Use of the Laser Diffraction Method for Assessment of the Pipette Method

Particle‐size distribution (PSD) is one of the most important and frequently measured soil parameters. The assumptions inherent in the use of Stokes' law in the standardized sieve–sedimentation method (SSM) are known to introduce bias into the estimates of PSD. We used the laser diffraction method (LDM) to quantitatively estimate the bias in the SSM. Pipette samples collected at standardized times during the SSM of three different soils were analyzed with LDM to detect the largest particles in suspension. Light microscopy was used to verify the LDM estimates. Particle sizes in suspension were compared as a function of time to the predicted sizes by Stokes' law. Our findings suggest that (i) real sedimentation in a soil suspension deviated from Stokes' law in all the soils investigated; (ii) larger particles then predicted by Stokes' law occurred in all fractions of all investigated soils where counts of anomalously larger particles ranged from several to around 40%; (iii) the longer the time of the sedimentation, the larger the deviation from Stokes' law with the effect of irregularly shaped particles becoming more evident; and (iv) sieving soil samples appears to not be an effective method for the separation of the sand fraction from finer fractions. Our observations on the deviation from Stokes' law has previously been widely discussed in the literature. However, quantification of this phenomenon, as described in this paper, should help to better estimate the uncertainty of the pipette method.