Environmental Engineering Reference
In-Depth Information
environments. Soils with the pH < 7.0 are termed acid soils, and soils with the pH > 7.0
are termed basic soils. Soil pH is a very important chemical parameter of soils and is
often termed the “master variable.” It influences soil-weathering processes, the type
of plants that can survive in a site, and the availability of nutrients and the activity
of soil microflora, just to name a few factors. Under certain chemical conditions, such
as high concentrations of sodium, soil pH can be as high as 9.5. Many typical eastern
landscaping plants require arid soils with pH less than those commonly found in desert
soils. Successful establishment and growth of these plants in desert soils typically
require adjustment of the pH.
Cation exchange capacity is an indication of a soil's ability to retain nutrients. Cation
exchange capacity is influenced by clay content, humus content, and pH.* Salt content
of a soil is often measured and reported, particularly in deserts and even more com-
monly in agricultural lands subjected to long-term irrigation. Due to low precipitation
and high evaporation, salts are not removed from soils as they are in other regions and
may actually accumulate on the surface. This is particularly common in enclosed dry
lakebeds in deserts known as playas , and salt accumulation can be related to irrigation
of agricultural lands, particularly when the water used is high in sodium or other major
cations. Salts in semiarid and arid regions are typically carbonates, sulfates, and chlo-
rides of calcium, magnesium, and sodium. A number of techniques and methods are
used to measure salt content. 21
2.8.8 Internal Soil Structure
Soils are composed of distinct layers termed horizons. Five primary or “master” horizons
are recognized, in order for increasing depth; they are given as follows: O—a layer com-
posed of organic materials—typically rare in desert soils; A—the first mineral horizon
darkened by decomposition residues of organic materials; E—horizon where materials are
leached from, usually light in color and rich in sands, and this horizon generally does not
occur in desert soils; B—this horizon is a region of accumulation in the soil profile and can
be enriched with clay, carbonates, or other materials; C—mineral horizon of little altera-
tion of parent materials; and R—consolidated bedrock.
Unlike other regions of the United States, desert soils exhibit fewer horizons, and these
horizons are often not distinct. This is due to low precipitation, little biological activity,
and low-organic matter production when compared to other regions. Many desert soils
have an A/C profile or an A/B/C with the B-horizon weakly developed because of little
movement of materials such as clays in the soil profile. More characteristic of desert
soils however is the accumulation of carbonates in the soil profile. Carbonates are water
soluble and are translocated in the profile upward by evaporation and downward by
the movement of water. Evidence of carbonate accumulation in the profile can be seen
as white lenticular masses, coatings on rocks contained within the soil, or as discrete
layers of indurated (hardened) carbonate (Figure 2.9). Soil horizons that have signifi-
cant accumulation of carbonates are designated with the lower case letter k (e.g., Bk). In
semiarid to arid regions, however, enough soil carbonate can accumulate in subsurface
horizons that it dominates the morphology of the horizon. These carbonate-rich hori-
zons are given the designation K. 10 In the southwestern United States, K horizons are
* For an introduction to the measurement of soil cation exchange capacity, readers are referred to Brady and
Weil. 21
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