Mountain Soils - Mountain Geography: Physical and Human Dimensions - page 314

The optimal zone for plant growth may occasionally be found in the subalpine zone,
as in the Australian Alps. The maximum elevation of 2,220 m on Mount Kosciusko co-
incides with broad, gentle upland surfaces. The timber-line is located at about 1,800
m. Lack of extensive glaciation, along with adequate moisture in the subalpine zone,
has favored soil development to an unusual degree. In fact, Costin (1955) called the
Australian Alps “soil mountains” to distinguish them from others, such as the European
Alps, where upland surfaces are predominantly peat or rock.
Andisols
Andisols (andosols in the FAO system) develop in volcanic ash or other volcanoclastic
parent material. They occur throughout the world on volcanoes and are especially prom-
inent in the Pacific Rim Ring of Fire, including the Andes, the Cascade Range, south-
ern Alaska, the Kamchatka Peninsula, Japan, and Hawai'i. They also occur in Iceland,
along the African rift zone, and in other volcanic regions. Clay minerals associated with
Andisols develop from microbial action and the alteration of minerals in place (Shoji et
al. 1993). These soils are typically porous, due to the presence of noncrystalline clays
and high concentrations of organic material (Chen et al. 1999). The clays bind readily
to organic matter and to phosphorus, making it unavailable to plants. Agriculture on
Andisols can be productive if phosphorus is added (Shoji et al. 1993). High water reten-
tion capacities make Andisols an important component of the water resource for people
living in high inter-Andean valleys (Podwojewski et al. 2002). Hofstede (1995) found soil
in an undisturbed grassland soil in the Colombian Andes to contain up to 2.5 grams of
water per gram of dry soil.
Humid Tropical Mountain Soils
Mountains of the humid tropics characteristically rise from lowlands supporting dense
tropical forests. As elevation increases, lowland forests become montane forests, which
become shorter and less dense until trees give way to shrubs, grasses, and herbaceous
plants at the highest levels. Old soils of most tropical lowlands are notoriously nutrient
poor; they exist in a hot and humid environment with excessive rainfall, rapid decom-
position of organic material, and intensive leaching of nutrients. Whereas most soils
serve as nutrient sources for plants, Oxisols (highly weathered lowland tropical soils)
are so leached that the living biomass of the forest, rather than the soil, is the major nu-
trient reservoir. As long as the forest remains, so do the nutrient reserves, which are re-
cycled through leaf fall, decomposition, leaching, and uptake by tree roots. If the forest
is removed, then the source of nutrients is removed. For this reason, most attempts to
grow annual crops on a large scale in the tropics have failed. The primary soil-forming
process on old surfaces in the lowland humid tropics, called laterization, is the selective
leaching of silica from the upper layers of the soil, leaving iron and aluminum, which im-
part red and yellow colors to the soil. Optimum conditions for laterization are high rain-
fall, high temperature, fluctuating water table, and the absence of soil organic material.
Oxides of iron and aluminum are relatively insoluble in the absence of humic acids, so