Use of densimetric techniques for fractionating soil organic constituents

Ultrasonic dispersion of soil was used as a pretreatment, prior to densimetric fractionation in a heavy liquid. A 2 to 3 minute sanification period, using a probe type instrument with a 125 watts power output, resulted in maximum recoveries of nonhumified materials with a minimum of mineral contaminants. Centrifuging the dispersed soil in a liquid of density 1.70 g/cc, resulted in the extraction of the undecomposed, or unaltered, plant residues with a C/N ratio of 25 : 1. Microscopic examination of this fraction showed the presence of irregularly shaped fragments of roots and insect bodies, with readily recognizable cellular structures. Fractionation, by centrifuging the sonified soil in a heavy liquid at a density of 2.0 g/cc, for one half hour, separated the partially humified and slightly altered residues in the soil with a C/N ratio of 17 : 1. The light fraction, so extracted, constituted 25% of the total organic carbon in the soil and had an ash content of 42%. It was comprised of materials with no recognizable cellular structures, in addition to some fragments still retaining some cellular structure. The dark coloured uniform masses, resembling charcoal, also present in this fraction, indicated the extraction of some humified materials. The use of the density gradient column, after sonic dispersion of soil aggregates, separated a number of fractions of closely related soil constituents. The continuous gradual change in density, from <2.00 to >2.70, made it feasible to accurately assess the composition, the form and the amount of organic matter present in each of the fractions. Approximately 20% of the organic content of the soil was present in the light fraction, containing the unhumified plant residues with a high C/N ratio. The largest proportion (80%) of the native soil organic matter was closely associated with the silt and clay fractions. The C/N ratio of 10 : 1 of these materials suggest that a greater part of the humified material was present in the organa-clay complex form. The effectiveness of the density-gradient fractionation was further tested by measuring the separation of added plant residues from amended soils. Coarse plant materials were readily separated from the inorganic soil system. However, the addition of finely ground radio-active plant residues resulted in an incomplete separation. Fifty-five percent of the initially added radio-activity present in the finely ground oat straw was recovered in the light fraction representing 5% by weight of the soil. The remainder of the radio-active plant material was tightly adsorbed to the silt and clay fractions of the soil. This could not be separated by the high frequency sonic vibrations utilized in this study. Slightly greater than 40% of the total soil weight was obtained in the heavier fractions. This was practically free of the added residues. The density gradient column, therefore, in association with an ultrasonic pretreatment offers a simple and reproducible technique for separating specific fractions from the soil. Although, it could not completely separate the finely ground plant materials added to the soil system, further refinements should make feasible the quantitative separation of the soil biomass and plant roots. In addition, it should make feasible the meaningful separation of unaltered, native soil organic matter based on its physical characteristics