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Isolation of Soil Particle-Size Fractions¹

ABSTRACT

Studies of the properties of soil particle-size fractions have been restricted by the lack of a method of quantitatively isolating these fractions without serious modification of their properties. A mild method of isolating soil particle-size fractions for such studies is described. It involves dispersion of the soil by ultrasonic vibration in water and subsequent isolation of the particle-size fractions by sieving, centrifugation, and filtration techniques. The method allows large-scale isolation of soil particle-size fractions without use of chemical reagents, and it gives almost quantitative recovery of soil material in the fractions isolated. Data reported show that the particle-size distribution values obtained for 13 soils after isolation of sand, silt, and clay fractions by the method described agreed closely with those obtained by particle-size analysis after chemical dispersion by the peroxide-sodium polyphosphate method.

Clay fractions isolated from soils by the physical method described had higher carbon, nitrogen, sulfur, and phosphorus contents and contained markedly higher percentages of the total carbon, nitrogen, sulfur, and phosphorus in the soils fractionated than did clay fractions isolated by another physical method that did not quantitatively separate clay-size material.

¹ Journal Paper no. J-7577 of the Iowa Agr. & Home Econ. Exp. Sta., Ames, Iowa. Project no. 1835. This work was supported in part by the Herman Frasch Foundation.

² Research Associate and Professor, respectively, Dep. of Agronomy, Iowa State Univ., Ames, Iowa 50010.

Received for publication May 9, 1973. Accepted for publication November 7, 1973.