Geology and Soils in Deserts of the Southwestern United States - Design with the Desert: Conservation and Sustainable Development - page 52

Mountains
Old alluvial surface
(fan terrace)
Young alluvial surface
River floodplain
Bedrock
Alluvium
FIGURE 2.10
Schematic representation of typical soil-landscape position correspondence in the Basin and Range geological
province. (Modified from Hendricks, D.M., Arizona Soils , University of Arizona College of Agriculture, Tucson,
AZ, 1986.)
2.10 Soils of the Built Environment
Most soils literature and classification schemes treat soil as a naturally occurring body
at the surface of the earth. Soils in the built environment are very different from those
of natural systems, and these soils are often termed “urban soils.” Definitions of what
constitutes urban soils usually include the following: (a) presence of a nonagricultural
man-made layer >50 cm thick; (b) significant mixing, filling, and removal of natural soil
materials; (c) possible contamination either by chemical or other wastes, such as bricks and
cement; and (d) occurrence in urban or suburban areas. There are several features that
make urban soils unique.
Urban soils are very heterogeneous in space due to human activities, which include
filling over existing soils, removing existing soils, and imposing a multitude of manage-
ment regimes in relatively small areas. Vertical heterogeneity reflects contrasting layers
from fill and deposition of waste products in the soil system. Soil structure in urban
soils is often weakened due to reduced inputs of organic matter. Organic matter contrib-
utes to soil structure formation by influencing soil acidity, biogeochemical weathering
of minerals, and presence or absence of soil biota. Soil structure in urban soils is also
subjected to repeated compaction in places such as parks where human activity can be
very intense.
Deterioration of soil structure and compaction leads to reduced aeration and drainage
because of the reduction in porosity. Soil-moisture movement and retention can also be
impeded by the presence of compacted layers within the profile. Proper infiltration and
aeration are critical for soil fauna and plants to thrive in urban environments. Organic mat-
ter inputs from plants represent a very important component of a healthy soil ecosystem
because it is both an energy and nutrient source for soil organisms. Plants benefit from the
nutrient content of organic matter after microbial processing. Organic-matter cycles are
truncated when surfaces are paved and when organic matter is removed by homeowners
or landscapers.