Geology and Soils in Deserts of the Southwestern United States - Design with the Desert: Conservation and Sustainable Development - page 49

2.8.5 Time
Soil formation in deserts is a slow process often requiring thousands of years. All of the
above factors are integrated over time. The soil-forming factors working together produce
the tremendous variety of soils we see on our landscapes. Ancient soil horizons, known
to geologists as paleosols , can sometimes be preserved in stratigraphic sequences. Paleosols
are typically recognized by the presence of root traces and burrows and can be very useful
in recognizing environmental change in the geological record. 17
Soils play a vital role in the natural and built environments. Some of the functions of
soils include (a) water retention, which provides water on site for plants and other organ-
isms and reduces downstream flood peaks; (b) nutrient processing, in which all nutrients
are processed through the soil system by the macrofauna and microfauna; and (c) habitat
that soil provides to many species of burrowing organisms and also forms a major portion
of plant habitat. 18 From the human perspective, soils are important as (a) the agricultural
medium, because almost all crops are grown in soils and (b) the foundation material that
soils provide for houses, buildings, and roads. 19
2.8.6 Soil Composition
A soil can be thought of as being composed of the components of mineral matter,
organic matter, air, and water. A typical loam soil consists of about 45% mineral matter
and 5% organic matter. The remaining pore space is occupied by about 25% water and
25% atmosphere. Soil solids consist of gravels, sands, silts, and clays, which typically are
referred to as the mineral matter . The percentage of sand, silt, and clay is the soil texture . The
clay fraction provides a soil with its ability to retain nutrients for later use by plants or
other organisms; sands and silts do not have a significant role in nutrient retention. Clays
also influence soil structure and strength, which control how difficult it is to deform the
soil. Soils high in clays are difficult to dig or work and form hard clods when dry. Organic
matter rarely exceeds 8% in mineral soils and consists of root, leaves, dead microorganisms,
and other detritus. Organic matter represents a very important nutrient source in soil
systems. In addition, organic matter contributes to soil structure and makes the soil easier
to work. Decomposition of organic matter yields nutrients and humus. Humus, like clay, is
important for the retention of nutrients in soils.
Pore space in soils is either filled with water or air. Pores play two critical roles in soils:
first, pores form the major pathway by which water and air enter and exchange with the
soil system; second, pores are where soil water is retained for future use by organisms,
providing room for root elongation and pathways for nonburrowing soil organisms. 20
Pore space is easily reduced by management activities resulting in a compacted soil.
Compaction of a soil is indicated by bulk density. Bulk density of soils ranges from about
1.0 to 1.5 Mg m −3 in natural soils to 1.65-2.0 Mg m −3 or higher if the soil is compacted. For
comparison, most rocks have bulk densities on the order of 2.65 Mg m −3 . Organic matter,
texture, and degree of compaction influence bulk density.
2.8.7 Soil Chemical Properties
Of the many chemical properties that a soil may possess, three of the most common
and important are pH, cation exchange capacity, and salt content. The measure of the
acidity of a soil is known as its pH . Although pH by definition ranges from 1.0 to 14.0,
soil pH typically ranges from 4.0 to 8.0 in most environments and 7.0 to 8.4 in desert