Soils of Arid Regions

Failure of a small dam constructed of dispersive soil, near Ramsgate, Natal, South Africa. 

A number of silty deposits formed under arid conditions are liable to undergo considerable volume reduction or collapse when wetted. Such metastability arises due to the loss of strength of interparticle bonds, resulting from increases in water content. Thus, infiltration of surface water, including that applied by irrigation, leakage from pipes and rise of the water table may cause large settlements to occur. 

A common feature of arid regions is the cementation of sediments by the precipitation of mineral matter from the groundwater. The species of salt held in solution, and also those precipitated, depends on the source of the water, as well as the prevailing temperature and humidity conditions. The process may lead to the development of various crusts or cretes in which unconsolidated deposits are cemented. The most commonly precipitated material is calcium carbonate (Netterburg, 1994). As the carbonate content increases in these soils, it first occurs as scattered concentrations of flaky habit, then as hard concretions. Once it exceeds 60%. the concentration becomes continuous. These deposits are referred to as calcrete (Fig. 5.9). The calcium carbonate in calcrete profiles decreases from top to base, as generally does the hardness. The development of calcrete is inhibited beyond a certain aridity since the low precipitation is unable to dissolve and drain calcium carbonate towards the water table. Consequently, in very arid climates, gypcrete may take the place of calcrete. 

Calcrete in the north of Namib-Nauluft Park, Namibia 234 

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Figure 1.3. Distributions of arid and semi-arid areas (Reprinted from Soils of Arid Regions, Dregne H.E., p 6, Copyright (1976), with permission from Elsevier)...

Dregne H.E. Soils of Arid Regions. Developments in Soil Science 6. Amsterdam, Oxford Elsevier, 1976. 

McFaddenL. D., Amundson R. G., and ChadwickO. A. (1991) Numerical modeling, chemical, and isotopic studies of carbonate accumulation in soils of arid regions. In Occurrence, Characteristics, and Genesis of Carboante, Gypsum, and Silica Accumulations in Soils (ed. W. D. Nettleon). Soil Science Society of America, Madison, WI, pp. 17-35. 

Thus, B(0H)3 is a Lewis add rather than a Bronsted acid (see Chapter 1). Because boron adsorbs most effectively in the pH 8 to 9 range on A1 and Fe oxides and silicate minerals, its availability is generally low in coarse-textured, acid-leached soils and in calcareous soils. Deficiency in add soils is the result of boron depletion by leaching, while deficiency in calcareous soils is caused by strong adsorption and predpitation as relatively insoluble Ca borate salts. In contrast, B toxicity is most commonly found in alkaline soils of arid regions these soils often contain high levels of Na which forms quite soluble borate salts. A lack of rainfall allows soluble borate to accumulate to phytotoxic levels. 

Except for much greater Cl accumulation in soils of arid regions, the soil chemistry of I and Br resembles that of Cl, except that I and Br are retained more strongly, especially by acid soils. The major input of I to soils appears to be atmospheric. Endemic iodine deficiency (goiter in humans) occurs in mountainous and continental areas isolated from the sea. Fortunately, supplementing NaCl with small amounts of I effectively supplies the I required in animal diets. Iodide and Br are both potentially toxic, but no natural cases have been reported. Bromide reactions in soils have been investigated as a tracer for die movement of water, nitrate, and soil solutions in soils. 

Soil pH - Soil pH is conceptuaDy the same as solution pH, the negative logarithm of the ion activity (understood to mean effective concentration). Soils of humid regions tend to become more acidic with time, whereas soils of arid regions remain more neutral or alkaline. Soil pH can sometimes be changed... 

Soluble salts of the soil Water in the soil should most properly be considered as the solvent for salts of the soil the result being the soil solution. In temperate climates and moderate rainfall areas, the soil solution is relatively dilute, with total dissolved salts ranging from 80 to 1 500 p.p.m. Regions of extensive rainfall show lower concentrations of soluble salts as the result of leaching action. Conversely, soils in arid regions are usually quite high in salts as these salts are carried to the surface layers of the soil by water movement due to surface evaporation. 

C. R. Kuske, S. M. Bams, and J. D. Busch, Diverse uncultivated bacterial groups from soils of arid southwestern United States that are present in many geographic regions. Appl. Environ. Microbiol. 65 3614 (1997). 

Total Mo in soils of China varies from 0.1-6 mg/kg with an average of 1.7 mg/kg. Arid and semi-arid soils have 0.2-4.2 mg/kg total Mo. Average Mo contents in arid and semi-arid regions are 1.58 mg/kg. In general, lower Mo contents are found in soils of arid and semi-arid regions than the humid and tropic/subtropic soils. In soils derived on loess, total Mo is from 0.21-1.45 mg/kg with an average of 0.62 mg/kg. Soils in the North... 

Han F.X., Binding, distribution and transformations of polluting trace elements in soils of arid and semi-arid regions receiving waste inputs. Ph.D. Dissertation, Hebrew University of Jerusalem. 1998. 

Reitemeier, R. F. (1946). Effects of moisture content on the dissolved and exchangeable ions ofsoils of arid regions. Soil Sri. 61, 195—214. 

The SAR magnitude reflects the quantity of sodium on the exchange sites of the soil. Most arid-region soils with SAR values of 15 have approximately 15% of their CEC loaded with sodium. This sodium load is known as the exchangeable sodium percentage or ESP. Soils with an ESP greater than 15 would be considered unproduc-... 

As discussed in Chapter 5, natural soil-forming processes of humid climates tend to acidify soils over the long term. However, in arid climates, because salts are not frequently leached from the soil by rainfall, salts and alkahnity tend to accumulate. This is because the potential evaporation and transpiration of water at the soil surface exceeds rainfall so that the net movement of water is upward in the soil profile. Consequently, salts accumulate at or near the surface of these arid-region soils. However, in localized cases, accumulation of salts and alkalinity arises from confinement, created by a physical barrier to water flow out of a depression in the landscape. Thus, salt problems are not restricted to soils of arid climates. 

Calcite (CaCOj), dolomite (CaMg(COf)2)> and apatite (CafiF,OH)[POfifi. Carbonates of clay size are rather rapidly leached from humid soils. In arid regions, calcite accumulates. The phosphate remains in the clay phase after apatite decomposition, although in the form of Ca phosphates in alkaline soils or A1 and Fe phosphates in acid soils. 

Soils are said to be sodic if they contain an excess of sodium. According to their content of salts and sodium, soils are grouped in saline-nonsodic, saline-sodic, nonsaline-sodic, and normal soils. In arid regions, soils rich in sodium are widely distributed due to salt accumulation as sodium chloride and sodium sulfate. The salt-affected soils are chemically characterized by the specific conductivity (mS-cm ) [S = Siemens] of the saturation extract and the saturation of the CEC (%) with sodium. The major cationic constituents of the soluble salts in saline soils are sodium, calcium, and magnesium. Precipitation of Ca and Mg as carbonates effects an increase in the proportion... 

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