Mountain Soils - Mountain Geography: Physical and Human Dimensions - page 298

Soil-forming Factors
The primary factors responsible for different kinds of soils are climate, biological
factors, topography, parent material, and time (Jenny 1941). The same factors affect soil
development in mountain as in nonmountain environments, but the intensity and relat-
ive importance of these factors differ between mountains and plains, and among moun-
tain locations. No one factor controls soil formation; rather, all are interrelated.
Climate
The primary climatic factors affecting soil development are temperature, precipitation,
and wind. Climate has significance far beyond itself, since it controls the distribution of
vegetation, which, in turn, influences the kind of soil that develops in any given area.
Temperature is an important control of the rates and types of weathering that cause
rock to break down. Physical weathering, considered dominant in cold climates, results
in a coarse soil texture, but there is increasing evidence for more chemical weathering
in mountains than was formerly thought (Thorn et al. 2001, 2011; Hall et al. 2002). One
study of basalt weathering in Hawaii (Brady et al. 1999) determined that weathering
rates were proportional to rainfall and less strongly affected by differences in temper-
ature.
The periodicity of temperature is one factor involved in weathering and soil form-
ation; others include rock type, presence of organic acids, and moisture availability
(Brady and Weil 2008). In tropical high mountains, the soil surface may freeze every
night, but warmth during the day can provide ample heat for chemical weathering to
produce a moderate amount of clay. By contrast, in midlatitude and polar mountains,
held in the grasp of winter for much of the year, chemical weathering is more restricted.
As discussed in Chapter 5, low temperatures and frost action are responsible for the
formation of various types of patterned ground. The daily frost of high tropical moun-
tains penetrates only a few centimeters below the surface, whereas the seasonal frost in
middle and high latitudes penetrates deeper. Consequently, rocks in tropical mountains
are more readily reduced to small aggregates, and the resulting patterns are of small
dimension; patterned ground in mid- and high latitudes occurs on a broader scale and
may contain larger rocks. Related slope processes operating in low-temperature moun-
tain climatic regimes include frost creep, solifluction, and other types of mass wasting,
also discussed in Chapter 5. These cause the movement and intermixing of surface lay-
ers, which disrupt the soil profile and the processes of soil formation.
Low temperatures limit biological activity. At low temperatures, less organic material
is added to the soil, soil organic matter decomposes very slowly, and soil fauna are rare
(Townsend et al. 1995; King et al. 2010). A soil temperature of 5°C (43°F) has been pro-
posed as the temperature below which biological activity in the soil becomes very slow
(Retzer 1974). At an alpine tundra research station at 3,750 m on Niwot Ridge in the Co-
lorado Rockies, soil temperatures remained above 5°C for the numbers of days shown
in Table 6.1. Site-specific soil temperatures vary with exposure, snow cover, and other
microsite conditions, but the Niwot Ridge data demonstrate the limited time available
for biological activity and the decrease of this activity period with soil depth.