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DIVISION S-1-SOIL PHYSICS

Soil Particle Concentrations and Size Analysis Using a Dielectric Method

^a USDA-ARS Southwest Watershed Research Center, P.O. Box 213, Tombstone, AZ 85638 USA
^b Dep. of Soil Science, Univ. of Wisconsin, 1525 Observatory Dr., Madison, WI 53706-1299 USA

gordon{at}tucson.ars.ag.gov

Limitations of traditional methods for particle-size analysis warrant the investigation of new techniques. An alternative method based on the difference between the dielectric constant of soil solids (4) and dispersing solution (81) was developed. We determined changes in suspended sediment concentrations (C) using a coaxial probe placed on the surface of a dispersed soil suspension by monitoring changes in the apparent dielectric constant with time following complete mixing. A single-point calibration for each sample was obtained using the known initial concentration. A refractive index (n) model of the suspension dielectric properties gave the slope of a C vs. n curve for changes in silt-size (0.002–0.05 mm) particles. A magnetic stirring rod was used to homogenize the dispersion, and temperature changes were minimized given the rapid measurement time. Using the dielectric method, particle-size distributions were measured on a 1- to 2-g sample with 400-s settling time because the effective depth of measurement was only 1.5 mm. Wet sieving was used to remove the sand fraction. Comparisons between the silt and clay fractions obtained using the dielectric and pipette methods were in agreement. The combination of speed, automation, small sample size, and nearly continuous data should be balanced against the higher cost of the equipment necessary for the dielectric method.

Abbreviations: TDR, time domain reflectometry

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