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Controls of Soil Formation
No soil development
because of very
steep slope
Transported soil
developed on
stream-deposited
sediment
Thinner soil
on steep slope
because of erosion
Residual soil is
developed on
bedrock
Thick soil
Unconsolidated deposits
Bedrock
FIGURE 5.15 The parent material for residual soils is the underlying bedrock, whereas transported soils
form on unconsolidated deposits. Also note that as slopes become steeper, soil becomes thinner. (Left
and center photos by E. J. Tarbuck; right photo by Grilly Bernard/Getty Images, Inc./Stone Allstock)
Time
Time is an important component of every
geological process, and soil formation is no
exception. The nature of soil is strongly
influenced by the length of time that
processes have been operating. If weathering has been going on for a comparatively
short time, the parent material strongly
influences the characteristics of the soil.
As weathering processes continue, the
influence of parent material on soil is overshadowed by the other soil-forming factors,
especially climate. The amount of time
required for various soils to evolve cannot
be specified, because the soil-forming
processes act at varying rates under
different circumstances. However, as a rule,
the longer a soil has been forming, the
thicker it becomes and the less it resembles
the parent material.