Soil particle size distribution – comparison between laser diffraction, integral suspension pressure and sedimentation (pipette) methods

The sedimentation (pipette) (SP) method has been in use for a long time as a solid reference method to estimate particle size distribution (PSD) in soil. The procedure is demanding, not the least concerning the manual extraction of soil fractions at given depth and time intervals during the sedimentation process and their subsequent drying and weighing. The more recent laser diffraction (LD) and integral suspension pressure (ISP) methods are promising alternatives. They have the advantage that the extraction-drying-weighing procedure for the finer soil fractions (clay and silt) is replaced by automatic registration of particle volumes (for LD) and pressures at given depth during the sedimentation process (for ISP). Due to these differences in measurement technics, PSD:s determined with LD and ISP methods often deviate more or less from PSD:s by SP method, which have implications for the matching with historical SP soil databases. We present some draft results of studies comparing the three methods on samples from agricultural soils in Sweden. The results show that there is still a need for further fine-tuning in the methodologies to align PSD composition from one method to the other.