Solonchaks (Sc)

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 extensive 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 section. 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 precipitation, at least during part of the year. Salts dissolved in the soil moisture remain behind after evaporation/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 leaching 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 system. 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 (temperature-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. Repeated Mirabilite-Thenardite transformation produc- es the soft and fluffy surface soil that is typical of a puffed Solonchak.

Another example of diurnally changing external Solonchaks concerns soils with a dominance of hy- groscopic salts such as CaCl2 or MgCl2, and to a lesser extent also NaCl. The resulting 'Sabakh' soils ('sabakh' is arabic for morning) are dark coloured and slippery in the morning as a result of moisture absorption during the night. The soils lose their dark colour again in the course of the day when the temperature rises and air humidity drops to a low value.

An example of an annual cycle in which the morphology of salt minerals plays a role is the formation of 'slick spots', isolated patches of very saline and soft mud in a field. Slick spots appear early in the dry season in shallow depressions (often hardly recognizable with the naked eye). The depressions are covered with a salt crust, e.g. a glass-like halite (NaCl) crust, that is so effective in sealing the underlying saline mud from the air that the soil remains wet throughout the dry season. Pores or cracks that can provide passage to rain or leaching water will not form. The crust may dissolve in a subsequent wet season but the unripe, impermeable mud remains saline and restores its protective crust as soon as the wet season is over. The untrafficable and very saline slick spots cannot be reclaimed with conven- tional (leaching) techniques. Characteristics of Solonchaks

Morphological characteristics The horizon differentiation of Solonchaks is normally determined by other factors than their high salt content. Many saline soils in waterlogged backswamps are Gleyic Solonchaks; without their salic horizon they would have been Gleysols. Likewise, Mollic Solonchaks have the appearance of a Cher- nozem, Kastanozem or Phaeozem, and Calcic and Gypsic Solonchaks are basically strongly saline Calcisols and Gypsisols. Saline Histosols, Vertisols and Fluvisols occur as well; they are not classified as Solonchaks because Histosols, Vertisols and Fluvisols key out before Solonchaks.

Solonchaks have a stable soil structure that is brought about by the high salt content of the soil but a typical structural expression of Solonchaks does not exist. Especially in heavy clays, very saline surface layers may exist without any clear efflorescence of salts. Examination with a lens reveals tiny crystals on the faces of crumb or granular structure elements. In extreme cases, very saline pseudo- sand may form that accumulates to clay dunes when exposed to strong winds. The other extreme occurs also: clayey ‘external Solonchaks’ that lose their surface structure when exposed to an occasional rain shower. The peptised surface layer will subsequently dry out to a hard crust. When the crust is still soft, it may be pushed upwards by gases escaping from the underlying mud; prints of gas bubbles remain visible when the crust is detached from the underlying wet soil. Recall that the surface layer of Sabakh soils is a muddy mixture of salt and soil particles during early morning hours. The fluffy top layer of puffed Solonchaks is a morphological feature that is exclusive to Solonchaks with a high content of sodium sulphate. The most common type of salt crust, however, is a loose cover of salt crystals.

The morphology of internal Solonchaks differs little from that of comparable non-saline soils. Solon- chaks have, perhaps, a somewhat stronger subsoil structure with, in very saline soils, tiny salt crystals on the faces of structure elements. With a salic horizon as the only common characteristic, there is considerable diversity among Solon- chaks and a detailed account of their hydrological, physical, chemical and biological properties is not well possible. A few general trends deserve attention in the present context.

Hydrological characteristics Internal Solonchaks are largely confined to areas that lie well above the drainage base. When leached, they may actually furnish (part of) the salts that accumulate in contiguous bottom land with external Solonchaks. Extremely saline soils with thick surface crusts occur in depressions that collect water from surrounding (higher) land in the winter but dry out in the warm season. Such soils are sometimes referred to as 'flooded' Solonchaks.

Altitude in m 10 1030 6

1020 colluvial 7 slope site numbers (volcanic) 9 Zanopa 8 Fan 1010 low centre of Basin

1000 Figure 5. Schematic cross-section of the Great Konya Basin, Turkey. Salinity patterns are influenced by topography and hy- drology. All concentrations are expressed in mmol/liter ‘saturation extract’ or groundwater. Click on a site number to see the saltprofile. Figure 5 presents a schematic cross-section through an inland basin with severe soil salinity: • here the water table is at shallow depth, strongly externally saline Gleyic Solonchaks occur (site 8). • Slightly above the base level (site 9), the soils are still strongly saline but the highest concentration of salts is at some depth in the soil. This results from a combination of upward salt transport from the groundwater through capillary rise and downward leaching of salt from the surface soil to the zone with the highest salt concentration. • In still higher areas with a deep water table (site 10), non-saline Calcisols and internally saline Calcic Solonchaks occur. • Sites 6 and 7 are located on an alluvial fan that drains freely to the low centre of the basin. Soils on the upper part of the fan are invariably non-saline; there is beginning salinization in the lower tract where the groundwater table is at shallow depth. (Note the different scales of the x-axis.)

Physical characteristics Solonchaks that dry out during part of the year tend to have strong structure elements. When the salt content is lowered by winter rains or irrigation water, soil structure tends to degrade, particularly if the salts contain sodium and/or magnesium compounds. Strong peptisation of clays at the onset of (winter) rains may make the surface soil virtually impermeable to water.

Chemical characteristics The salt content of Solonchaks is normally judged by the 'Electric Conductivity of a saturation extract' (ECe). The ECe value is obtained by puddling an aliquot of water-saturated soil and subsequently measuring the electrical resistance between two electrodes submerged in (some of) the saturation extract. The reciprocal value of the resistance measured is the ECe, expressed in mho/cm (in older liter- ature) or dS/m (S stands for 'Siemens'). As a rule of thumb ( sic!), a soil extract or water sample contains some 0.6 grams of dissolved salts per liter for every dS/m measured. o A salic soil horizon has an ECe value in excess of 15 dS/m at 25 C at some time of the year, or more than 8 dS/m if the soil-pH (H2O,1:1) is greater than 8.5 (alkaline carbonate soils) or less than 3.5 (acid sulphate soils). Extracts of saturated soil pastes are used in base laboratory work; for quick orientation, the electric conductivity is often determined on 1:1 or 1:5 soil extracts (EC1 or EC5). Values obtained with different methods cannot always be compared, inter alia because a 'suspension effect' (different at different dilution ratios) influences the outcome of the conductivity measurement.

Biological characteristics Faunal activity is depressed in most Solonchaks and ceases entirely in soils with 3 percent salt or more. In severely salt-affected lands, the vegetation is sparse and limited to halophytic shrubs, herbs and grasses that tolerate severe physiological drought (and can cope with periods of excessive wetness in areas with seasonally flooded Solonchaks). Management and use of Solonchaks

Excessive accumulation of salts in soil affects plant growth in two ways: 1 The salts aggravate drought stress because dissolved electrolytes create an ‘osmotic potential’ that affects water uptake by plants. Before any water can be taken up from the soil, plants must compensate the combined forces of the soil’s ‘matrix potential’, i.e. the force with which the soil matrix retains water, and the osmotic potential. As a rule of thumb (sic!) the osmotic potential of a soil solution (in hPa) amounts to some 650 * EC (in dS/m). The total potential that can be com- pensated by plants (known as the ‘ critical leaf water head’) varies strongly between plant species. Plant species that stem from the humid tropics have a comparatively low ‘critical leaf water head’. Green peppers, for instance, can compensate a total soil moisture potential (matrix plus osmotic forces) of only some 3,500 hPa whereas cotton, a crop that evolved in arid and semi-arid climates, survives some 25,000 hPa!. Table 2 presents a widely used key for grading of salt-affected soils with attention for the harmful effects of soil salinity on crop performance.

Table 2. Indicative soil salinity classes and implications for crop performance. o ECe at 25 C Salt Concentration Effect on crops (dS/m) (cmol/l) (percent) < 2.0 < 2 mostly negligible 2.0 - 4.0 2 - 4 < 0.15 some damage to sensitive crops 4.0 - 8.0 4 - 8 0.15 - 0.35 serious damage to most crops 8.0 - 15.0 8 - 15 0.35 - 0.65 only tolerant crops succeed > 15.0 > 15 > 0.65 few crops survive Note that Table 2 gives merely an indication: the damage done to a particular crop depends as much on the moisture content of the rooted soil as on the salt content of the saturated soil (extract). Farmers on Solonchaks know this and adapt their cultivation methods accordingly. An example: plants on furrow-irrigated fields are not planted on the top of the ridges but at half height. This ensures that the roots benefit from the irrigation water while salt accumulation is strongest near the top of the ridge, away from the root systems.

2 Dissolved salts upset the balance of ions in the soil solution; nutrients are proportionally less available. Antagonistic effects are known to exist, for example, between sodium and potassium, between sodium and calcium and between magnesium and potassium. In higher concentrations, the salts may be directly toxic to plants. Very harmful in this respect are sodium ions and chloride ions (disturb N-metabolism).

Strongly salt-affected soils have little agricultural value: they are used for extensive grazing of sheep, goats, camels and cattle or lie idle. Only after the salts have been flushed from the soil (which then ceases to be a Solonchak) may good yields be hoped for. Application of irrigation water must not only satisfy the needs of the crop but excess water must be applied above the irrigation requirement to main- tain a downward water flow in the soil and flush excess salts from the root zone. Irrigation of crops in arid and semi-arid regions must be accompanied by drainage whereby drainage facilities should be de- signed to keep the groundwater table below the critical depth.