Effect of Manual Sample Dosing Techniques on Soil Particle Size Distribution Measured via Laser Diffraction

Accurate determination of the soil particle size distribution (PSD) is critical for a wide range of environmental, agronomic, and geotechnical applications. Laser diffraction method (LDM) has gained popularity because of its speed and reproducibility; however, it remains sensitive to sample preparation and introduction methods. This study evaluated the impact of three manual dosing techniques on PSD results obtained via laser diffraction for seven USDA-classified soil types, with the pipette method used as a reference. Each technique (A: pipetted suspension; B: semiliquid paste; C: dried material) was applied to 1050 measurements. The results revealed a systematic underestimation of clay and overestimation of silt fractions across all LDM techniques, with Technique A yielding the highest relative standard deviation (average RSD for clay: 16.8%; sand: 26.9%). Techniques B and C showed markedly better repeatability (clay RSDs: 7.1% and 10.2%, respectively), with silt exhibiting the highest measurement precision overall (mean RSD: 6.7%). One-way analysis of variance (ANOVA) confirmed that the choice of dosing technique significantly affected the measured clay fraction (p < 0.001), whereas no statistically significant differences were found for silt or sand. All the laser-based techniques misclassified the soil texture in the USDA triangle, with most samples shifting to silt-dominated groups regardless of the true origin. These findings highlight that while LDM itself introduces systematic biases in PSD estimation, the choice of manual dosing technique—particularly uncontrolled suspension pipetting (Technique A)—further amplifies measurement variability, rendering it unsuitable for high-precision applications. These findings highlight the strong influence of manual dosing on LDM outcomes and confirm the unsuitability of uncontrolled suspension pipetting (Technique A) in precision analysis. Recommendations are provided for standardized manual procedures that can improve reproducibility and classification accuracy in soil laboratories.