Comparison of American Society of Testing Materials and Soil Science Society of America hydrometer methods for particle-size analysis

Particle-size analysis (PSA) is widely used in both soil science and engineering. Soil classification schemes are built on PSA values and recent developments in pedotransfer functions rely on PSA to estimate soil hydraulic properties. Because PSA is method dependent, the standardization of experimental procedures is important for the comparison of reported results. A study was conducted to compare the American Society of Testing Materials (ASTM) hydrometer method (D422) for PSA with the hydrometer method published by the Soil Science Society of America (SSSA). Tests on soils ranging in texture from sand to sandy clay loam were conducted at temperatures ranging from 20 to 30 degrees C. The main difference between methods is the temperature correction, with the ASTM method relying on an empirical correction and the SSSA method using a blank hydrometer reading. Identical texture estimates for all but one of 48 total samples was observed between methods. Percentage of fines, silt, and clay demonstrated relatively consistent values between methods. The ASTM and SSSA methods were compared at values of D50, D30, and D10 (i.e., effective particle diameter values when the size-distributions have dropped to percentages of less than 50, 30, and 10, respectively). Excellent agreement was found between methods for D50 and D30 values (correlations above 0.99). Less agreement was found for D10 (correlation 0.989) values, but still reasonably good. The results suggest that for the range of soil textures evaluated in this study, ASTM and SSSA methods can be used interchangeably for textural analysis.