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Set #7
Mineral Soils conditioned by a (semi-)Arid Climate

Major landforms in (semi-)arid regions
Solonchaks
Solonetz
Gypsisols
Durisols
Calcisols

Major landforms in (semi-)arid regions

Figure 1
Desert areas of the world

Arid and semi-arid regions are distinguished on the basis of their annual precipitation sums and include:

  1. Deserts with an annual precipitation sum <50 mm/year and devoid of vegetation,
  2. Arid regions with 50-250 mm/year precipitation and sparse vegetation, and
  3. Semi-arid regions with a precipitation sum of 250 to 500 mm per annum and a steppe savannah/prairie/pampa vegetation.

Most deserts and (semi-)arid regions occur between 10░ and 35░ latitude (e.g. Sahara desert, Kalahari desert), in the interior parts of continents (e.g. Australia, Gobi desert) and in rain shadow areas in fold belts (e.g. Peru, Nepal). Large parts of the arctic tundra receive less then 250 mm precipitation per annum and qualify as `arid regions' too. Figure 1 presents a sketch map of the desert areas of the world.

Important geomorphic processes in the dry regions of the world differ from those in more humid environments:

  1. streams are intermittent or ephemeral (and have very irregular discharges),
  2. mass-wasting processes and unconfined sheet floods are prominent,
  3. many rivers do not debauch into the sea but end in inland depressions without outlet,
  4. salt lakes are a common landscape feature,
  5. aeolian processes play an important role, particularly in areas below the 150 mm/year isohyet, and
  6. physical weathering processes are prominent whereas hydrolysis of minerals is subdued.

Polar and subtropical fronts have shifted southwards in the (geologically) recent past and many regions that are arid today once had a more humid climate. Conversely, many of the present humid regions were much drier in glacial periods, especially between 20,000 and 13,000 BP when aeolian processes influenced land formation more than at present.

Mass wasting, fluvial processes and aeolian processes are the most important landform-shaping factors in arid and semi-arid regions. This chapter will solely discuss mass wasting and fluvial and lacustrine landforms in arid environments; sandy aeolian deposits were treated in an earlier chapter and loess deposits will be dealt with later when the major landforms of steppes and prairie regions will be discussed.

Mass-wasting processes

Mass-wasting processes are associated with strongly accidented terrain, e.g. where tectonic uplift has created mountains and in areas with steep fault scarps or incised valleys. Mass wasting often produces erosion landforms, such as residual hills or mountains that remain as isolated features in a low-relief plain. The residual elements consist normally of weathering-resistant rocks (e.g. Uluru sandstone, Australia) or are capped with a layer of resistant rock protecting the underlying softer rock from erosion (e.g. Utah, Great Monument National Park). Such a residual hill or table mountain is called an `inselberg' or `mesa'. Mountain foot slopes with a low slope angle and consisting of bedrock covered with a thin blanket of debris are termed `pediments'. Contrary to what it is often thought, pediments are erosional landforms because material is moved down the slope. Ultimately, severe erosion may create multiple, deeply incised valleys, in particular in areas with soft sedimentary rocks such as shale or marls, and create a `badland'.

The only depositional landform associated with mass wasting is the `talus cone' or `rock debris cone'. In barren deserts or mountains, temperature differences between day and night can be considerable and this frequently results in thermal disintegration of rocks. Salt crystals in the fissures may accelerate the process. Detached fragments of rocks and stones accumulate in debris cones at the foot of an inselberg or mountain.

Fluvial landforms in arid and semi-arid regions

Fluvial processes in arid regions produce typical landforms. These are different in high-relief and low-relief areas.

High-relief areas

Where a mountain front borders on a level plain, for instance at a major fault scarp or rift valley boundary, `alluvial fans' are likely to form. These form upon deposition of weathering products at the slope break. Debris flows and sheet floods during occasional heavy downpours are discharged from the hinterland via feeder canyons. Erosion products accumulate at the exit point (at the border between hinterland and plain) in a typical half-circular cone, the alluvial fan. The cone has a steep gradient and a pattern of unconfined channels that shift over the depositional body. As the water velocity of the protruding river becomes less, its sediment load can no longer be carried and much of it is deposited right at the entrance to the fan. This rapidly blocks the channel, which then sweeps left and right to evade the obstacle. The result is a low-angle sediment cone. The sediments tend to be coarser at the `proximal' end of the fan (close to the fan head or apex) than at the `distal' part (the fan toe, far into the plain.

Low-relief areas

Episodic heavy downpours in low-relief areas are often followed by overland flash floods and debris flows that follow existing depressions in the landscape. Such arid-region fluvial valleys are called `wadis'. Many wadis that are now found in desert regions formed during a more humid climatic episode between 13,000 and 8,000 years BP, at the transition from the Last Glacial to the Early Holocene. Wadis in desert regions carry water only after torrential rainstorms that normally occur once in a few years. At the onset of the rains, water can still infiltrate into the soil. As the downpours continue, the supply of water soon exceeds the infiltration capacity of the soil and excess water is discharged as surface run off: a `flash flood' is set in motion. Slaking and caking of the soil surface enhance surface run-off towards the wadis that become torrential braided streams with high sediment loads. These braided streams have only one channel, but multiple bars. After the downpour, the river will completely dry up again until the next event. Many wadis connect with dry, salty basins where individual floodplains merge into extensive `playas'. These are salty lakes with properties that will be outlined in the next paragraph.

Many inland depression areas in deserts are former lake areas in which open water was present during the early Holocene. Former river delta sequences and coastline features (e.g. coastal terraces) may still be visible.

Lacustrine landforms in arid and semi-arid regions

If a low-lying basin has no outlet, incoming water from (flash) floods evaporates inside the basin where its dissolved salts accumulate in the lowest parts. First, CaCO3 and MgCO3 precipitate as calcite, aragonite or dolomite. As the brine becomes further concentrated, gypsum (CaSO4.2H2O) segregates, and still later, when the lake is almost dry, halite (NaCl) and other highly soluble salts. Such salt lakes indicate that annual evapotranspiration is greater than the sum of incoming floodwater and precipitation. When such a `playa' dries out, the muddy lake floor shrinks and cracks. Accumulated salts crystallise and form crusts on top of the playa floor and in cracks in the surface soil. Much of the accumulated salts stem from (evaporitic marine sediments) outside the basin; many Mesozoic (Triassic, Jurassic) and Tertiary sediments are very rich in evaporites.

It depends on local hydrographic conditions whether a playa is wet around the year or dries out. A playa may stay (almost) permanently wet if it is part of a closed basin that is under the influence of groundwater. Some playas such as the Dead Sea are fed by perennial rivers and will not dry out either but their water is so salty that salts precipitate. Laminated evaporites of considerable thickness can form in this way, with lamination reflecting the periodicity of the seasons.

The largest evaporite basin formed in recent geological history is the Mediterranean basin. A closed, or almost closed, basin formed when mountain building blocked the Strait of Gibraltar some 6 million years ago. Before the Strait opened again, half a million years later, a layer of 1 kilometre of evaporites had accumulated on the basin floor.

Many lakes in present-day arid regions were freshwater lakes in the wet period between 12,000 and 8,000 BP. Terraces and/or shorelines from that period extend well above the present lake or lacustrine plain. The same lakes were completely dry in the arid Late Pleniglacial period (20,000-13,000 BP). Even a comparatively minor climate change can upset sedimentation regimes in arid lands.

Arid and semi-arid regions harbour a wide variety of soils that occur also in more humid environments (e.g. Leptosols, Regosols, Arenosols, Fluvisols). Typical dry-zone soils are soils whose formation was conditioned by aridity; accumulation and/or redistribution of anorganic compounds mark such soils.

High levels of soluble salts characterize Solonchaks; these soils are particularly common in closed depressions such as playas and inland basins. Solonetz are not marked by a high salt content, but by a high proportion of sodium ions in the soil solution and adsorbed at the cation exchange sites on clay and silt particles. Solonetz occur predominantly in temperate and subtropical, semi-arid region1s. Gypsisols shows signs of substantial accumulation of gypsum in the upper metre of soil; Calcisols are marked by accumulation of calcium carbonate. Gypsisols and Calcisols are found in a wide range of landforms, including pediments, lake bottoms, terraces and alluvial fans. Durisols with a `duric' or `petroduric' horizon that is hardened by silica (SiO2) cementation, are not exclusive to arid regions but most Durisols occur in drylands, e.g. alongside Calcisols or Gypsisols.

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. lacking 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*.

# See Annex 1 for key to all Reference Soil Groups

@ Diagnostic horizon, property or material; see Annex 2 for full definition.

* Qualifier for naming soil units; see Annex 3 for full definition.

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 grasses and/or halophytic herbs, and in poorly managed irrigation areas. Solonchaks 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 low volume 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 taken as diagnostic. Solonchaks are most extensive in the Northern Hemisphere, notably in arid and semi-arid parts of northern Africa, the Middle East, the former USSR and central Asia; they are also widespread in Australia and the Americas. Figure 1 shows the major occurrences of Solonchaks in the world.

Figure 1
Solonchaks world-wide

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

The most extensive occurrences of Solonchaks are in inland areas where evapotranspiration is considerably greater than precipitation, at least during a greater 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

Content and depth of accumulated salt(s)

Figure 2 reveals 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 through 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 danger that harmful (quantities of) salts will accumulate in the rooted surface soil, depends on soil physical characteristics but also on the climate. 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 Solonchaks, expressed in mole anhydrous salt per kg H2O, as a function of the soil temperature (Braitsch, 1962)

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 (sulphates). Figure 3 presents a diagram of a common situation with Solonchaks in bottomland that receives water (and salts) from adjacent uplands. Much soil salinity is man-induced through irrigation in combination with inadequate drainage.

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 distinguish between:

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

Sulphate soils

Chloride-sulphate s.

Sulphate-chloride s.

Chloride soils


Soda soils

Sulphate-soda soils

Soda-sulphate soils

Cl-/SO4- -           <0.5                <0.2                 <0.2

Cl-/SO4- -            0.5-1.0             0.2-1.0              0.2-1.0

Cl-/SO4- -            1.0-5.0             1.0-2.0              1.0-5.0

Cl-/SO4- -            >5.0                >2.0                 >5.0

CO3- -/SO4- -                                                      <0.05

CO3- -/SO4- -                                                      0.05-0.16

CO3- -/SO4- -                                                      >0.16

Mineralogy of salt efflorescences

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). Source: Driessen & v.d. Linden, 1970

The morphology of saline soils is to some extent conditioned by the mineralogy of salts in the soil. Figure 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.

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 is high, crystalline sodium sulphate is present in the surface soil as -shaped mirabilite (Na2SO4.10H2O). See Figure 4.

Figure 4
Stability diagram of minerals in an NaCl-saturated NaCl-Na2SO4-MgCl2-H2O system.

Source: Braitsch, 1962

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 transformations produce the soft and fluffy surface soil that characterizes a puffed Solonchak.

Another example of diurnally changing external Solonchaks concerns soils with a dominance of hygroscopic 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 recognisable 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 conventional (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 may have the appearance of a Chernozem, 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 accounted for 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' may 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. Solonchaks 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 Solonchaks and a detailed account of their hydrological, physical, chemical and biological properties is not well possible. A few general trends:

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 bottomland 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 also referred to as `flooded' Solonchaks.

Figure 5 presents a schematic cross-section through an inland basin with severe soil salinity:

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 may 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 considering the ECe-value, i.e. the `Electric Conductivity of a saturation extract'. 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 literature) 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.

A salic soil horizon has an ECe value in excess of 15 dS/m at 25 oC 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 (EC12 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).

Land use and management 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 compensated 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

ECe at 25 oC
(dS/m)

Salt Concentration
(cmol/l)          (percent)

Effect on crops

<2.0

2.0-4.0

4.0-8.0

8.0-15.0

>15

<2

2-4                   <0.15

4-8                  0.15-0.35

8-15                0.35-0.65

>15                  >0.65

mostly negligible

some damage to sensitive crops

serious damage to most crops

only tolerant crops succeed

few crops survive

Figure 5
Schematic cross-section of the Great Konya Basin, Turkey. Note that topography and hydrology influence salinity patterns. All concentrations are expressed in mM/liter `saturation extract' or groundwater

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. An example: plants on furrow-irrigated fields are not planted on the crest 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.

  1. 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 maintain 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 designed to keep the groundwater table below the critical depth.

Solonetz (SN)

The Reference Soil Group of the Solonetz accommodates soils with a dense, strongly structured, clay illuviation horizon that has a high proportion of adsorbed sodium and/or magnesium ions. The name `Solonetz' (from R. sol, salt, and etz, strongly expressed) has become somewhat confusing now that most saline soils, with or without a high proportion of adsorbed sodium ions, key out as Solonchaks in the WRB key. Solonetz that contain free soda (Na2CO3) are strongly alkaline (field pH > 8.5). Internationally, Solonetz are referred to as `alkali soils' and `sodic soils', `Sols sodiques Ó horizon B et Solonetz solodisÚs' (France), Natrustalfs, Natrustolls, Natrixeralfs, Natrargids or Nadurargids (USA) and as Solonetz (USSR, Canada, FAO).

Definition of Solonetz#

Soils having a natric@ horizon within 100 cm from the soil surface.

Common soil units:

Vertic*, Salic*, Gleyic*, Mollic*, Alcalic*, Gypsic*, Duric*, Calcic*, Stagnic*, Humic*, Albic*, Takyric*, Yermic*, Aridic*, Magnesic*, Haplic*.

# See Annex 1 for key to all Reference Soil Groups.

@ Diagnostic horizon, property or material; see Annex 2 for full definition.

* Qualifier for naming soil units; see Annex 3 for full definition.

Summary description of Solonetz

Connotation: Soils with a high content of exchangeable sodium and/or magnesium ions; from R. sol, salt, and etz, strongly expressed.

Parent material: unconsolidated materials, mostly fine-textured sediments.

Environment: Solonetz are normally associated with flat lands in a climate with hot, dry summers, or with (former) coastal deposits that contain a high proportion of sodium ions. Major concentrations of Solonetz are in flat or gently sloping grasslands with loess/loam or clay in semi-arid, temperate and subtropical regions.

Profile development: ABtnC and AEBtnC profiles with a black or brown surface soil over a natric horizon that starts at less than 100 cm from the soil surface. Well-developed Solonetz can have a (beginning) albic eluviation horizon directly over a natric horizon with strong round-topped columnar structure elements. A calcic or gypsic horizon may be present below the natric horizon. Many Solonetz have a field-pH above 8.5 indicative of the presence of free sodium carbonate.

Use: high levels of exchangeable sodium ions affect arable cropping, either directly (Na-toxicity) or indirectly, e.g. because of structure deterioration when soil material with a high proportion of adsorbed sodium and/or magnesium ions is wetted. Many Solonetz in temperate regions have a humus-rich surface soil and can (still) be used for arable farming or grazing; Solonetz in semi-arid regions are mostly used as range land or lie idle.

Regional distribution of Solonetz

Solonetz occur predominantly in areas with a steppe climate (dry summers and an annual precipitation sum of not more than 400 to 500 mm), in particular in flat lands with impeded vertical and lateral drainage. Smaller occurrences are found on inherently saline parent materials (e.g. marine clays or saline alluvial deposits). Worldwide, Solonetz cover some 135 million hectares. Major Solonetz areas are found in the Ukraine, Russia, Kazakhstan, Hungary, Bulgaria, Rumania, China, USA, Canada, South Africa and Australia.

In the past, Solonetz were frequently lumped into one broad soil group with Solonchaks: the "salt-affected soils". However, Solonetz need not be saline and Solonetz and Solonchaks often have quite different morphological and physico-chemical properties, and consequently also different management requirements. At present, Solonetz and Solonchaks are separated at a high taxonomic level in most national soil classification systems.

Figure 1
Solonetz world-wide

Associations with other Reference Soil Groups

Solonetz are frequently associated with:

Micro-relief, periodical water logging, and the spatial variability of soil and groundwater salinity determine lateral soil sequences in regions with Solonetz.

Genesis of Solonetz

The essential characteristic of Solonetz is their natric subsurface horizon, which shows signs of clay translocation and has an `Exchangeable Sodium Percentage' (ESP) of 15 or greater in the upper 40 cm of the horizon. The ESP, defined as `100 * exchangeable Na / CEC', reflects the chemical composition of the soil solution in equilibrium with the solid soil material under conditions as prevailed during the CEC determination. The WRB definition of a natric horizon waives the requirement of ESP > 15 in the upper 40 cm of the natric horizon. It suffices that soil at that depth contains "more exchangeable Mg plus Na than Ca plus exchange acidity (at pH 8.2)" if ESP > 15 in some sub-horizon within 200 cm of the surface.

The sodium that is responsible for the high ESP-value may originate from areas with a marine history. Many Solonetz in inland areas contain sodium sulphates (Na2SO4.xH2O) or Na2CO3.xH2O (`soda') as the dominant sodium compound. It is widely thought that soda can form in two ways:

Excess bicarbonate is in practice always sodium bicarbonate, which is eventually transformed to Na2CO3. The biological formation of soda from sodium sulphate is said to follow the sequence Na2SO4 Ó Na2S Ó Na2CO3 + H2S, whereby hydrogen sulfide gas leaves the system. This reaction requires (periods of) anaerobic conditions and the presence of organic matter in addition to sodium sulphate.

The formation of a natric horizon is not (yet) properly researched but seems furthered by annual fluctuations in temperature and soil moisture content. The solubility of common sodium and magnesium compounds in soil such as Na2SO4.10H2O, Na2CO3.10H2O and MgSO4.7H2O, increases sharply over the temperature range from 0 to 30 oC (see under Solonchaks; Figure 2). Rapid accumulation of these compounds in the surface soil during dry and hot summer seasons is followed by much slower leaching during the wet but cold winter season. Hysteresis between rapid accumulation and slow discharge of sodium and magnesium compounds in the (sub)surface soil is certainly to be expected in regions with a continental climate where summers are dry and warm and winter precipitation is largely snow that melts in early spring (leaching water temperature close to freezing point). The fact that major Solonetz areas are found in the dry interior parts of North America, Eurasia and Australia seems to confirm this hypothesis.

The presence of `free' soda in soil is associated with a field-pH > 8.5. Under such conditions, organic matter tends to dissolve and move through the soil body with moving soil moisture. The remaining mineral soil material is bleached and in the extreme case a clear eluvial horizon may form directly over the dense natric subsurface horizon. Black spots of accumulated organic matter can be seen in many Solonetz, at some depth in the natric horizon. The dense natric (clay) illuviation horizon poses an obstacle to water percolating downward at the beginning of a wet season. Rain water or snowmelt contains little sodium, if any. This causes a sudden drop in the ionic strength and sodium concentration of the soil moisture at the wetting front. As a consequence, the water films (`double layers') around individual clay plates become thicker, which weakens the bonds between negatively charged sides of clay plates and positively charged `ends' of other plates. Soil aggregation is thus weakened and the soil material disperses. This process is held accountable for the rounded tops of (columnar) structure elements in mature natric horizons. Where the surface soil is subsequently lost because of erosion, the exposed natric horizon shows a characteristic `cobblestone' pattern. Black flakes of translocated organic matter can often be seen on top of the exposed natric horizon alongside whitish, bleached mineral particles. It has been reported that in extreme cases silica and alumina will even dissolve from silicate clays at the upper boundary of the natric horizon.

Note that not all Solonetz contain soda and have a high field-pH! Solonetz can also form through progressive leaching of salt-affected soil. Even soils that were initially rich in calcium may eventually develop a natric horizon. Prolonged leaching and exchange of adsorbed Na+ by H+ will ultimately produce a bleached eluvial horizon with a low pH. Such strongly degraded soils are known as `Solods'.

Characteristics of Solonetz

Morphological characteristics

`Typical' Solonetz feature a thin, loose litter layer resting on black humified material about 2-3 cm thick. The surface horizon is brown, granular and shallow but can also be more than 25 cm thick; it is easily eroded away. If still present, it normally overlies a brown to black, coarse columnar or prismatic, natric subsurface horizon. Structure elements in the natric horizon may be covered by thick, dark cutans of clay and/or translocated organic matter, especially if the soil reaction is strongly alkaline. The rounded tops of columnar structure elements may be covered with bleached, powdery fine sand or silt. In strongly degrading Solonetz, a bleached `albic horizon' may even be present between the surface horizon and the natric horizon. The natric horizon grades with depth into massive subsoil.

Hydrological characteristics

Clayey Solonetz are nearly always slowly permeable to water. Rapid slaking of surface soil during rain showers (or surface inundation) and subsequent ponding of water on top of dry (sic!) soil is a common problem. Shallow drainage gullies are common even in (nearly) flat depression areas, which demonstrates how rapid dispersion of surface soil material is conducive to water erosion of Solonetz.

Physical characteristics

Most Solonetz are very hard in the dry season and sticky when wet. Clayey Solonetz tend to become lumpy at the surface when ploughed, particularly where the shallow surface horizon was lost and the top of the natric horizon became exposed. The dense natric horizon hinders downward percolation of water and root penetration. There are strong indications that a high percentage of exchangeable magnesium affects the soil structure in a similar manner as a high ESP.

Chemical characteristics

The strong sodium saturation of Solonetz is harmful to plants in several ways.

The impression exists that sensitive crops (e.g. beans) develop true sodium toxicity symptoms already at low ESP-values whereas tolerant crops such as cotton become stunted at much higher ESP, mainly because of sodium-induced adverse physical soil conditions.

Management and use of Solonetz

The suitability of `virgin' Solonetz for agricultural uses is almost entirely dictated by the depth and properties of the surface soil. A `deep' (say >25 cm) humus-rich surface soil is needed for successful arable crop production. Unfortunately, most Solonetz have only a much shallower surface horizon, or have lost the surface horizon altogether.

Solonetz amelioration has two basic elements:

  1. improvement of the porosity of the (sub)surface soil, and
  2. lowering of the ESP.

Most reclamation attempts start with incorporation of gypsum or, exceptionally, calcium chloride in the soil. Where lime or gypsum occur at shallow depth in the soil body, deep ploughing (mixing the carbonate or gypsum containing subsoil with the surface soil) may make expensive amendments superfluous. Traditional reclamation strategies start with the planting of a sodium-resistant crop, e.g. Rhodes grass, to gradually improve the permeability of the soil. Once a functioning pore system is in place, sodium ions are carefully leached from the soil with `good quality' (calcium-rich) water.

An extreme reclamation method, which was developed in Armenia and successfully applied to Calcic Solonetz soils in the Arax river valley, uses diluted sulphuric acid (a waste product of the metallurgical industry) to dissolve CaCO3 contained in the soil. This adds calcium ions to the soil solution, which repel sodium ions from the soil's exchange complex. The practice improves soil aggregation and soil permeability. The resulting sodium sulphate (in the soil solution) is subsequently flushed out of the soil.

By and large, Solonetz are problem soils when used for arable agriculture. The prospects for crop production on Solonetz are largely dictated by the thickness of the humus-rich surface layer. Deep ploughing can improve Solonetz in areas where lime or gypsum is present at shallow depth in the soil. This strategy and the use of ameliorants such as gypsum were found to be the most effective on Solonetz under irrigation. Ameliorated Solonetz can produce a fair crop food grain or forage. The majority of the world's Solonetz was never reclaimed and is used for extensive grazing or lies idle.

A word of caution

Soil analytical laboratories determine the Exchangeable Sodium Percentage (ESP) of soil material in a number of steps. First, `adsorbed bases' are determined by bringing an aliquot of the soil material in contact with a strong electrolyte solution such as 1 M NH4-acetate. After `equilibrium' is established, repelled `bases' (Na+ and Mg2+-ions and others) are determined in the acetate solution. Next, the exchange capacity of the soil material is determined by exposing the same aliquot of soil to another electrolyte solution that is buffered to a constant pH-value, e.g. pH 7.0 or pH 8.2. The ESP-value is calculated by multiplying the quantity of repelled Na+ (first electrolyte solution) by 100 and dividing the result by the quantity of the repelled replacement cation (determined in second electrolyte solution).

The cation exchange properties of many soil materials are in part pH-dependent. This has consequences: the actual ESP-value under field conditions is overestimated if the field-pH exceeds the value of the buffered (second) electrolyte solution and is underestimated if the field-pH is lower. It follows that (widely used) generic tables that suggest orders of crop yield depression as a function of measured ESP-values overestimate damage if the field-pH exceeds the pH of the buffered electrolyte solution and underestimate damage is the field-pH is lower. This explains why cotton can be produced in the Gezira region of Sudan (field-pH > 8.5 and a measured ESP-value of 35%) even though tables published in the United States indicate a maximum tolerable ESP-level of only 16% (at a field-pH close to 7.0). Generic tables on the damage inflicted by high sodium levels are to be used with great caution!

Gypsisols (GY)

Gypsisols are soils with substantial secondary accumulation of gypsum (CaSO4.2H2O). They are found in the driest parts of the arid climate zone, which explains why leading soil classification systems labeled them `Desert soils' (USSR), Aridisols (USDA Soil Taxonomy), Yermosols or Xerosols (FAO, 1974).

Definition of Gypsisols#

Soils having

  1. a gypsic@ or petrogypsic@ horizon within 100 cm from the surface; and
  2. no diagnostic horizons other than an ochric@ horizon, a cambic@ horizon, an argic@ horizon permeated with gypsum or calcium carbonate, a vertic@ horizon, or a calcic@ or petrocalcic@ horizon underlying the gypsic@ or petrogypsic@ horizon.

Common soil units:

Petric*, Hypergypsic*, Leptic*, Vertic*, Endosalic*, Duric*, Calcic*, Luvic*, Takyric*, Yermic*, Aridic*, Hyperochric*, Skeletic*, Sodic*, Arzic*, Haplic*.

# See Annex 1 for key to all Reference Soil Groups

@ Diagnostic horizon, property or material; see Annex 2 for full definition.

* Qualifier for naming soil units; see Annex 3 for full definition.

Summary description of Gypsisols

Connotation: soils with substantial secondary accumulation of calcium sulphate; from L. gypsum, gypsum.

Parent material: mostly unconsolidated alluvial, colluvial or aeolian deposits of base-rich weathering material.

Environment: predominantly level to hilly land and depression areas (e.g. former inland lakes) in arid regions. The natural vegetation is sparse and dominated by xerophytic shrubs and trees and/or ephemeral grasses.

Profile development: AB(t)C profiles with a yellowish brown ochric surface horizon over a pale brown or whitish cambic or (relic ?) argic subsurface horizon. Accumulation of calcium sulphate, with or without carbonates, is concentrated in and below the B-horizon.

Use: Deep Gypsisols located close to water resources can be planted to a wide range of crops. Yields are severely depressed where a petrogypsic horizon occurs at shallow depth. Nutrient imbalance, stoniness, and uneven subsidence of the land surface upon dissolution of gypsum in percolating (irrigation) water are further limitations. Irrigation canals must be lined to prevent the canal walls from caving in. Large areas of Gypsisols are in use for low volume grazing.

Regional distribution of Gypsisols

Gypsisols are exclusive to arid regions; their world-wide extent is probably of the order of 100 million hectares. Major occurrences are in and around Mesopotamia, in desert areas in the Middle East and adjacent central Asian republics, in the Libyan and Namib deserts, in southeast and central Australia and in the southwestern USA. Figure 1 presents an overview of major Gypsisol areas.

Figure 1
Gypsisols world-wide

Associations with other Reference Soil Groups

Gypsisols occur in the same climatic zone as Calcisols. Note that presence of a gypsic or petrogypsic horizon is diagnostic for Gypsisols but that accumulation of gypsum occurs also in other Reference Soils. Vertisols, Solonchaks, Gleysols or Kastanozems with clear signs of gypsum accumulation intergrade with the Gypsisol Reference Group but do not key out as Gypsisols because of diagnostic properties other than a gypsic or petrogypsic horizon.

Genesis of Gypsisols

Most Gypsisols formed when gypsum, dissolved from gypsiferous parent materials, moved through the soil with the soil moisture and precipitated in an accumulation layer. Where soil moisture moves predominantly upward (i.e. where a net evaporation surplus exists for an extended period each year), a gypsic or petrogypsic horizon occurs at shallower depth than a layer with lime accumulation (if present). Gypsum is leached from the surface soil in wet winter seasons. In arid regions with hot, dry summers, gypsum (CaSO4.2H2O) dehydrates to loose, powdery hemihydrate (CaSO4.0.5H2O), which reverts to gypsum during the moist winter. The so-formed (highly irregular) gypsum crystals may cluster together to compact layers or surface crusts that can become tens of centimeters thick. Gypsum precipitates in the soil body as fine, white, powdery crystals in former root channels (`gypsum pseudomycelium') or in pockets, or as coarse crystalline `gypsum sand', or in strongly cemented petrogypsic horizons. In places it forms pendants below pebbles and stones or rosettes (`desert roses').

The accumulated gypsum is rarely formed in situ, but there are exceptions. `Intrazonal' Gypsisols (formed under a dominant influence of local material or relief) have been reported from sites where sulphate-rich groundwater occurred at shallow depth. Another example was reported from areas with pyritic sediments in southwest Siberia where sulphate ions, formed when sulphides oxidized upon forced drainage of the land, precipitated as gypsum at depths of 20 to 150 cm below the surface of the soil. In the Republic of Georgia, gypsum was seen to form where saline, Na2SO4-containing, seepage water came in contact with dolomite weathering. By and large, however, the gypsum in Gypsisols originates from Triassic, Jurassic and Cretaceous evaporites or from (predominantly) Miocene gypsum deposits.

Characteristics of Gypsisols

Morphological characteristics

The `typical' Gypsisol has 20 to 40 cm of yellowish brown, loamy or clayey surface soil over a pale brown subsurface soil with distinct white gypsum pockets and/or pseudo-mycelium. The surface layer consists of strongly de-gypsified weathering residues and has a low organic matter content and a weak, subangular blocky structure. Gypsum accumulation is most pronounced in the subsurface layer or slightly deeper and can be anything from a gypsic horizon with a soft, powdery and highly porous mixture of gypsum, lime and clay, to a hard and massive petrogypsic horizon of almost pure, coarse gypsum crystals.

Hydrological characteristics

Gypsisols feature a wide range of hydraulic properties. Saturated hydraulic conductivity values vary from 5 to >500 cm/d. Infiltration of surface water is almost zero in severely encrusted soils. By contrast, very high percolation losses occur in soils in which dissolution of gypsum has widened fissures, holes and cracks to interconnected subterranean cavities. Infilling of the cavities with surface soil material makes it necessary to level the land surface each year. This makes the valuable topsoil ever shallower. See Figure 2.

Figure 2
Cavity formation, uneven subsidence, and stripping of the surface soil upon prolonged irrigation of shallow Gypsisols. Source: Van Alphen & Romero, 1971

Physical characteristics

Most de-gypsified surface layers contain more than 40 percent clay and have an `available' water holding capacity of 25 to 40 percent (by volume). Surface soils with more than 15 percent gypsum have seldom more than 15 percent clay and their retention of `available' soil moisture does not exceed 25 volume percent.

Loamy surface soil slakes easily and subsequently dries to a finely platy crust at the surface that hinders infiltration of rainwater and promotes sheet wash and gully erosion.

Chemical characteristics

Small quantities of gypsum will not harm plants but gypsum contents of more than 25 percent, as common in gypsiferous subsoil, upset the nutrient balance and lower the availability of essential plant nutrients such as phosphorus, potassium and magnesium.

The total element contents of Gypsisol surface horizons are typically less than 2500 mg N/kg, 1000 mg P2O5/kg (of which less than 60 mg/kg is considered `available'), and 2000 mg K2O/kg: application of fertilizers is required for good yields. The cation exchange capacity (CEC) is conditioned by the clay content of the soil material; it is typically around 20 cmol(+)/kg in the surface soil and around 10 cmol(+)/kg deeper down. The exchange complex is saturated with bases.

Management and use of Gypsisols

Gypsisols that contain only little gypsum in the upper 30 cm soil layer can be used for production of small grains, cotton, alfalfa, etc. Dry farming on deep Gypsisols makes use of fallow years and other water harvesting techniques but is rarely rewarding under the adverse climate conditions. Many Gypsisols in (young) alluvial and colluvial deposits have relatively little gypsum. Such soils can be very productive if carefully irrigated. Even soils containing 25 percent powdery gypsum or more could still produce excellent yields of alfalfa hay (10 tons per hectare), wheat, apricots, dates, maize and grapes if irrigated at high rates in combination with forced drainage. Irrigated agriculture on Gypsisols is plagued by quick dissolution of soil gypsum resulting in irregular subsidence of the land surface, caving in canal walls, and corrosion of concrete structures. Dissolution of gypsum might also reduce the depth of a petrogypsic horizon to the extent that the hard pan obstructs root growth, and/or interferes with water supply to the crop and with soil drainage. Large areas with Gypsisols are in use for extensive grazing.

Durisols (DU)

The Reference Soil Group of the Durisols is represented in arid and semi-arid environments and holds very shallow to moderately deep, free-draining soils that contain cemented secondary silica (SiO2) in the upper metre of soil. Durisols are internationally known as "hardpan soils" (Australia) or "dorbank" (South Africa) or they represent the "duripan phase" of other soils, e.g. of Calcisols (FAO).

Definition of Durisols#

Soils having a duric@ or petroduric@ horizon within 100 cm from the surface.

Common soil units:

Petric*, Leptic*, Vertic*, Gypsic*, Calcic*, Luvic*, Arenic*, Hyperduric*, Takyric*, Yermic*,

Aridic*, Hyperochric*, Chromic*, Haplic*.

# See Annex 1 for key to all Reference Soil Groups

@ Diagnostic horizon, property or material; see Annex 2 for full definition.

* Qualifier for naming soil units; see Annex 3 for full definition.

Summary description of Durisols

Connotation: soils with hardened secondary silica; from L. durus, hard.

Parent material: mainly alluvial and colluvial deposits of all texture classes.

Environment: level and slightly sloping alluvial plains, terraces and gently sloping piedmont plains in arid, semi-arid and Mediterranean regions.

Profile development: AC or ABC profiles; eroded Durisols with exposed petroduric horizons are common in (gently) sloping terrain.

Use: most Durisols can only be used for extensive grazing. Arable cropping of Durisols is limited to areas where irrigation water is available (a continuous petroduric horizon at shallow depth must be broken up).

Regional distribution of Durisols

Extensive areas of Durisols occur in Australia, in South Africa/Namibia and in the USA (notably in Nevada, California and Arizona); minor occurrences have been reported from Central and South America and from Kuwait. Durisols are a new introduction in international soil classification and have not often been mapped as such. A precise indication of their extent is not (yet) available. Figure 1 presents a sketch map of their main occurrences.

Associations with other Reference Soil Groups

Durisols are confined to dry regions, where they occur in association with Gypsisols, Calcisols, Solonchaks, Solonetz, Vertisols, Arenosols, Cambisols and, more rarely, Planosols or Kastanozems. In places, Durisols occur together with Andosols. In areas with silica-capped mesas, Durisols may be found in lower parts of the landscape.

Figure 1
Durisols world-wide

Genesis of Durisols

Most Durisols occur in strongly weathered alluvial or colluvial parent material. It is generally believed that duric and petroduric horizons form by downward translocation of clay and silica, even in regions with a very low annual rainfall sum. Periodic flooding and wetting of the surface soil during occasional heavy downpours promote leaching and acidification of the upper soil layer; the leached silica accumulates deeper in the soil where it hardens as the soil dries out. The consistent occurrence of a cemented hardpan (a petroduric horizon, often referred to as a `duripan') at shallow depth, even beneath surfaces on which new soil material regularly accumulates, is accepted as evidence that silica translocation is still taking place. The consistent positive correlation between the depth of the hardpan and the permeability of the overlying soil is a further indication that hardpan formation is not a paleo-feature.

Duric and petroduric horizons, with their active cementation, must not be confused with `silcrete', i.e. hardened, silica-cemented lumps or continuous layers of soil material that formed under a different climate than that of today. Most silcrete stems is of (early) Tertiary age. It is commonly associated with silica-rich parent rocks such as quartz sandstone, but occurs also on weathered igneous rocks, and in the lower layers of strongly leached, red, tropical soils. In places, silcrete has become exposed after erosion of the surface soil; the hardened silcrete cap protects the soil from further erosion.

Characteristics of Durisols

Morphological characteristics

Most Durisols are well-drained, medium to coarse-textured soils. They have either a petroduric horizon or a duric horizon within 100 cm from the surface. A petroduric horizon is a subsurface horizon cemented by secondary silica (presumably amorphous and microcrystalline forms of SiO2), commonly with accessory cements such as calcium carbonate and/or iron oxides. A duric horizon contains indurated nodules, (`durinodes') that are cemented by silica. Dry fragments of a petroduric or duric horizon do not slake upon prolonged soaking in water or in hydrochloric acid.

Petroduric horizons range in thickness from 10 cm to more than 4 m. Two main morphological types are distinguished, i.e. massive `duripans', and `duripans' with a platy or laminated structure. The plates or `laminae' are between a few mm and 15 cm thick. Pores and surfaces of plates are coated with amorphous `opal' or microcrystalline silica. Roots tend to grow in between the plates or form a mat on top of the petroduric horizon. Rodents are capable of burrowing through the pan(s); their burrows are later filled in with soil material from shallower horizons. Roots and water can enter the underlying horizons through these passages, which improve root growth and soil moisture retention.

The `durinodes' in a duric horizon show normally a pattern of roughly concentric layers when viewed in cross section. Duric horizons are less common than petroduric horizons of which they are considered to be the predecessor.

A typical Durisol profile has a red (brown) to grayish brown, non-calcareous surface soil on top of a duric or petroduric horizon. Durisols may have an argic, cambic or calcic horizon above the (petro)duric horizon. If unconsolidated materials underlie the (petro)duric horizon, these are normally weakly structured and calcareous or gypsiferous. In many instances the material is calcareous immediately below the (petro)duric horizon and gypsiferous at greater depth.

Hydrological characteristics

The water storage capacity of Durisols with a petroduric horizon depends mainly on the depth and composition of the soil above this `duripan'. The petroduric horizon obstructs vertical water movement. Data on soil moisture stored between 333 and 15,000 hPa soil suction (often wrongly perceived as `available' soil moisture), suggest that any value between (almost) 0 and 15 % moisture may be expected. In less strongly cemented duric horizons one may find between 5 and 15 % `available' moisture.

Physical characteristics

The texture class of petroduric and duric horizons can range from sand to sandy clayloam. Textures finer than sandy clayloam are rare; sandy loam appears to be the most common material. The bulk density of petroduric and duric horizons is between 1.2 and 2.0 kg dm-3; values between 1.3 and 1.7 kg dm-3 are most common. Petroduric horizons tend to be denser (bulk density between 1.6 and 2.0 kg dm-3) than duric horizons.

Petroduric and duric horizons are normally (but not exclusively) `massive', i.e. without structure. The dry consistence of `duripans' is typically hard or extremely hard. The dry consistence of duric horizons varies between soft and very hard but `durinodes' are usually (extremely) hard.

Chemical characteristics

The pH(H2O) of petroduric and duric horizons may be as low as 5.0 or as high as 10.0 but values are typically between 7.5 and 9.0. The electrical conductivity is typically less than 4 dS m-1; higher values are not uncommon ("Hyposalic" and "Salic" soil units). Many Durisols have high levels of exchangeable sodium and low contents of carbon and extractable iron. The (nominal) base saturation is usually well in excess of 50 %.

Land use and management of Durisols

The agricultural use of Durisols is limited to extensive grazing. Durisols in `natural' environments generally support enough vegetation to contain erosion but elsewhere erosion of the surface soil is widespread.

Stable landscapes occur in dry regions where Durisols were eroded down to their resistant `duripan'. Durisols may be cultivated with some success if sufficient irrigation water is available. Note that the petroduric horizon must be broken up, or removed altogether, if it forms a barrier to root and water penetration. Excess levels of soluble salts may build up in Durisols in low-lying areas. Hard `duripan' material is used in road construction.

Calcisols (CL)

The Reference Soil Group of the Calcisols accommodates soils in which there is substantial secondary accumulation of lime. Calcisols are common in calcareous parent materials and widespread in arid and semi-arid environments. Formerly Calcisols were internationally known as `Desert soils' and `Takyrs'.

Definition of Calcisols#

Soils having

  1. a calcic@ or petrocalcic@ horizon within 100 cm of the surface; and
  2. no diagnostic horizons other than an ochric@ or cambic@ horizon, an argic@ horizon which is calcareous, a vertic@ horizon, or a gypsic@ horizon.

Common soil units:

Petric*, Hypercalcic*, Leptic*, Vertic*, Endosalic*, Gleyic*, Luvic*, Takyric*, Yermic*, Aridic*, Hyperochric*, Skeletic* , Sodic*, Haplic*.

# See Annex 1 for key to all Reference Soil Groups

@ Diagnostic horizon, property or material; see Annex 2 for full definition.

* Qualifier for naming soil units; see Annex 3 for full definition.

Summary description of Calcisols

Connotation: soils with substantial secondary accumulation of lime; from L. calcarius, calcareous

Parent material: mostly alluvial, colluvial and aeolian deposits of base-rich weathering material.

Environment: level to hilly land in arid and semi-arid regions. The natural vegetation is sparse and dominated by xerophytic shrubs and trees and/or ephemeral grasses.

Profile development: `typical' Calcisols have ABC or AB(t)C-profiles with a pale brown ochric surface horizon over a cambic or argic subsurface horizon. Finely textured subsurface horizons may develop some or all of the characteristics of a vertic horizon. Substantial secondary accumulation of lime occurs within 100 cm from the surface.

Use: dryness, and in places also stoniness and/or the presence of a shallow petrocalcic horizon, limit the suitability of Calcisols for agriculture. If irrigated, drained (to prevent salinisation) and fertilised, Calcisols can be highly productive under a wide variety of crops. Hilly areas with Calcisols are predominantly used for low volume grazing of cattle, sheep and goats.

Regional distribution of Calcisols

It is difficult to quantify the worldwide extent of Calcisols with any measure of accuracy. Many Calcisols occur together with Solonchaks that are actually salt-affected Calcisols and/or with other soils with secondary accumulation of lime that do not key out as Calcisols. The total Calcisol area may well amount to some 1 billion hectares, nearly all of it in the arid and semi-arid (sub)tropics of both hemispheres. Figure 1 gives an indication of the regional distribution of Calcisols.

Figure 1
Calcisols world-wide

Associations with other Reference Soil Groups

Calcisols can occur in association with a variety of soils, many of them with signs of secondary redistribution of carbonates. Lateral transitions in the field are primarily associated with differences in relief, climate and/or geology. Cross-sections through landscapes with Calcisols normally show a gradual transition from shallow soils with rather diffuse signs of lime redistribution (at the highest parts) to deeper soils that are also richer in carbonates. There is great variation in the expression of lime redistribution; common forms include filled-in pores that show up as `pseudomycelia', pockets of soft lime, soft and hard nodules, and layered, platy or compact, consolidated `calcrete'. Studies of calcic horizons suggest that both lateral and vertical redistribution of lime have occurred and that lateral movement of lime is not without significance. Soils found in association with Calcisols range from shallow Leptosols (at the highest parts of the landscape) to Vertisols at the lower end of slopes and in bottomlands. Calcisols in depression areas are frequently associated with Solonchaks and Gleysols. Piedmont plains with Calcisols in semi-arid subtropical regions may grade into areas with Chernozems or Kastanozems with a deep groundwater table.

Genesis of Calcisols

Many Calcisols are old soils if counted in years but their development was slowed down by recurrent periods of drought in which such important soil forming processes as chemical weathering, accumulation of organic matter and translocation of clay came to a virtual standstill. As a result, only an ochric surface horizon could develop and the modification of subsoil layers did not advance beyond the formation of a cambic subsurface horizon. Many Calcisols are `polygenetic': their formation took different courses during different geologic eras with different climates. The argic subsurface horizon of many Calcisols is widely considered to be a relic from eras with a more humid climate than at present.

The most prominent soil forming process in Calcisols - the process from which the soils derived their name - is the translocation of calcium carbonate from the surface horizon to an accumulation layer at some depth. In eroding land or in land that is intensively homogenised by burrowing animals, lime concretions may occur right at the surface of the soil. However, it is more common to find the surface horizon wholly or partly de-calcified.

Dissolution of calcite (CaCO3) and subsequent accumulation in a calcic or petrocalcic horizon is governed by two factors:

  1. the CO2-pressure of the soil air, and
  2. the concentrations of dissolved ions in the soil moisture.

The following equilibria are involved (the pH-ranges over which the equilibria are in operation are shown in Figure 2):

CO2 + H2O = H2CO3o

H2CO3o = HCO3- + H+

HCO3- = CO32- + H+

For all practical purposes, the dissolution and precipitation of calcite in soils (pH <9) can be viewed as follows:

    

CaCO3 + H2CO3 = Ca2+ + 2 HCO3-

An increase in the CO2-content of the soil-air drives the reaction to the right: calcite dissolves and the concentrations of Ca2+- and HCO3--ions in the soil solution rise. Alternatively, calcite dissolves if (rain) water with a low Ca2+-concentration flushes the soil.

Precipitation of calcite occurs if the reaction is driven to the left, e.g. by a lowering of the CO2-pressure (with a consequent rise in pH), or by an increase in ion concentrations to the point where the solubility product of dissolved calcium carbonate is exceeded.

Figure 2
Solubility of calcite at different CO2-pressures and corresponding pH-values.

Source: Bolt & Bruggenwert, 1979

The formation of a calcic horizon is now easily understood: the partial CO2-pressure of the soil-air is normally highest in the A-horizon where root activity and respiration by micro-organisms cause CO2 contents to be 10 to 100 times higher than in the atmospheric air. As a consequence, calcite dissolves and Ca2+- and HCO3--ions move downward with percolating soil moisture, particularly during and directly after a rain shower. The water may take up more dissolved calcite on its way down.

Evaporation of water and a decrease in partial CO2-pressure deeper in the profile (fewer roots and less soil organic matter and microorganisms) cause saturation of the soil solution and precipitation of calcite. The precipitated calcite is not or only partly transported back with ascending water because much of this water moves in the vapour phase. (The water table in Calcisols is normally deep; where there is capillary rise to the solum, calcite accumulates at the depth where the capillary water evaporates.)

Calcite precipitation is not (always) evenly distributed over the soil matrix. Root channels and wormholes that are connected with the outside air act as ventilation shafts in which the partial CO2-pressure is much less than in the soil around it. When Ca(HCO3)2-containing soil moisture reaches such a channel, it loses CO2 and calcite precipitates on the channel walls. Where narrow root channels become filled with calcite, so-called `pseudomycelium' forms. Other characteristic forms of calcium carbonate accumulation in Calcisols are nodules of soft or hard lime (`calcrete'), platy or continous layers of calcrete and calcite `pendants' or `beards' below pebbles.

High soil temperature and high soil-pH enhance dissolution of silica from feldspars, ferromagnesian minerals, etc. Where there is (or was) sufficient moisture in some period of the year to enable translocation of dissolved silica, this may have furthered the hardening of the layer with calcite accumulation. However, cementation of a petrocalcic horizon is in first instance by calcium and magnesium carbonates.

Characteristics of Calcisols

Morphological characteristics

Most Calcisols have a thin (=<10 cm) brown or pale brown surface horizon over a slightly darker subsurface horizon and/or a yellowish brown subsoil that is speckled with white calcite mottles. The organic matter content of the surface soil is low, in line with the sparse vegetation and rapid decomposition of vegetal debris. The surface soil is crumb or granular, but platy structures can occur as well, possibly enhanced by a high percentage of adsorbed magnesium. Most subsurface soils have a blocky structure; the structure elements are coarser, stronger and often more reddish in colour in an argic horizon than in subsurface soils without clay accumulation.

The highest calcite concentration is normally found in the deeper subsurface soil and in the subsoil. Burrowing animals homogenize the soil and bring hardened carbonate nodules to the surface; their filled-in burrows (`krotovinas') may extend deep into the subsoil.

Hydrological characteristics

Most Calcisols are well drained and are wet only in part of the (short) rainy season when there is just enough downward percolation to flush soluble salts to the deep subsoil. One reason why Calcisols as a taxonomic unit have good drainage properties is that carbonate-rich soils in wet positions (depressions, seepage areas) quickly develop a salic horizon (long dry summers!) and key out as Solonchaks.

Physical characteristics

Most Calcisols have a medium or fine texture and good water holding properties. Slaking and crust formation may hinder the infiltration of rain and irrigation water, particularly where surface soils are silty. Surface run-off over the bare soil causes sheet wash and gully erosion and, in places, exposure of a petrocalcic horizon.

Chemical characteristics

Most Calcisols contain only 1 or 2 percent organic matter but many are rich in plant nutrients. The pH(H2O;1:1) is near-neutral in the surface soil and slightly higher at a depth of 80 to 100 cm where the carbonate content may be 25 percent or more. The nominal cation exchange capacity of typical Calcisols is highest in the surface soil (10 to 25 cmol(+)/kg) and slightly less at some depth. The exchange complex is completely saturated with bases; Ca2+ and Mg2+ make up more than 90 percent of all adsorbed cations.

Land use and management of Calcisols

Vast areas of `natural' Calcisols are under shrubs, grasses and herbs and are used for extensive grazing. Drought-tolerant crops such as sunflower might be grown rain-fed, preferably after one or a few fallow years, but Calcisols reach their full productive capacity only when carefully irrigated. Extensive areas of Calcisols in the Mediterranean zone are used for production of irrigated winter wheat, melons, and cotton. Fodder crops such as `el sabeem' (sorghum bicolor), Rhodes grass and alfalfa, are tolerant of high calcium levels. A score of vegetable crops have successfully been grown on irrigated Calcisols fertilised with nitrogen, phosphorus and trace elements (Fe, Zn).

Furrow irrigation is superior to basin irrigation on slaking Calcisols because it reduces surface crusting/caking and seedling mortality; pulse crops in particular are very vulnerable in the seedling stage. In places, arable farming is hindered by stoniness of the surface soil and/or a petrocalcic horizon at shallow depth. Citrus is reportedly sensitive to high levels of `active CaCO3' i.e. finely divided calcium carbonate particles in the soil matrix.

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