Mountain Soils - Mountain Geography: Physical and Human Dimensions - page 317

FIGURE 6.8 This diagram shows an idealized cross section of plant community patterns from north-
to south-facing slopes in the desert environment of the Santa Catalina Mountains, Arizona. Soils
would be expected to follow the same pattern if the vegetation zones remained the same over
long periods of time. (Adapted from Whittaker et al. 1968: 441.)
Alfisols and Ultisols have well-defined horizons. Alfisols, which have developed in
many mid- and low-latitude environments, cover about 13 percent of the land area of the
United States, including parts of the northern Rockies (Brady and Weil 2008). Ultisols
are more weathered than Alfisols, but less strongly weathered than Oxisols and less
acidic than Spodosols. They are relatively old soils, formed in humid tropical and sub-
tropical environments. The key diagnostic feature of Vertisols is the presence of clays
that swell when wet and shrink, forming cracks, when dry. Vertisols typically occupy
gentle slopes and lowland or valley sites with poor drainage; however, they are occa-
sionally found on steep slopes (Soil Survey Staff 2010). Mountain locations of Vertisols
include areas of Ecuador, Bolivia, and Ethiopia.
Recognition of the influence of human activities on soils prompted an international
effort to develop a system for classifying anthropogenic (affected by human activities)
soils—anthrosols (Bryant and Galbraith 2003; IUSS Working Group WRB 2007). Human
activities in mountain regions, notably mining, terracing, agriculture, traffic, waste dis-
posal, and settlement, alter the parent material, chemistry, fertility, texture, structure,
and rate of development of horizons in mountain soils. Thus, the identification and clas-
sification of anthrosols is important for mountain regions.
Potential and Limitations of Mountain Soils
With the exception of intermediate elevations in arid and humid tropical mountains,
soils become less developed and less productive as elevation increases. They also be-