Comparison of Laser Diffraction versus Composite Sieve and Specific Gravity Tests for Particle Size Distributions across a Range of Soil Textures

Accurate characterization of soil texture is critical to predicting saturated and unsaturated flows in phreatic and vadose zones. This study compared grain size analysis methods across a range of soil textures. Soil cores were extracted from five Florida sites at three depths. After removing organic matter, mineral grain size distributions were analyzed using (1) a composite dry sieve and hydrometer method (SHM) and (2) a laser diffraction method (LDM). The soils were tested in a CILAS 1090 particle size analyzer using a 5% solution of sodium hexametaphosphate. Analysis indicated that LDM detected greater proportions of fine material than SHM, with 27% greater detection of clay and silt. Relative variation by method became increasingly pronounced as soil texture became finer. For instance, the coarsest soil tested differed between methods by 0.9%, 13%, and 14% for clay, silt, and sand, respectively, while the finest soil varied by 1.4%, 66%, and 67%.