Agriculture Reference
In-Depth Information
SOIL COMPONENTS INDEPENDENT
2.1.
SOIL SOLIDS
Soil scientists define sand particles 2 mm in diameter as the largest “regular”
component of soil. Many soils contain gravel, rocks, and other materials, that
are larger than 2 mm in diameter but because of their relatively large size have
limited surface area and provide little in the way of chemical reactivity in soil.
For this reason these particles will not be considered. Particles smaller than
2 mm are divided into groups in numerous ways by different disciplines and
researchers; however, for our purposes it will suffice to view them as belong-
ing to three groups: sand, silt, and clay.
2.1.1.
Sand
The sand fraction of soil is composed of particles 2-0.02 mm in diameter.
1
It
is derived from the physical breakdown of rock. Wave action in oceans and
lakes, tumbling in a river, or grinding in a glacier are common ways sand is
produced. Heating and cooling, wetting and drying, and freezing and thawing
are also common physical ways that sand-size particles are produced. Plant
roots growing through cracks in rocks also break them down into smaller par-
ticles that may eventually become sand size. Many soil scientists find it con-
venient to subdivide sand in to fine, medium, and coarse or even more groups.
The surface area of each group is different; however, the surfaces and the
chemical reactions occurring on them are not.
In terms of chemistry, all sand is assumed to be silica, the empirical formula
of which is SiO
2
or silicon dioxide and can be considered the anhydride
2
of
silicic acid, H
4
SiO
4
. However, in the environment in general and soil specifi-
cally, silica is polymerized to form sheets and three-dimensional structures
such as sand particles. Here each silicon atom is tetrahedrally surrounded by
four oxygen atoms (see Figure 2.1) and each oxygen also has two lone pairs
of electrons. All surfaces of silica structures will have electrons that attract any
positive or partially positive species near the surface. Thus water, through its
partially positive hydrogens, and any positive species it contains will be
attracted to the surfaces of silica. Figure 2.1 also shows a water molecule illus-
trating the partially positive hydrogens and partially negative oxygens and the
lone pair of electrons on the oxygens attached to silicon.
At broken edges it is possible to find oxygen atoms that are attached to
silicon but have no silicon bonding partner. These are often associated with
hydrogen forming hydroxy (—OH) groups. The bonds and orbitals of silicon,
1
This is the international Soil Science Society definition. The USDA defines sand as ranging from
2 to 0.05 mm in diameter.
2
An anhydride is a compound that has lost the elements of water.
