Laser diffraction, transmission electron microscopy and image analysis to evaluate a bimodal Gaussian model for particle size distribution in soils

The Shiozawa and Campbell Gaussian bimodal model describes the particle size distribution (PSD) in soils as a weighted sum of two fractions: the primary minerals (sand and silt) and the secondary minerals (clay) fraction, each described by a Gaussian function. This model was developed and tested using traditional sedimentation techniques analysis for PSD such as sieving and hydrometer. Because of the lack of particle size distribution data in the clay range, Shiozawa and Campbell set the mean and the standard deviation in the clay fraction as a constant. Today, the availability of laser diffraction (LD) techniques makes it possible to overcome this limit and test the model by using a soil dataset that includes the clay fraction distribution. This paper describes the results of the test of the Shiozawa and Campbell Gaussian bimodal model on eight samples, six of them from different locations in Washington State (USA) and two from a hillside area of Northern Italy. PSD analysis was performed with sedimentation techniques, small-angle laser diffraction apparatus and transmission electron microscopy, the latter allowing measurement of very fine particles (sizes down to 0.05 μm). To test the effect of the PSD technique on the particle-size measurement and therefore on the model reliability, a comparison between sedimentation techniques and LD was performed. Moreover a validation of the LD method in the clay range was performed by comparison of LD to Transmission Electron Microscopy and Image Analysis methodologies. The results from the bimodal model showed that the model provides a good characterization of PSD for five of the eight samples analyzed only, revealing that more complex distributions are required for a loam, a silt loam and for a clay soil, where multimodal modes were found. The comparison between sedimentation technique and LD showed that the volume percentage of the clay-size fraction obtained by laser diffraction was lower than the mass percentage of the clay-size fraction measured by pipette. The silt fraction displayed the opposite trend. Transmission Electron Microscopy and Image Analysis of the clay fraction showed that Laser Diffraction provides an overestimation of the mean diameter in the clay fraction, when particles are assumed to be represented as spheres.