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Abstract Five surface soils (0–20 cm) with different amounts of fine particles were separated into clay–, silt– and three sand–size fractions using ultrasonic dispersion and sedimentation in water. The conditions for achieving an adequate fractionation were examined as previous studies showed substantial differences in methodology. Size fractions were analyzed for C and N. Adequate dispersions of the soils were obtained when more than 1600 joules were dissipated per ml of soil suspension (soil/water ratio, 1/5). Recovery of soil solids ranged from 97.8 to 98.8%. The particle size distributions obtained by the method described agreed with those of a standard method of soil particle size analysis. Compared with whole soil contents, the clay and silt fractions were enriched in C and N whereas the sand fractions all were depleted. The organic matter enrichment of the clay and silt fraction decreased with increasing fraction dry weight. The C enrichment factor ranged from 3.3 to 16.4 for the clay fraction and from 1.8 to 9.9 for the silt fraction. Similar values were obtained for the N enrichment factor. The proportion of soil organic matter associated with the clay fraction increased with increasing soil clay content, whereas organic matter in the silt fraction tended to decrease with increasing soil silt content. The sand–size fraction contained less than 10% of the soil C, whereas the clay contained from 49 to 69%, and the silt from 22 to 38%. From 47 to 78% of the soil N was found in the clay fraction. The silt fraction accounted for 20 to 33% of the soil N.
Bent T. Christensen (Tue,) studied this question.