Geoscience Reference
In-Depth Information
was initially weighed, vigorously mixed with an excess of water, and allowed to
settle for a couple of hours, the muddy water above the soil sediment was poured
out. With the muddy water containing only fi ne clay particles, it was believed that
they would always fl oat and not sink into the sediment. Another excess of clean
water was poured into the soil sediment and vigorously mixed, and again after a
couple hours, the muddy water was poured out. This mixing and washing process
continued until the water above soil sediment was clear and no longer muddy after
a couple of hours of settling. After drying the soil sediment, it was weighed, and the
difference between the fi rst and second weights was assumed to be the weight of
clay in the sampled soil. After hydraulics, the new subject of mechanics, was devel-
oped from the end of the eighteenth century to the start of the nineteenth century
from Newton's three laws of motion, physicists formulated the velocity of sedimen-
tation of spherical particles in water with a mathematical equation. With sedimenta-
tion velocity depending upon the square of a spherical radius, we know that if the
radius is 1 and the velocity is 1 as well, then we also know that a sphere of radius 10
has a velocity equal to 100. Soil chemists, who fi nally specialized in soil physics,
applied the equation for determining the size of soil particles.
The fi rst objective classifi cation of soil fractions was elaborated by the Swedish
soil chemist Atterberg who assumed even distributions of particles within the fol-
lowing fractions indicated as equivalent sphere diameter: 2-0.6, 0.6-0.2, 0.2-0.06,
0.06-0.02, 0.02-0.006, 0.006-0.002, and<0.002 mm. Having considered their
practical utility, he eventually simplifi ed his classifi cation system to
coarse sand
having particle sizes 2.0-0.2 mm,
fi ne sand
with size ranges of 0.2-0.02 mm,
silt
characterized by the size 0.02-0.002 mm, and
clay
with size <0.002 mm. The
boundary between sand and silt varies in national classifi cations. The US pedologic
classifi cations have set it at either 0.05 mm in the past or 0.02 mm in the recent
Keys
to Soil Taxonomy
. In an oversimplifi cation we could say that sand particles are rec-
ognizable by naked eye, silt particles are visible by means of lens and optical micro-
scope, and clay particles are examined by an electron microscope. While the general
properties of sand and clay fractions are obvious, the silt fraction is characterized as
the range from the boundary when sand assumes some claylike properties to the
boundary where the main properties of clay come to an end. The latter boundary is
demonstrated whenever Brownian movement in water is no longer identifi ed for
particles larger than 0.002 mm. The Atterberg system of particle fractions was fur-
ther modifi ed and slightly differs in the USA from the European system or from that
of the International Union of Soil Science. More about the recent classifi cation of
soils according to their content of sand, silt, and clay will be discussed in one of the
next chapters, in
5.2.3
Soil's First Name: Texture. Because the systems are not
everywhere the same and manifest slight differences in various individual countries,
we shall simplify them into only fi ve groups before we describe briefl y the domi-
nant properties of each of them.
