Geoscience Reference
In-Depth Information
Fig. 5.11
Particle size distribution curve: (
a
) is silt; (
b
) is silty clay loam
and more complex soil taxonomic determination will be explained in the second
half of our topic.
Because the verbal designation of soil texture is not yet unifi ed in all countries,
it is advantageous to plot the results of our measurements in a graph where the size
of particles is plotted in logarithmic scale on the horizontal axis and the percentage
of particles on the vertical axis. The curve is a summation curve, usually referred to
as cumulative particle size distribution curve. We explain the principle of reading
and explaining this curve by the following example. For a particle size of 0.02 mm,
the curve passes through a cumulative distribution value of 85 %, and for a particle
size of 0.002 mm, the curve passes through a cumulative distribution value of 40 %.
Thus, the silt content is (85-40) = 45 %. The soil texture is close to that of a loam.
The more numerous are the groups of particles defi ned according to their size, i.e.,
the narrower is the size of particles in the group, the more smooth is the curve plot-
ted in the graph (Fig.
5.11
).
The principle of measuring the percentage of individual soil particle fractions is
based upon the sedimentation velocity of individual particles in stagnant water.
Following the most frequently used procedure, we carefully stir soil in water, use a
chemical procedure to break up every aggregate into individually separated parti-
cles, pour this suspension into a calibrated cylinder, add water, mix it well, and
measure the velocities of particles by inserting a specially designed densitometer - a
hydrometer - into the suspension. The density of the suspension decreases with
time as particles of different sizes individually settle at different rates toward the
bottom of the cylinder. During early stages of measurement, the density decreases
quickly because the large sand particles settle quickly. Silt particles, because of their
smaller size, settle more slowly and are kept in suspension for longer times. Clay
particles, the smallest, move very slowly and even after a day remain in suspension
