Solonchaks (Sc)
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SOLONCHAKS (SC) The Reference Soil Group of the Solonchaks includes soils that have a high concentration of ‘soluble salts’ at some time in the year. Solonchaks are largely confined to the arid and semi-arid climatic zones and to coastal regions in all climates. Common international names are 'saline soils' and 'salt-affected soils'. Definition of Solonchaks Soils, 1 having a salic horizon starting within 50 cm from the soil surface; and 2 having no diagnostic horizons other than a histic, mollic, ochric, takyric, yermic, calcic, cambic, duric, gypsic or vertic horizon. Common soil units: Histic, Gelic, Vertic, Gleyic, Mollic, Gypsic, Duric, Calcic, Petrosalic, Hypersalic, Stagnic, Takyric, Yermic, Aridic, Hyperochric, Aceric, Chloridic, Sulphatic, Carbonatic, Sodic, Haplic. Summary description of Solonchaks Connotation: saline soils; from R. sol, salt, and R. chak, salty area. Parent material: virtually any unconsolidated soil material. Environment: arid and semi-arid regions, notably in seasonally or permanently waterlogged areas with a vegetation of grasses and/or halophytic herbs, and in inadequately managed irrigation areas. Solon- chaks in coastal areas occur in all climates. Profile development: mostly AC- or ABC-profiles, often with gleyic properties at some depth. In low- lying areas with a shallow water table, salt accumulation is strongest at the surface of the soil (‘external Solonchaks’). Solonchaks with a deep water table have the greatest accumulation of salts at some depth below the surface (‘internal Solonchaks’). Use: Solonchaks have limited potential for cultivation of salt tolerant crops. Many are used for exten- sive grazing or are not used for agriculture at all. Regional distribution of Solonchaks The total extent of Solonchaks in the world is estimated to be between 260 million (Dudal, 1990) and 340 million hectares (Szabolcs, 1989), depending on the level of salinity that is adopted as diagnostic. Solonchaks are most extensive in the northern hemisphere, notably in the arid and semi-arid parts of northern Africa, the Middle East, the former USSR and central Asia; they are also widespread in Aus- tralia and the Americas. Figure 1 shows the major occurrences of Solonchaks in the world. Dominant Associated Inclusions Miscellaneous lands Figure 1. Solonchaks worldwide Associations with other Reference Soil Groups Solonchaks have in common that they have a ‘high’ salt content in some part or all of the control sec- tion. Note that also other Reference Soil Groups than Solochaks may have a salic horizon. Such soil groups have other properties that are considered more characteristic than the salic horizon and key out before the Solonchaks, e.g. Histosols, Vertisols and Fluvisols; their salic soil units are intergrades to Solonchaks. Genesis of Solonchaks Most Solonchaks occur in inland areas where evapotranspiration is considerably greater than precipi- tation, at least during part of the year. Salts dissolved in the soil moisture remain behind after evapo- ration/transpiration of the water and accumulate at the surface of the soil ('external Solonchaks') or at some depth ('internal Solonchaks'). The Reference Soil Group of the Solonchaks is heterogeneous by nature. Solonchaks may differ in: * the content and depth of salts in the soil, * the composition of accumulated salts, * the mineralogy of salt efflorescences. Content and depth of accumulated salt(s) Figure 2 shows that the solubility of most salts is temperature-dependent. The solubility product is greater in the warm dry season when there is a net upward water flux from the groundwater table to the surface soil, than in the cooler wet season when salts are leached from the surface soil by surplus rainfall. This hysteresis between (rapid) influx of salts in the soil and (slow) discharge is conducive to net accumulation of salts (and development of a salic soil horizon) in seasonally dry regions. External Solonchaks form in depression areas with strong capillary rise of saline groundwater and in poorly managed irrigation areas where salts imported with irrigation water are not properly discharged to a drainage system. Internal Solonchaks develop where the water table is deeper and capillary rise cannot fully replenish evaporation losses in the dry season. Internal Solonchaks may also form through leach- ing of salts from the surface to deeper layers, e.g by surplus irrigation or by natural flushing of the soil during wet spells. The 'critical depth' of the groundwater, i.e. the depth below which there is little dan- ger that harmful (quantities of) salts will accumulate in the rooted surface soil, depends on soil physical characteristics but also on the evaporative demand of the atmosphere. The USDA Soil Survey Staff considers a depth of 6 feet critical, "especially if the surface is barren and capillary rise is moderate to high". Figure 2. Solubility of common salts in 8 Solonchaks, expressed in mole anhydrous 1 salt per kg H O, as a function of the soil 7 2 temperature (After Braitsch, 1962) 2 1. MgCl .6H O 6 2 2 6 2. NaCl 5 3. KCl 5 4. CaCl2.6H2O 9 3 8 5. CaCl2.4H2O 6. CaCl .2H O 4 11 2 2 7. MgSO4.7H2O mol/kg 4 8. MgSO4.6H2O 13 3 9. MgSO4.H2O 7 12 10. Na2CO3.10H2O 11. Na CO .H O 2 14 2 3 2 15 12. Na2SO4.10H2O 10 16 13. Na2SO4 1 14. NaHCO3 18 17 15. CaSO4 16. CaSO .2H O 0 4 2 17. K2SO4.2H2O 0 20 40 60 80 100 18. K2SO4 Temp. (°C) Composition of accumulated salts Salts in areas with strongly saline soils are more often than not imported with river water from far-away catchment areas or with seepage water or surface run off from nearby uplands. Accumulated salts can often be traced to deeper geological strata of marine origin (chlorides) or to volcanic deposits (sul- phates). Figure 3 presents a diagram of a common situation with Solonchaks in bottomland that re- ceives water (and salts) from adjacent uplands. Much soil salinity is man-induced through irrigation in combination with inadequate drainage. Precipitation UPLANDS STRUCTURAL TERRACES run off BAJADAS ALLUVIAL FAN A MARL PLAIN run off evaporation B springs saltcrust deep NEOGENE LACUSTRINE infiltration losses LIMESTONE MARL deep seepage A = watertable in spring May) B = watertable in summer (September) Figure 3. Schematic representation of import and redistribution of salts in the Great Konya Basin, Turkey. Watertable A in spring (May); B in autumn (September) (After Driessen & v.d. Linden, 1970). French soil scientists differentiate saline soils by the dominant cations in the soil, in particular the ratio of bivalent and monovalent cations (Duchaufour, 1988; Loyer et al., 1989). For practical reasons they dis- tinguish between: • calcium dominated saline soils, characterized by a dominance of calcium and magnesium over sodium and potassium. The ratio of (Ca2++Mg2+)/(Na++K+) is between 1 and 4 and the Ca2+/Mg2+-ratio is 1 or greater. It is widely believed that the structure of calcium dominated soils remains stable even when the salts are flushed out of the soil. • sodium dominated saline soils, in which the ratio of (Ca2++Mg2+)/(Na++K+) in the soil solution is less than 1. The structure of these soils tends to degrade when the salts are flushed out of the soil. • magnesium dominated saline soils, in which the ratio of (Ca2++Mg2+)/(Na++K+) in the soil solution is greater than 1, the Ca2+/Mg2+-ratio equals 1 or less, and the Na+/Mg2+-ratio is less than 1. Desalinization of such soils provokes hydrolysis of adsorbed Mg++-ions, which is generally associated with degradation of the soil structure. Russian soil scientists characterize ‘salt provinces’ on the basis of anion ratios. See Table 1. Table 1. Classification of saline soils based on anion ratios (Plyusnin, 1964) Pljusnin Rosanov Sadovnikov - 2- Sulphate soils Cl /SO4 < 0.5 < 0.2 < 0.2 - 2- Chloride-sulphate s. Cl /SO4 0.5 - 1.0 0.2 - 1.0 0.2 - 1.0 - 2- Sulphate-chloride s. Cl /SO4 1.0 - 5.0 1.0 - 2.0 1.0 - 5.0 - 2- Chloride soils Cl /SO4 > 5.0 > 2.0 > 5.0 2- 2- Soda soils CO3 /SO4 < 0.05 2- 2- Sulphate-soda soils CO3 /SO4 0.05 - 0.16 2- 2- Soda-sulphate soils CO3 /SO4 > 0.16 Mineralogy of salt efflorescences The morphology of saline soils is in many cases conditioned by the mineralogy of salts in the soil. Fig- ure 4 presents the stability diagram of minerals in an NaCl-saturated NaCl-Na2SO4-MgCl2-H2O sys- tem. The diagram demonstrates that diurnal temperature fluctuations may already induce mineralogical transformations. oC 90 Vanthoffite Figure 4. Stability diagram of Na6Mg D’ Ansite minerals in an NaCl-saturated (SO ) Bischofite 4 4 Na21MgCl3(SO4)10 NaCl-Na2SO4-MgCl2-H2O . MgCl2 Loeweite system (After Braitsch, 1962). 70 6H O Na12Mg7 2 . (SO4)13 15H O Kieserite 2 . MgSO4 50 H2O Bloedite Thenardite Na2Mg(SO4)2 . Na2SO4 4H2O 30 Hexahydrite . MgSO4 6H2O 10 Epsomite Mirabilite . Na SO . 10H O MgSO4 7H2O 2 4 2 Mg 100 80 60 40 20 0 SO4 An example of a specific type of Solonchak which forms under the influence of diurnal (tempera- ture-induced) fluctuations in the morphology of salts is the 'puffed Solonchak', an externally saline soil in which the greater part of all salt consists of sodium sulphate. At night, when the temperature at the soil surface is low and air humidity high, crystalline sodium sulphate is present in the surface soil as needle-shaped Mirabilite (Na2SO4.10H2O). See Figure 4. The needle-shaped Mirabilite crystals push fine soil aggregates apart when they are formed. When the temperature rises again during the day, Mirabilite is re-converted to water-free Thenardite (Na2SO4) crystals that have the appearance of fine flour.