Geoscience Reference
In-Depth Information
When the weathering is very strong and when it is realized in acid conditions, the
original crystal lattice could be completely disturbed and the “central” cations like
Fe
3+
and Al
3+
are pulled out to form sesquioxides Fe
2
O
3
and Al
2
O
3
and eventually
transported at least from the place of origin. The transport of sesquioxides is depen-
dent upon several soil conditions as we demonstrate later in Sect.
12.2
about soil
horizons.
5.2
The Old and New Minerals
5.2.1
Size of Soil Particles
The study of soil particles, their size, shape, and role upon soil properties belongs to
the branch of soil science called soil texture.
Soils are comprised of particles having a huge disparity of sizes. Gravel, the
remainder of stones after weathering having individual sizes larger than 2 mm, is
also called the skeleton of a soil. We have the opportunity to separate the skeleton
from a soil sample by sifting the fi ne particles through a sieve with circular openings
2 mm in diameter. Observations and careful experiments have shown that gravel
composing the skeleton does not support plants - indeed, plants do not even start to
grow in particles greater than 2 mm. On the one hand, mixtures of weathered par-
ticles less than 2 mm defi nitely provide some partial support for plants. The size of
2 mm is not an especially accurate value and only represents a reasonable boundary
obtained by convention for a life-supporting environment. If gravel prevails in soils,
the volume suitable for rooting of plants is strongly reduced, the amount of water
being supplied to roots is very small especially during periods without rain, and a
feeble import of nutrients usually persists. On the other hand, the presence of even
a small, hardly noticeable gravel skeleton can have a positive, highly benefi cial role
providing relatively big pores along the boundary between its gravel and the sur-
rounding fi ne soil. Frequently, these big pores are not fi lled with water even if the
bulk of the soil is fully water saturated. In such cases, the big pores form a network
through which air can penetrate into the waterlogged soil and provide an opportu-
nity for aerobic processes to exist in the neighborhood of the gravel grains. These
large pores provide still another advantage by making it easier for water from espe-
cially heavy rains to infi ltrate the soil surface. Further on in this chapter, we deal
with characteristics of soils without gravel grains, simply with fi ne soil.
The size distribution of particles in the fi ne soil is truly remarkable. Extending
from as large as 2 mm to as small as less than 0.002 mm (i.e., 2
m or 2 microme-
ters), their diameters have a range greater than a thousandfold. The coarse fraction
is sand, the fi nest fraction is clay, and between them is silt. The size of these frac-
tions was originally estimated empirically. For example, after a sample of dry soil
μ
