SIZE FRACTIONATION OF SOIL ORGANO-MINERAL COMPLEXES USING ULTRASONIC DISPERSION

Potential artifacts created by soil fractionation procedures are frequently ignored when studying the interaction of natural and synthetic organic compounds with clay-sized materials. Ultrasonic dispersion treatments of 0-7.66 kJ applied to soil in water were used in combination with centrifugation and sedimentation techniques to obtain sand, silt, coarse clay, and medium clay fractions from soils with 19.2 to 44.2 g organic C kg-1 soil. No more than 1.37 kJ of sonication energy was needed for maximum yields of silt-sized materials, whereas sand yields decreased monotonically and clay yields increased at a decreasing rate with energy inputs up to 7.66 kJ. Coulter counter analysis of silt-sized fractions showed that no redistribution in particle-size occurred within this fraction with increased sonication energy. Changes in total C and N concentrations in the silt-sized fractions showed no consistent trends with increased sonication energy. Total C and N concentrations increased in the coarse clay-sized fractions and decreased in the medium clay-sized fractions with a greater input of sonication energy. Diffuse reflectance Fourier transform infrared analysis did not reveal any qualitative changes in the coarse clay fractions, although changes in the medium clay fractions of the two soils highest in organic C occurred when energy was increased from 0.84 to 2.94 kJ. We conclude that excessive sonication energies may cause undesirable artifacts, especially in the medium clay-sized fraction. It is suggested that a standard sonication procedure to obtain adequate particle size yields and limit artifacts in the fractionated organo-mineral complexes include the application of no more than 3-5 kJ of energy to 10 g soil in 50 mL of water.