Dispersing sandy soil for the measurement of particle size distributions using optical laser diffraction

Despite the speed and accuracy of particle size distribution measurement by optical laser diffraction instruments, the dispersion of the sample, especially of soil aggregates, remains time-consuming. The Malvern Instruments obscuration measurement was used to calculate the dispersion ratio as an indicator of soil disaggregation. It enabled rapid and direct comparison between measurements made using different amounts of soil, dispersed using different fluids and dispersion methods. The dispersion ratio was necessary to select the most suitable media and method of dispersion for the soil samples and was useful for identifying grain breakage and flocculation. A standard for size distribution measurements involved a soil sample suspended in tap water and dispersed using ultrasonic action for 3 min. Reproducibility of grain size distribution measurements on the Malvern laser sizer showed that variation caused by resampling was greater than variation in the optical measurement provided that the measurement sweeps for the instrument were appropriately set. Blending size distributions for material measured using two different focal lenses was problematic. It is suggested that the laser light source Coulter LS-100 may be a more suitable instrument when many sample measurements are required.