processes. Given sufficient amounts of time to evolve, it can be said the nature of
the soil is highly reflective of climate.
Topography can speed or slow soil evolution. Soil depth and chemistry are pro-
foundly affected by slope and there is a vegetative feedback in that various plant
communities are optimized at various elevations. The topography does not need
to be extreme for there to be significant soil differences. Modest hillslopes make
for major soil differences on scales of tens to hundreds of meters.
Biota are the plant and animal forms associated with the soil. Plants range
from microscopic to the largest trees. Their importance includes anchoring of soils
via root systems, storage and exchange of energy and nutrients, and the increase
of infiltration of rainwater. Animals enrich soils through their surface and below
surface droppings (especially earthworms) and stir nutrients through the digging
of burrows. Clearly, the nature of the biota influences the development of soils.
Even though soil formation is slow compared to human lifetimes, soils can be
seriously degraded or lost in a matter of a few years. Societies have declined with
concomitant degradation of their soil resources. The story of European and Asiatic
ports becoming silted and unusable because of erosion from agriculture is all too
common in historic times.Desertificationimplies the degradation of soils and is
of great concern in the Sahel, south of the Sahara. The Dust Bowl of the U.S. Great
Plains is a prima facie example of the interaction of overuse of the soil resource
with natural drought.312 Soils
Permafrost
Permafrost is soil that is frozen except for a brief period during the summer. In much of
Russia in the region known as Siberia, and in Alaska and northern Canada, vast stretches of
permafrost are present. It may also be encountered in alpinelocationsaround the world. In
Siberia and North America this frozen zone can extend to enormous depths. In some places
in Siberia, the permafrost may extend over a thousand feet into the soil and subsoil. Such an
environment presents huge difficulties for permanent human settlement, as well as significant
engineering and construction challenges. Buildings must be elevated on pillars in permafrost
areas, otherwise the weight of the building generates heat that melts the soil underneath and
causes the structure to become unstable, lean, or shift, and potentially collapse. Roads,
railroads, water lines, and other infrastructure is also extremely difficult and expensive to build
and maintain. Agriculture is possible for only a few months in the summer in permafrost
regions, and then the crops produced must mature quickly to be harvested before the winter
sets in and the soil freezes again. Such regions typically have quite low population densities
because of the harsh environmental conditions found there.