THE GLOBAL DISTRIBUTION AND CHARACTER OF SOILS - Discovering Physical Geography - page 290

Global distribution of Spodosols

(a) (c)

Figure 11.27 Spodosols. (a) Generalized map of Spodosols on
Earth. These soils occur on about 3% of Earth's ice-free land area.
( Source: U.S. Department of Agriculture.) (b) A typical Spodosol
landscape in the Great Lakes region. (c) A well-developed Spo-
dosol in the Great Lakes region. Note the thick E horizon, which
has distinct tongues (red arrows) that extend deeper into the soil.
These features developed because water preferentially followed
tree roots as it moved downward. The dark horizon (white arrows)
directly below the E horizon is a Bhs horizon rich in eluviated
organic acids and iron. (b)

illuviated organic matter, aluminum, and iron. If sufficient
time or eluviation has transpired, the Bs horizon will actu-
ally become cemented with eluviated organic material called
ortstein (Figure 11.27c). In these instances, the B horizon
will have a Bhs designation to reflect the presence of both
humic acids and sesquioxides.

matter is very slow as a result of the cool and moist environ-
mental conditions.
Given the organic nature of Hisotols, the major hori-
zons of Histosols are O horizons that are very dark in color
(Figure 11.28c). Because these soils are extremely fertile, they
are actively cultivated for crops, such as mint or cranberries, in
places where they are easily accessible. They are also excellent
soils for sod farms and certified organic farms. These soils are
also used as mulch and a low-grade fuel source. In the American
Midwest, Histosols are commonly referred to as peat or muck .

Histosols Although the vast majority of soils possess a dis-
tinct mineral component, some soils are almost entirely organic.
The organic soils usually fall into the soil order Histosols . His-
tosols are found on about 1% of Earth's ice-free land surfaces
(Figure 11.28a), most of which are in cool, moist environments.
These soils form in isolated shallow lakes, ponds, wetlands,
and bogs (Figure 11.28b) where the decomposition of organic

Gelisols In 1998 the soil taxonomy system was revised to
include a new soil order: Gelisols . Gelisols develop in very
cold climates at high latitudes or high mountain elevations and

Histosols Organic soils that form in cool, wet environments
where organic carbon decomposes very slowly.

Gelisols Soils in subarctic and arctic environments that con-
tain permafrost within 2 m of the surface.