Applicability of laser diffraction method for soil particle size distribution analysis of five soil orders in the water erosion region of China

Soil particle size distribution (PSD) is a critical parameter for soil loss estimation in many water erosion models. The laser diffraction method (LDM) has been increasingly applied for soil PSD determination in recent decades, and its applicability and reliability need systematical examinations. The objective was to evaluate the performance of LDM for PSD determination of five soil orders in the water erosion region of China relative to the traditional sieve-pipette method (SPM). A total of 465 soil samples were collected in the five water erosion subregions of China, which correspond to five soil orders of Mollisols, Alfisols, Entisols, Inceptisols, and Oxisols. All the samples were analyzed with LDM for the four size fractions widely employed in water erosion models (i.e., 100 to 2,000, 53 to 100, 2 to 53, and <2 μm), and the corresponding results were compared to those obtained by SPM. The comparisons show varying results between LDM and SPM for the sand content, or the two sand-sized fractions of 100 to 2,000 and 53 to 100 μm. LDM consistently overestimated the silt content and underestimated the clay fraction, yet the magnitude of either the overestimation or underestimation varied among soil orders. The PSD discrepancies shifted the textural classes of 44.8% to 96.2% of the soil samples and, on average, increased the soil erodibility K factor by 0.010 to 0.034 t MJ^–1 h mm^–1 for different soil orders. The optimal clay/silt and silt/sand boundaries of LDM were derived to match the size fractions measured by SPM. Nevertheless, except for the silt/sand boundary of 58.2 μm for one soil order, each of the other threshold boundaries held a Lin’s concordance correlation coefficient less than 0.8, suggesting moderate or poor agreement between LDM and SPM. Linear regression models were also established to convert the four size fractions between the two methods for all the samples as well as for each soil order. However, not all the conversion models were statistically significant at the level of 0.05. Those that were significant varied among soil orders for each size fraction. These findings hold important practical implications for soil PSD determination and water erosion modeling in China and elsewhere.