Environmental Soil Chemistry - Chapter 10

10 The Chemistry of Saline and Sodic Soils Introduction ceans contain about 97.3% of the Earth’s water, continents about 2.8%, and the atmosphere about 0.001% (Todd, 1970). About 77.2% of the water associated with continents occurs in ice caps and glaciers and about 22% is groundwater. The remaining 0.8% occurs as surface waters (lakes and rivers). The land surface of the Earth is 13.2 ´ 109 ha; of this area, 7 ´ 109 ha is arable and only 1.5 ´ 109 ha is cultivated (Massoud, 1981). Of the cultivated land, approximately 0.34 ´ 109 ha (23%) is saline and 0.56 ´ 109 ha (37%) is sodic, containing excessive levels of Na+. Salinity can be defined as “the concentration of dissolved mineral salts present in waters and soils on a unit volume or weight basis” (Tanji, 1990b). Figure 10.1 and Table 10.1 show the global distribution of salt-affected soils. Salt-affected soils can be classified as saline, sodic, and saline–sodic soils. Briefly, saline soils are plagued by high levels of soluble salts, sodic soils have high levels of exchangeable sodium, and saline–sodic soils have high contents of both soluble salts and exchangeable sodium. These soils will be described more completely later. 285 286 10 The Chemistry of Saline and Sodic Soils FIGURE 10.1. Global distribution of salt-affected soils. Reprinted with permission from Szabolcs, I. (1989). “Salt-Affected Soils.” CRC Press, Boca Raton, FL. TABLE 10.1. Global Distribution of Salt-Affected Soilsa Area in millions of ha Continent North America Central America South America Africa South Asia North and Central Asia Southeast Asia Australasia Europe Total Saline Sodic (alkali) Total 6.2 9.6 15.8 2.0 — 2.0 69.4 59.6 129.0 53.5 27.0 80.5 83.3 1.8 85.1 91.6 120.1 211.7 20.0 — 20.0 17.4 340.0 357.4 7.8 22.9 30.7 351.5 581.0 932.2 a From I. Szabolcs, “Review of Research on Salt-Affected Soils.” Copyright © 1979 UNESCO, Paris. “Salt-Affected Soils.” Copyright © 1989 CRC Press. Reprinted by permission of CRC Press. Salt-affected soils occur most often in arid and semiarid climates but they can also be found in areas where the climate and mobility of salts cause saline waters and soils for short periods of time (Tanji, 1990b). However, for the most part, in humid regions salt-affected soils are not a problem because rainfall is sufficient to leach excess salts out of the soil, into groundwater, and eventually into the ocean. Some salt-affected soils may occur along seacoasts or river delta regions where seawater has inundated the soil (Richards, 1954). Causes of Soil Salinity 287 Causes of Soil Salinity Soluble Salts In arid and semiarid climates, there is not enough water to leach soluble salts from the soil. Consequently, the soluble salts accumulate, resulting in salt-affected soils. The major cations and anions of concern in saline soils and waters are Na+, Ca2+, Mg2+, and K+, and the primary anions are Cl–, SO2–, HCO– , CO2– , and NO– . In hypersaline waters or brines, B, Sr, Li, SiO , Rb, F, Mo, Mn, Ba, and Al (since the pH is high Al would be in the Al(OH)– form) may also be present (Tanji, 1990b). Bicarbonate ions result from the reaction of carbon dioxide in water. The source of the carbon dioxide is either the atmosphere or respiration from plant roots or other soil organisms. Carbonate ions are normally found only at pH ³ 9.5. Boron results from weathering of boron-containing minerals such as tourmaline (Richards, 1954). When soluble salts accumulate, Na+ often becomes the dominant counterion on the soil exchanger phase, causing the soil to become dispersed. This results in a number of physical problems such as poor drainage. The predominance of Na+ on the exchanger phase may occur due to Ca2+ and Mg2+ precipitating as CaSO , CaCO , and CaMg(CO ) . Sodium then replaces exchangeable Ca2+ and Mg2+ on the exchanger phase. Evapotranspiration An additional factor in causing salt-affected soils is the high potential evapo-transpiration in these areas, which increases the concentration of salts in both soils and surface waters. It has been estimated that evaporation losses can range from 50 to 90% in arid regions, resulting in 2- to 20-fold increases in soluble salts (Cope, 1958; Yaalon, 1963). Drainage Poor drainage can also cause salinity and may be due to a high water table or to low soil permeability caused by sodicity (high sodium content) of water. Soil permeability is “the ease with which gases, liquids or plant roots penetrate or pass through a bulk mass of soil or a layer of soil” (Glossary of Soil Science Terms, 1997). As a result of the poor drainage, salt lakes can form like those in the western United States. Irrigation of nonsaline soils with saline water can also cause salinity problems. These soils may be level, well drained, and located near a stream. However, after they are irrigated with saline water drainage may become poor and the water table may rise. Irrigation Water Quality An important factor affecting soil salinity is the quality of irrigation water. If the irrigation water contains high levels of soluble salts, Na, B, and trace elements, serious effects on plants and animals can result (Ayers and Westcot, 1976). 288 10 The Chemistry of Saline and Sodic Soils Salinity problems are common in irrigated lands, with approximately one-third of the irrigated land in the United States seriously salt-affected (Rhoades, 1993). In some countries it may be as high as 50% (Postel, 1989). Areas affected include humid climate areas such as Holland, Sweden, Hungary, and Russia, and arid and semiarid regions such as the southwestern United States, Australia, India, and the Middle East. About 100,000 acres of irrigated land each year are no longer productive because of salinity (Yaron, 1981). One of the major problems in these irrigated areas is that the irrigation waters contain dissolved salts, and when the soils are irrigated the salts accu-mulate unless they are leached out. Saline irrigation water, low soil permeability, inadequate drainage, low rainfall, and poor irrigation management all cause salts to accumulate in soils, which deleteriously affects crop growth and yields. The salts must be leached out for crop production. However, it is the leach-ing out of these salts, resulting in saline drainage waters, that causes pollution of waters, a major concern in saline environments. The presence of selenium and other toxic elements (Cr, Hg) in subsurface drainage waters is also a problem in irrigated areas. Selenium (resulting from shale parent material) in drainage waters has caused massive death and deformity to fish and waterfowl in the Kesterson Reservoir of California. Sources of Soluble Salts The major sources of soluble salts in soils are weathering of primary minerals and native rocks, residual fossil salts, atmospheric deposition, saline irrigation and drainage waters, saline groundwater, seawater intrusion, additions of inorganic and organic fertilizers, sludges and sewage effluents, brines from natural salt deposits, and brines from oil and gas fields and mining (Jurinak and Suarez, 1990; Tanji, 1990b). As primary minerals in soils and exposed rocks weather the processes of hydrolysis, hydration, oxidation, and carbonation occur and soluble salts are released. The primary source of soluble salts is fossil salts derived from prior salt deposits or from entrapped solutions found in earlier marine sediments. Salts from atmospheric deposition, both as dry and wet deposition, can range from 100 to 200 kg year–1 ha–1 along seacoasts and from 10 to 20 kg year–1 ha–1 in interior areas of low rainfall. The composition of the salt varies with distance from the source. At the coast it is primarily NaCl. The salts become higher in Ca2+ and Mg2+ farther inland (Bresler et al., 1982). Important Salinity and Sodicity Parameters The parameters determined to characterize salt-affected soils depend primarily on the concentrations of salts in the soil solution and the amount of exchange-able Na+ on the soil. Exchangeable Na+ is determined by exchanging the Na+ Important Salinity and Sodicity Parameters 289 from the soil with another ion such as Ca2+ and then measuring the Na+ in solution by flame photometry or spectrometry (e.g., atomic absorption or inductively coupled plasma emission spectrometries). The concentration of salts in the solution phase can be characterized by several indices (Table 10.2) and can be measured by evaporation, or using electroconductometric or spectrometric techniques. Total Dissolved Solids (TDS) Total dissolved solids (TDS) can be measured by evaporating a known volume of water from the solid material to dryness and weighing the residue. However, this measurement is variable since in a particular sample various salts exist in varying hydration states, depending on the amount of drying. Thus, if different conditions are employed, different values for TDS will result (Bresler et al., 1982). TDS is a useful parameter for measuring the osmotic potential, –to, an index of the salt tolerance of crops. For irrigation waters in the range of 5–1000 mg liter–1 TDS, the relationship between osmotic potential and TDS is (Bresler et al., 1982) –to » –5.6 ´ 10–4 ´ TDS (mg liter–1). (10.1) Without the minus sign for osmotic potential in Eq. (10.1), one could also use the same equation to determine osmotic pressure (to) values. Further details on osmotic potential and osmotic pressure, as they affect plant growth, will be discussed later in this chapter. The TDS (in mg liter–1) can also be estimated by measuring an extremely important salinity index, electrical conductivity (EC), which is discussed below, to determine the effects of salts on plant growth. The TDS may be estimated by multiplying EC (dS m–1) by 640 (for EC between 0.1 and 5.0 dS m–1) for lesser saline soils and a factor of 800 (for EC > 5.0 dS m–1) for hypersaline samples. The 640 and 800 are factors based on large data sets relating EC to TDS. To obtain the total concentration of soluble cations (TSC) or total con-centration of soluble anions (TSA), EC (dS m–1) is usually multiplied by a factor of 0.1 for mol liter–1 and a factor of 10 for mmol liter–1 (Tanji, 1990b). TABLE 10.2. Salinity Parameters Salinity index Units of measurement Total dissolved solids (TDS) or total soluble salt concentration (TSS) Total concentration of soluble cations (TSC) Total concentration of soluble anions (TSA) Electrical conductivity (EC) mg liter–1 molc liter–1 molc liter–1 dS m–1 = mmhos cm–1 (higher saline soils); dS m–1 ´ 10–3 or mS cm–1 = mmhos cm–1 (lower saline soils) ... - tailieumienphi.vn