IMPACT OF REFRACTIVE INDEX ON DETERMINING THE PARTICLE SIZE DISTRIBUTION OF SOIL SAMPLES IN THE VALLEY AND RIDGE AND PIEDMONT PROVINCES OF VIRGINIA

Particle size analysis provides fundamental information on size and distribution of the soil mass fraction, which is important in determining soil texture. Advancement of laser diffraction technology (LDPSA) has led to production of devices that can be used to determine particle size distribution (PSD) in a dispersed solution. In LDPSA the most appropriate refractive index (RI) is sample dependent, and thus the RI decision could lead to variation in reported PSD for the same sample. The present study investigated the impact of using different RI on PSD measurements of three soil samples collected from the Valley and Ridge, and Piedmont provinces of Virginia. The study also compared the traditional sedimentation techniques (sieve, pipette, and hydrometer) versus the LDPSA (LS 13-320 particle size analyzer) on PSD. The three soil samples represent three different soil series (Frederick, Mill Rock and Tatum) with varying percentages of sand, silt, and clay. Soil samples were initially disaggregated and sieved through #10 sieves (2 mm). Four sub samples were then prepared for each soil sample and each sub sample was subjected to different RI (RI of quartz, kaolinite, and mica). A randomized complete block experimental design with sub sampling was used to estimate the RI effects and to compare the LDPSA and sedimentation techniques. Results of the study revealed that there was a significant difference between PSD of the clay fraction when different RI were used. Nevertheless, this was not evident for silt and sand fractions. In addition, the LDPSA underestimated the clay fraction while overestimated the silt fraction compared to the hydrometer and pipette methods. Conversely, the sand fraction was much more consistent across the LDPSA and sedimentation techniques. Use of LDPSA technique can reduce the time and labor required for particle size analysis. However, the selection of the suitable RI based on the mineralogy of the soil sample is vital to ensure more consistent outcome.

Key words: LDPSA, Refractive index, PSD