Geoscience Reference
In-Depth Information
Soil Horizon Development
The formation of soil horizons is complex because it in-
volves the internal organization of sediment through a
combination of additions, depletions, transformations, and
translocations over a period of time. This organization is
best understood when it is viewed in an animated way that
demonstrates the progressive development of soil. To do
so, go to the
Geo Media Library
and select
Soil Horizon
Development
. Once you complete the animation, be sure to
answer the questions at the end to test your understanding
of this concept.
case, the inclusion of the lowercase “t” means that the horizon is
texturally distinctive, through the illuviation of clay at that level,
from the rest of the soil. Another way to make this distinction is
to say that the B horizon is likely to be an
argillic horizon
with a
strong blocky structure. This structure developed because abun-
dant clay was added to this part of the soil through eluviation.
One of the ways in which soil scientists study and man-
age soils is in association with the concept of soil classifica-
tion. In general, classification is a common practice in many
scientific disciplines because it makes it easier to remember
attributes about the phenomena under study. Classification also
provides a basis from which one category can be compared
with another. Given the predictable pedogenic processes and
factors that operate on Earth, soils are particularly well suited
for classification.
Although many different classification systems are used
around the world, the one used in the United States is called
soil taxonomy
. This system is based on the existing proper-
ties of a soil, such as color, texture, structure, and mineral
content, which can be measured. Many other soil classifica-
tion systems are genetic; that is, the soil scientist attempts
to reconstruct how the soil evolved, even though the initial
environmental conditions may not be known. Because soil
taxonomy is a generic, logically based system, the classifi-
cation scheme is hierarchical with several levels of general
classifications, each with sublevels below it. With each suc-
cessively lower level, fewer and fewer similarities appear
between soil types.
The highest level within soil taxonomy is the
soil order
.
Twelve soil orders occur around the world, each one distin-
guished on the basis of diagnostic horizons (Table 11.1) that
meet certain criteria. These diagnostic horizons are the
epipedon
(from the Greek word
epi
, meaning “over” or “upon”), which is
the horizon formed at the soil surface, and the subsurface hori-
zons formed by removal or accumulation of material.
Soil orders are subdivided into several categories,
which, from highest to lowest, are
suborders, great
groups,
families,
and
series
. To give you an idea of this hierarchy's
Soil Science and Classification
The previous section discussed the basic model of soil forma-
tion and the various horizons that evolve over time. Given that
model, you can now examine some of the soil variability on
Earth, which is known because of the research conducted by
scientists working in the field of
soil science
. This scientific
discipline specifically deals with soils as natural resources,
including their physical, chemical, and biological properties.
Soil science is a discipline particularly relevant to humans
because we use soils in countless ways. Of particular impor-
tance is that soils are the medium in which we grow our food.
Thus, understanding the properties of soils is essential to indus-
trialized agriculture and directly affects the quality and price
of the food we buy. Soils are also the fabric in which we build
our homes, buildings, highways, and myriad other structures.
As a result, engineers must have a good understanding of soil
characteristics in order to build structures that are properly sup-
ported. Soils are also important filters for surface and ground-
waters and are thus highly relevant to hydrology, which focuses
on the way that water moves on Earth. People who study soils
are called
soil scientists
(Figure 11.1). The goal of soil scien-
tists is to improve our understanding of the Earth's soils in order
to preserve and efficiently utilize them.
A B horizon that is enriched in eluviated
Soil taxonomy
The method of soil classification that is based
on the physical, chemical, and biological properties of the soil.
Argillic horizon
clay.
Soil science
The study of soil as a natural resource through
understanding of its physical, chemical, and biological
properties.
Soil order
A group of 12 distinctive soils differentiated at the
most general level.
Epipedon
The uppermost horizon or horizons of a soil that is
used for diagnostic purposes.
