%). Even before saturation of the detector, an interaction was detected when combinations of small and large particles were mixed such that the proportion of small particles tended to be over-estimated and large particles under-estimated. Percentage laser light transmission measurements were inadequate to guide development of calibrations to correct for threshold/saturation limits and light competition problems. It was concluded that the geometric change in numbers of particles as the size decreases in a given weight of sample is too large to allow the use of current laser diffraction instrumentation for particle analyses of soil and related samples where the sizes in the distribution are from clay to sand or even with a narrower range of sizes (e.g., clay or silt).