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Dispersion of Soils by an Ultrasonic Method that Eliminates Probe Contact¹

ABSTRACT

A cup-type ultrasonic vibrator was evaluated for use in dispersing soils and was calibrated to the standard chemical dispersion plus shaking method. The "cup horn" device isolates the soil suspension from contact with, and potential contamination from, the Ti alloy of the ultrasonic probe-tip or "horn." Five soils (2 A horizons and 3 B horizons) representing four soil orders were tested. The soils varied in texture (16–48% clay), clay mineralogy (mixed to kaolinitic), pH (4.9–7.0), and free-iron (1.9–5.0%). Twenty-minute cup horn treatment at 200 watts yielded clay contents that were 0 to 9.5% lower than those from the standard method; 25-minute treatment yielded values 1.5% lower to 21.5% higher than the standard method. Medium and coarse sand fractions in the soils were lower, and very fine sand and silt fractions tended to be higher after cup horn treatment than after the standard treatment. In a second test, approximately 11% (absolute) of sand samples (50–250, 250–2000 µm), and 6% (absolute) of coarse-silt samples (20–50 µm) shifted to finer fractions after cup horn treatments. Part of this shift was the result of grain abrasion and part was the result of removal of coatings on grains left intact by the standard dispersion treatment. Four of the five soils formed stable dispersions when sonicated in pH 9.5 Na₂CO₃ solution. The fifth, a very strongly acid soil, flocculated unless titrated to pH 8.5 to 9.5 using 0.1M NaOH after ultrasonic treatment. Coefficients of variation of replicated sand, silt, and clay determinations after cup horn treatment ranged from < 1 to 4%; coefficients of variation of replicated determinations after the standard treatment ranged from 2 to 5%.

¹ Contribution from the Dep. of Land, Air, and Water Resources, Univ. of California, Davis, CA 95616.

² Assistant Professor, Dep. of Agronomy and Soils, Washington State Univ., Pullman, WA 99164 (formerly Research Assistant, Davis, CA); Research Assistant, and Associate Professor, Davis, CA.

Received for publication May 9, 1983. Accepted for publication February 22, 1984.