Mountain Soils - Mountain Geography: Physical and Human Dimensions - page 316

tent of humid tropical mountain soils at higher elevations are the lower rates of weath-
ering that occur with cooler temperatures and the relative youthfulness of the moun-
tain landforms. Many tropical mountains formed only during the late Tertiary period
(i.e., about 70 million to 1 million years ago). In New Guinea, for example, soils old
enough to be strongly weathered are restricted to elevations below 2,100 m. On the oth-
er hand, weathering is sufficiently rapid in the humid tropics so that Entisols, the least
weathered soils, are relatively scarce except on the highest summits and ridges.
Although usually associated with coniferous forests of midlatitude and subpolar
mountains, podzolization also operates at higher altitudes in the humid tropics (Schawe
et al. 2007). This involves the selective leaching of iron and aluminum, leaving silica be-
hind in the upper layers of the soil. As organic matter increases, creating humic acids,
and temperatures decrease, podzolization prevails, resulting in increasing acidity and
more distinct soil horizons with increasing altitude. These tendencies only hold true
up to the timberline; beyond this point, nocturnal low temperatures limit the develop-
ment of vegetation and soils (Hedberg 1996). In the tropics, as long as soils are fertile
and temperatures favorable, annual crops are more successful at intermediate altitudes
than in the lowlands. In fact, maize, common beans, and some types of squash were
first domesticated and grown in neotropical highlands. Agriculturally suitable soils plus
a moderate climate at intermediate altitudes often result in denser human populations
in tropical highlands than in lowlands, as discussed in Chapters 10 and 11.
Other Mountain Soils
Four soil orders of the U.S. Soil Taxonomy system—Gelisols, Vertisols, Ultisols, and
Alfisols—also occur on mountains as well as in nonmountainous regions. Gelisols, which
are associated with permafrost environments, are strongly affected by changes in
volume caused by the freezing and thawing of water. Permafrost presents a barrier to
downward movement of water in Gelisols, and the disturbances of freezing and thawing
may prevent the formation of diagnostic horizons.