Evaluation and correction of measurement using diffraction method for soil particle size distribution

Abstract: Particle size distribution (PSD) is one of the most fundamental physical properties of the soil, which provides researchers the key basics for the spatial variability of digital soil mapping. Earlier, the soil PSD was mostly derived from the classical sieve-pipette method (SPM). However, the process of SPM analysis is tedious and time-consuming especially for the fine-textured soils. Based on the laser diffraction technique, laser diffraction method (LDM) provides an effective method to determine soil PSD, and its popularity increases for soil researches involving a large number of samples. However, results from the LDM and traditional SPM are different. The present study is aimed to assess the suitability of LDM as a routine method for determining soil PSD, and establish a simplified protocol for transforming the LDM results into traditional SPM ones. The soil samples (a total number of 23) from 13 Chinese provinces or autonomous region were analyzed and the results indicated that: 1) Compared to the SPM in the condition of limited sample numbers and large between-sample variation, LDM underestimate of clay content in soil samples, and overestimate of silt content in soil samples, the relative errors for clay and silt fractions were 36.33% and 36.51% respectively; 2) a linear relationship was established for the measured results of SPM and LDM, the detemination coefficients were 0.91, 0.90 and 0.79; through model transformation, the relative error of the LDM measured results decreased to 16.25% (for clay content), 12.83% (for silt content); 3) upon using the Mie theory to determine the soil PSD, the results of the present study also indicated that the precision could be improved and the discrepancies between the PSD obtained from the SPM and LDM could be decreased with slight modifications in SRI values; A soil particle refractive index (SRI) of 1.50 and soil particle absorption index (SAI) of 0.01 were found to be optimal for the Mie theory model. With relatively limited sample numbers and apparent textural difference between the samples, the distinct incompatibilities were observed in the present work between the PSD obtained by the LDM and SPM. However, depending on the specific research purpose, the deviations between the LDM and SPM may be considerably reduced with the increase of sample capacity or decrease of the spatial scale. This study has contributed to the development of the systematization and standardization for soil comparison. Also, it can help to establish the criteria of soil texture classification based on LDM results.