Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/6/1999
Publication Date: N/A
Interpretive Summary: Soil particle size analysis involves measuring the distribution of solid soil granules in various size ranges. This soil analysis provides information that is fundamental to our understanding of the structural and physical aspects of soil, its nutrient cycling properties, and water holding capacity. Standard methods for soil particle size analysis that were developed in the 1920's and 1930's are laborious and time consuming. These considerations warrant the development of improved methods based on modern technology. By adapting and programming an electromagnetic soil water content sensor, a particle size analysis system was constructed that has distinct advantages over traditional methods. The underlying theory, materials, and methods used for this purpose are described in detail in this paper. The new method allows the analysis to be conducted in about 7 minutes that used to require 8 hours. The new method also provides much more data per sample analysis. The combination of speed, automation, and small sample size must be balanced against the higher cost of the equipment that is needed. However, the labor savings alone could rapidly outweigh these costs for a government or university laboratory that regularly conducts particle size analysis, provided that the theories and results presented herein are born out in future studies.
Technical Abstract: 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 over 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 to 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/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.