Mountain Soils - Mountain Geography: Physical and Human Dimensions - page 297

CHAPTER SIX
Mountain Soils
LARRY W. PRICE and CAROL P. HARDEN
Soil is so common that most people assume they know what it is. The scientific definition
of soil depends on who uses the term. To an engineer, soil is the unconsolidated material
at the surface of the Earth, whereas to a biologist, soil is alive with living organisms that
contribute to its physical and biological characteristics. For our purpose of examining the
broad range of mountain soils on Earth, soil will be considered as the uppermost layer of
the Earth's surface, in which organisms live, and which has physical, chemical, biologic-
al, and mineralogical properties that differ from those of the underlying parent material.
Soils form by the weathering and breakdown of rocks and other mineral materials, in-
cluding volcanic ash and landslide deposits, in combination with the movement of water
and the activity and decay of plants and animals. Soil consists of mineral particles, liv-
ing and nonliving organic matter, the spaces (pores) between solid particles, and the gas
and liquid in those spaces. Functionally, soil is the interface between the mineral earth,
the atmosphere, water, and living things. It may look like “dirt,” but it supports life on
our planet and controls the storage and runoff of freshwater. A typical soil consists of
distinct layers called soil horizons, which, taken together, form the soil profile. These lay-
ers are distinguished by differences in their color, texture, structure, organic matter, clay
content, and pH (acidity). Horizons form slowly over time, as water percolating through
the soil dissolves available chemicals and carries them downward. This process, called
leaching, leads to the removal of water-soluble elements in the upper layers of soil and
the enrichment of lower horizons where the leached elements are redeposited. Tiny sol-
id particles may also be moved downward by percolating water until redeposition takes
place. Thus, the upper layers continually lose material to the lower layers.
The uppermost soil layers (“A” horizons) contain the most organic material and are
darkest in color, while the middle layers (“B” horizons) are primarily composed of mineral
fragments. The lowest part of the soil profile (“C” horizon) consists of partially weathered
parent material. Well-defined horizons are the product of soil development under long-
persisting, undisturbed conditions; hence they are uncommon in mountain soils. At high
altitude, cold conditions restrict the pace of biological activity and the movement and
geochemical activity of water, so soils form slowly. Mountain soils are characteristically
shallow, rocky, acidic, infertile, and immature. The diversity of mountain landscapes has
the effect of creating a discontinuous and heterogeneous patchwork of microenviron-
ments characterized by continually changing and contrasting site conditions. It is within
this framework that mountain soils develop and, accordingly, reflect the nature of their
origin.