Geoscience Reference
In-Depth Information
regolith is not rock and is not soil. Rather, it lies somewhere
between the two and is the material in which the soil forms.
Often, regolith production at the bedrock interface occurs at
the same time that soil development is occurring in the upper
part of the weathered zone.
The second kind of parent material is
transported parent
material
. As the name implies, transported parent material dif-
fers from residual parent material because it has been moved
from one place to another. Remember that residual parent
material is regolith that forms
in place
as bedrock weathers.
Although the sediments that comprise this bedrock were depos-
ited after being moved from someplace else, the deposition
happened such a long time ago (millions of years ago) that,
as far as regolith production is concerned, it is considered to
have occurred in one place. On the other hand, transported par-
ent material is sediment that was moved relatively recently by
water or glaciers. Or perhaps wind blew sand to form dunes
like those pictured in Figure 11.9. If this sediment is left undis-
turbed for many years, plants and animals will begin to live in
the uppermost part of the deposit and a soil will form through
the combined processes of additions, depletions, translocations,
and transformations.
most important aspects of climate that affect soil forma-
tion because they influence the kind and rate of biological
and chemical reactions that occur in the soil. In general,
more reactions occur in warmer and wetter places than in
colder and drier regions. As a result, soils tend to be better
developed and thicker in warm, humid regions than in
cold, dry locations.
Climate also strongly influences the process of transloca-
tion in a soil. Remember that translocation refers to the move-
ment of organic and mineral matter from the upper part of the
soil into a deeper zone. Most of this movement occurs when
water percolates through the soil after it falls at the surface.
To visualize this process, imagine, for example, the gradual
melting of snow that occurs after a big winter storm passes
through an area. As the snow melts, some of it evaporates into
the atmosphere, but much of it is absorbed into the ground.
When spring arrives, plants rapidly take up some of this soil
water, but a certain percentage of the water continues to move
slowly downward within the parent material under the force
of gravity. As this water comes into contact with minerals in
the soil or sediment, it dissolves them—much as water dis-
solves sugar—and carries the dissolved particulates deeper
into the soil through the process of eluviation. At some point,
the water stops flowing downward and the dissolved minerals
recrystallize in the zone of illuviation. Simply put, more elu-
viation occurs in humid climates than in arid regions because
more water percolates through the soil. Figure 11.10 shows in
general terms the depth to which eluviation penetrates and the
resulting depth of the illuvial zone in humid, semi-arid, and
arid climate regimes.
2.
Climate
Climate is a critical variable in the context of
soil development. Temperature and precipitation are the
3.
Organisms
Organisms collectively are made up of the
plant and animal life that resides within soil. The im-
portance of organisms in soil formation simply cannot
be overestimated. At a fundamental level, organisms
give life to soil and contribute greatly to the notion
that soil is significantly more than
just dirt
. At a more
complex level, organisms have a symbiotic relationship
with soil in that they not only acquire their food from
it, but also help regulate the soil environment in which
they live.
Organisms contribute to soil health and formation in
many different ways. One way is when plant roots penetrate
deeply into the ground. This penetration creates passage-
ways in the soil in which water and oxygen can flow. Many
small animals, such as ants, ground squirrels, prairie dogs,
and other rodents, spend the majority of their lives burrow-
ing through the soil (Figure 11.11). Through this ongoing
process, the soil is continually mixed in a process called
bioturbation
. The mixed soil is softened and aerated,
Figure 11.9 Example of transported parent material.
These
sand dunes along the shore of Lake Michigan consist of sand
deposits that were blown there by the wind. Note the grass grow-
ing on parts of the dunes. The establishment of grass begins
the process of soil formation by providing organic material to be
added to the sand parent material.
Transported parent material
Parent material such as glacial
or stream sediments that has recently been deposited and in
which soil forms.
The mixing of soil by plants or animals.
Bioturbation
