Characterization of particle-size distribution in soils with a fragmentation model

Particle-size distributions (PSDs) of soils are often used to estimate other soil properties, such as soil moisture characteristics and hydraulic conductivities. Prediction of hydraulic properties from soil texture requires an accurate characterization of PSDs. The objective of this study was to test the validity of a mass-based fragmentation model to describe PSDs in soils. Wet sieving, pipette, and light-diffraction techniques were used to obtain PSDs of 19 soils in the range of 0.05 to 2000 μm. Light diffraction allows determination of smaller particle sizes than the classical sedimentation methods, and provides a high resolution of the PSD. The measured data were analyzed with a mass-based model originating from fragmentation processes, which yields a power-law relation between mass and size of soil particles. It was found that a single power-law exponent could not characterize the PSD across the whole range of the measurements. Three main power-law domains were identified. The boundaries between the three domains were located at particle diameters of 0.51±0.15 and 85.3±25.3 μm. The exponent of the power law describing the domain between 0.51 and 85.3 μm was correlated with the clay and sand contents of the soil sample, indicating some relationship between power-law exponent and textural class. Two simple equations are derived to calculate the parameters of the fragmentation model of the domain between 0.51 and 85.3 μm from mass fractions of clay and silt.