Making soil particle size analysis by laser diffraction compatible with standard soil texture determination methods

The standard sieving, pipette, and hydrometer methods for soil particle size analysis (PSA) have three main drawbacks: (i) the procedures are tedious, (ii) the procedures are time consuming, (iii) and the results are protocol dependent. Laser diffraction PSA delivers rapid results using standardized procedures, but so far it has been difficult to reconcile results with those from standard sedimentation methods. The objective of this study was to develop a protocol that would permit direct usage of laser diffraction PSA and render results compatible with current methods. The protocol was developed using 54 standard soil samples from different textural classes. Regression of the laser diffraction PSA against the hydrometer/pipette method yielded R² values of 0.92/0.9, 0.92/0.94, and 0.99/0.99 and RMSE values of 0.04/0.05, 0.07/0.06 and 0.05/0.03 for clay, silt, and sand, respectively. These statistics are comparable to those obtained by regressing results of the hydrometer against the sieve and pipette methods. A key factor in securing accurate and precise results was limiting the particle size range of the samples by wet sieving the sand fraction. This created representative samples and stable soil dispersed suspensions, allowing accurate estimations of particle size distribution for clay and silt fractions without empirical transformations. Results obtained with the proposed protocol matched those of standard sedimentation analyses for a wide range of soils, encouraging further adoption of laser diffraction for soil PSA.