Mountain Soils - Mountain Geography: Physical and Human Dimensions - page 304

the same (Bushnell 1942). The term catena (chain) refers to the predictable patterns
of soil types along a slope ( toposequence is also used). Topographic differences also
produce differences in other soil-forming factors. Mid-slope soils, for example, are less
leached than soils on summits (Birkeland 1984). Soil properties change with elevation
and exposure, but is this because of topography or changes in climate and vegetation?
Topography is generally viewed as a relatively passive soil-forming factor, while climate
and vegetation play active roles in soil development. In any event, topography is critical
because gradient and drainage directly affect soil development.
Parent Material
The inorganic stuff from which soils are made affects soil characteristics. It may be
solid bedrock or unconsolidated materials such as glacial deposits, talus, blockfields,
mudflows, alluvial fans, windblown deposits, or even materials transported by people.
Soils develop more rapidly on unconsolidated material than on solid bedrock, but much
depends on the nature of the rock and the site conditions. Volcanic ash weathers very
rapidly; its silicate minerals readily break down into clay. For this reason, volcanic soils
tend to be highly productive. Within bedrock, soft and weak rocks (sedimentary) usually
weather more rapidly than hard and resistant rocks (igneous or metamorphic). Fine-
grained rocks, such as shale or sandstone, typically break into fine particles; coarse-
grained rocks, such as granite, break into coarse particles. Some rocks weather relat-
ively rapidly into clays; others do not. Thus, the type of parent material affects soil tex-
ture, soil structure, moisture-holding ability, and base-exchange capacity (a measure of
fertility).
Parent material also affects soil chemistry. Many igneous and metamorphic rocks are
acidic, whereas sedimentary rocks tend to be basic. Some rocks undergo such strong re-
actions that they form distinctive soils in almost all climates. Limestone, for example, is
famous for forming the rendzina (included in FAO leptosol group), a humus-rich carbon-
ate soil that occurs in several mountain regions (Kubiena 1970). Other rock types with
unusual chemical properties—including serpentine, gypsum, and calamite (an ore of
zinc)—limit soil development by inhibiting plant growth and metabolism (Billings 1950;
Brady et al. 2005). The effects of these rock types may be so strong that their distribu-
tion can be plotted by mapping the vegetation.
Parent material affects soil properties primarily in the early stages of soil develop-
ment. Mountains are relatively young geologically; glacial advances and other erosion-
al processes have exposed and created extensive new surfaces. For this reason, parent
material plays a more important role in the distribution of soil types in mountains than
in the surrounding lowlands.
Time
Time, like topography, is often considered a passive factor in soil formation because it
makes no direct contribution to soil characteristics; nevertheless, soil profile develop-
ment requires time. The older a soil, the more advanced soil formation will be. If a lava
flow buries a mountain slope, soil formation begins anew and the former soil ( paleosol )