Environmental Engineering Reference
In-Depth Information
FIGURE 1.2 A soil survey map of several soils showing different soil types
with differing slopes and erosion. The first two letters represent a
mapping unit (different soil), the following capital letter represents
the slope, and the last number represents the amount of erosion.
In the United States soils are classified as belonging to one of twelve soil orders, * which
are distinguished by the number and type of horizons they contain. Some orders, such as
Gelisols and Aridisols, are also distinguished by their location. Gelisols are frozen soils
that occur and develop in frigid regions and contain a permanent frozen layer. All soils in
arid or desert regions are called Aridisols. Other soils that have one very distinguishing
characteristic are the Andisols, which develop from glassy volcanic ash, and the
Vertisols, which develop cracks 30 cm wide and 1 meter deep when dry.
A complete list of the twelve soil orders is given in Table 1.1, along with an indication
of a characteristic important in the sampling of each.
* Other countries have systems similar to the U.S. system, but often have more orders.
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