Is the laser diffraction method reliable for soil particle size distribution analysis?

The laser diffraction method (LDM) has been increasingly applied for quantifying soil particle size distribution (PSD), owing to its advantages of rapid analysis, high reproducibility, and continuous PSD measurement for a wide range of size fractions. However, some ambiguities exist regarding the comparability of results with those obtained using other classical methods. The objective of the current study was to evaluate LDM-derived PSDs via comparisons with PSDs obtained with the standard sieve–pipette method (SPM) and from the absolute method of microscopy. A total of 277 soil samples were collected at different soil depths in a typical cropland in the northeast mountainous region of Beijing and analyzed with both SPM and LDM. Due to time and labor constraints, scanning electron microscopy (SEM) was performed on 100 samples randomly selected for the PSDs within the clay fraction withdrawn by SPM. The results manifested on the average 18.9% underestimation of clay content and 25.3% overestimation of silt content by LDM compared to SPM. These disagreements directly caused the shifts of soil texture class in 44.8% of the soil samples. Significant linear regression equations were generated to convert LDM–derived sand and silt contents to SPM–derived ones (p < 0.01). The linear conversions for the clay content were only significant for the calibration samples, but possessed negative coefficients of determination for the validation set. According to SEM, silt-sized particles were wrongly included in the clay fraction identified by SPM. Eliminating such particles, the clay contents corrected by SEM were significantly lower when assuming the shape of clay particles < 2 µm as plates or discs with constant thickness–diameter ratio of 1/10, and higher when considering the clay particles as spheres for volume calculation, in contrast to those measured by LDM (P < 0.01). Detailed volume-based PSDs within the clay fraction were further compared between SEM and LDM, revealing dissimilar PSD patterns but statistically similar median particle diameters. These findings suggest the effectiveness of LDM in soil PSD determination. Future work is needed to systematically quantify the impact of other possible factors such as clay mineralogy and refractive index on LDM-derived PSDs.