Measuring Particle Size Distribution Using Laser Diffraction: Implications for Predicting Soil Hydraulic Properties

Methods to predict soil hydraulic properties frequently require information on the particle size distribution (PSD). The objectives of this study were to investigate various protocols for rapidly measuring PSD using the laser diffraction technique, compare the obtained PSD with those determined using the traditional hydrometer-and-sieves method (HSM), and assess the accuracy of soil hydraulic properties predicted from the measured PSD. Ten soil samples encompassing a wide textural range were analyzed using the HSM and 3 different laser diffraction methods (LDM1, LDM2, and LDM3). In LDM1, the soil sample was thoroughly mixed before analysis. In LDM2, the sand fraction was sieved out and analyzed separately from the silt-clay fraction. LDM3 was similar to LDM2 except that the silt-clay fraction was diluted so that a large sample volume could be used while maintaining an acceptable level of obscuration. LDM2 and LDM3 improved the agreement between the PSD with the HSM in comparison to LDM1, without the need of altering the Mie theory parameters or the use of scaling factors. Moreover, a reasonable prediction of measured saturated hydraulic conductivity and water retention curve was achieved when using the PSD from LDM2 and LDM3, in conjunction with bulk density information.