Use of densimetric techniques for fractionating soil organic constituents
Date
1967-10
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ORCID
Type
Degree Level
Masters
Abstract
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.
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Degree
Master of Science (M.Sc.)
Department
Soil Science
Program
Soil Science