Mountain Soils - Mountain Geography: Physical and Human Dimensions - page 313

FIGURE 6.6 Two soil profiles from the foreland of the Lewis Glacier on Mount Kenya. The deeper
profile on the left shows an Inceptisol, developed on late glacial deposits, while the profile on
the right, with less vertical development, is an Entisol developed on younger neoglacial deposits.
(From Mahaney 1990.).
In arid and semiarid regions, the increase of moisture with increasing altitude pro-
motes soil development by supporting more complete plant cover and greater produc-
tion of organic material and by intensifying the chemical and physical weathering pro-
cesses. Aridisols are characteristically light in color and low in organic matter. Evapor-
ation of soil moisture can deposit soluble salts in the upper horizons or at the surface.
Arid soils are generally alkaline (Fig. 6.7), but higher-elevation soils, with more mois-
ture, more organic matter, and more microorganisms, are darker in color and less al-
kaline. The nitrogen content and base-exchange capacity of soils also increase with in-
creasing elevation in arid mountains (Hanawalt and Whittaker 1977).
Marked differences in exposure to the sun may occur and affect soil and vegetation
on north- and south-facing slopes (Whittaker et al. 1968; Istanbulluoglu et al. 2008).
The elevation of a soil zone may differ by tens of meters between slopes of different
aspects (surfaces facing different directions) (Fig. 6.8). In hot, dry regions, the more
favorable sites for plants usually extend to higher elevations on poleward-facing slopes
and shaded areas. Conditions for plant and soil development in arid mountains are op-
timal only in a narrow vertical zone between the low-temperature high altitudes and
the arid low altitudes. If the topography within this optimal zone includes steep slopes
and narrow ridges, the area favorable to soil development will be relatively small; if it
consists of broad and gentle upland slopes, the area favorable to soil development will
be considerably larger. Examples of broad, gently sloped areas situated within favor-
able climatic zones at higher altitudes include the Tibetan Plateau and the Altiplano of
Bolivia and Peru. Broad uplands also occur on the gentle dip slopes of fault blocks in the
Basin and Range mountains of the western United States.