Aridisols

 

Aridisols

Summary:

  • Vegetation: Present vegetation - species adapted to arid climate; former vegetation - not specified
  • Climate: Arid regions (cold and warm deserts); cryid or frigid - thermic or hypothermic soil temperature regime
  • Soil moisture regime: aridic, torric
  • Major soil property: crusts, desert pavement, accumulation of material such as clay, CaCO3, or salts
  • Diagnostic horizons: cambic, argillic, calcic, petrocalcic, natric, gypsic, petrogypsic, salic
  • Epipedon: ochric, anthropic
  • Major processes: weathering, silication, calcification, hardening, salinization, solodization, deflation

Aridisols - Environmental Conditions
Climate
: Arid regions including cold polar, cool temperate and warm deserts, which occupy about 36% of the land surface based on climate and about 35% based on vegetation. Aridisols may also occur in semi-arid areas outside of zones broadly classified as arid - e.g. where local conditions impose aridity - steep, south-facing slopes in N-hemisphere, physical properties that limit water infiltration. Aridisols are classified on the basis of their soil moisture regime (more specifically referenced to the soil moisture control section), which is dry in all parts >50% of the time in most years, and not moist for as much as 90 consecutive days when the soil is warm enough (>80C) for plant growth. In an aridic (and torric) soil moisture regime, potential evapotranspiration greatly exceeds precipitation during most of the year. In most years, little or no water percolates through the soil. This hydrologic regime has a distinctive influence on the development of such soils. During Quarternary time, most deserts have changed back and forth from cooler-moister, to warmer-more-arid climates, therefore, change in climatic conditions have to be considered when talking about Aridisols.  

Vegetation: Present vegetation comprises species adapted to dry climate such as cactus (Cactaceae), mesquite (Prosopis), creosotebush (Larrea), Yucca (Yucca), sagebrush (Artemisia), or shadscale (Atirplex). Species have to live in an environment with sparse organic matter, low microbial population, and lack of nutrients such as nitrogen and phosphorous. Use of Aridisols is limited because of lack of water, low biotic activity and low nutrient status. Irrigation can be used to improve crop growth on Aridisols, but issues of internal permeability, salinization and alkalization arising from the irrigation water should be addressed.  

Relief: They form on plain terraces and on steep slopes  

Parent Material: They occur on land surfaces of Pleistocene or greater age, therefore, they occur on parent material such as crystalline rocks. Aridisols do develop on fluvial and eolian materials, extensively in large deserts such as the Gobi, Namib, or Kalahari desert. Aridisols occur on gypsiferous material formed from marine sedimentary rocks, on unconsolidated sediments, or limestone.  

Time: Most Arisisols are found on landscapes that are relatively old and stable (up to more than million years).    

 

Aridisols - Processes
In arid regions chemical and physical reactions operate in the same way as in humid regions, although with less intensity and at shallower depths. Physical weathering such as weathering due to the crystallization of salts or thermal expansion and contraction of the constituents minerals is favored in arid regions. Chemical weathering is retarded because of lack in water although the importance of chemical weathering has been proved in many pedological research studies. Because of sparse vegetation and low humification rates little humus accumulates in the typic Aridisol, i.e., in many Soils ochric epipedons are found.  

Evidence of leaching below the average depth of water storage is commonly observed in Aridisols and is explained by: (i) more humid paleoclimates, and/or (ii) the influence of occasional, exceptionally large precipitation events. Examination of soil forming processes in arid zones invariably requires consideration of possible paleoclimatic influences (i.e. some features in the soil may have formed under conditions quite different from those operating at present), the periodic occurrence of large precipitation events that can punctuate the otherwise dry environment of these regions and local variation in factors that prescribe soil genesis. It seems to be contradictory that horizons accumulated with clay, sodium, salts, gypsum, or silica occurs in Aridisols which is associated with illuviation of those materials. A prerequisite for leaching or eluviation/illuviation is rainfall. Aridisols occur on landscapes that are more than one million years old, a time scale that has allowed for development of accumulations of clay, carbonates, and silica.

A predominant influence on soil formation in arid zones is that potential evapotranspiration greatly exceeds precipitation during most of the year. Thus, drainage of water through the soil is limited. The occurrence of horizons enriched in secondary minerals is strongly controlled by the distinct hydrology of arid regions which favors limited leaching from the solum. The source of secondary enrichment may be atmospheric, from groundwater and weathering of soil minerals. Thus, in evaluating the occurrence and significance of enrichment it is important to evaluate mineral source(s), hydrology and relative age of the soil-landscape. Relative age is important because many of the processes of enrichment are necessarily time dependent. The processes associated with the accumulation of materials in Aridisols are: (i) lessivage or eluviation/illuviation of clays - argillic horizons, (ii) silication , i.e., the accumulation of silica - duripans, (iii) calcification , i.e., the accumulation of CaCO 3 - calcic or petrocalcic horizons. The hardening of soil material may lead to a decrease in volume of voids by infilling with salts and silica. This process is responsible for the formation of petrocalcic, petrogypsic horizons or duripans.  

The composition of the initial material in which some Aridisols are forming that contain argillic, natric and calcic horizons does not readily explain their internal enrichment in phyllosilicate clays or carbonates. Thus, it has been suggested that aeolian inputs may explain this enrichment. However, in some settings subsurface water enriched with clays and especially carbonates may also account for formation of Bt, Btn and Bk(m) horizons.  

Soluble salt accumulation (salinization) is usually associated with depressional landscape positions, such as playas, and a source of saline ground water. Saline accumulations such as sulfates and chlorites of Ca, Mg, K, and Na are also associated with some irrigated agricultural areas. The accumulation of Na salts is called solodization. The accumulation of salts is often associated with a natural or artificially high water table (irrigation) feeding capillary water to, or near to the soil surface where salt accumulates upon evaporation. Salinization of irrigated agricultural areas in semi-arid and arid areas is a problem that has plagued the human race since the dawn of 'civilization'.  

Rubification, i.e, the reddening of the soil due to oxidation of Fe-bearing minerals is often observed in Aridisols. Soil moisture conditions in arid regions favors oxidation over redoxidation.  

The processes deflation and deposition are responsible for the development of 'desert pavement' (surface pebble layers). Deflation is the sorting out, lifting, and removal of loose, dry, fine grained soil particles by the turbulent action of the wind. It is assumed that vertical sorting of stones, i.e., the gradual upward migration of pebbles that have been heaved up by swelling clay, with local supplement action by frost, growth of salt crystalls, and expansion of entrapped air, with preferential collapse of fines into voids too small to accept pebbles during subsequent desiccation support developing a surface pebble layer. The pavement serves as a dust trap but inhibits loss of soil particles by wind erosion.    

 

Aridisols - Properties
Pedogenic processes produced numerous soil features associated with dry climate: (i) crusts, (ii) desert pavement, (iii) cambic horizons, (iv) argillic horizons, (v) natric horizons, (vi) carbonate accumulations (calcic and petrocalcic horizons), (vii) duripans, (viii) salic and gypsic horizons.  

(i) Crusts are surficial layers generally less than 10- to 20-cm thick. They are dominated by fine material composed of compound polygonally prismatic and platy fragments that are coherent when dry. When silt particles dominate they may exhibit vesicular porosity. The distinctive morphology of crusts probably results from repeated wetting and drying, entrapped air during wetting likely accounts for vesicle formation. The impact of soil crusts to infiltration is high, because crusts slow the permeability to water in contrast to rapid infiltration that happens in uncrusted soils.  

(ii) Desert pavement is a surface pebble layer. Several pathways, which probably operate over tens of thousands of years may account for the same end product, these include: (a) removal of fine particles from surface by wind/water, leaving a 'lag' of coarser fragments, (b) vertical sorting of coarse fragments towards surface via wet/dry, freeze/thaw, and uplift by swelling clay, salt growth, air entrapment below, concomitant downward movement of fines, and (c) over time, pavement becomes 'flat' and covered with a thin veneer of 'varnish', composed of Fe, Mn and silicate clays, microbiological processes may contribute to its formation in some settings.  

(iii) Cambic horizons (Bw) have a texture of loamy very fine sand or finer and contain some weatherable minerals. They are characterized by the alteration or removal of mineral material as indicated by mottling or gray colors, stronger chromas or redder hues than in underlying horizons. Carbonates are leached out in low-carbonate parent material, whereas in highly calcareous parent materials, evidence of carbonate removal may take the form of carbonate coatings on undersides of pebbles in the cambic horizon.  

(iv) Argillic horizons (horizons enriched in clay - Bt) may form due to in situ weathering or illuviation of clay in the Bt horizon. Carbonates have to be leached before illuvial clay can accumulate in argillic horizons because clay flocculates in the presence of carbonates.  

(v) Natric horizons (n in combination with any master horizon) satisfy the requirements of an argillic horizon, but also has prismatic, columnar, or blocky structure, and > 15 % saturation with exchangeable Na +. Sodium has characteristic effects of soil physical properties. In the presence of Na clay and humus disperse into individual hydrated particles instead of remaining flocculated. Sodic soils readily lose their structure, deflocculation occurs, the soil structure is destroyed, and pores clog at the surface, therefore the permeability at the surface is reduced.  

(vi) Calcic and petrocalcic horizons (Bk and Bkm or Ck and Ckm) show an accumulation of carbonate and they commonly lie below argillic and cambic horizons in Aridisols. Generally, carbonates are leached out before clay are translocated to form an argillic horizon. Calcic horizons develop over time into petrocalcic horizons, which are indurated calcic horizons cemented by calcium carbonate and in some places with magnesium carbonate. Petrocalcic horizons cannot be penetrated with a spade or auger when dry and the cemented layer is impenetrable to roots.  

(vii) Duripans (Bqm or Cqm) are subsurface soil horizons cemented by illuvial silica, usually opal or microcrystalline forms, to the degree that less than 50 % of the volume of air-dry fragments will slake in water or HCl. Often the duripans in Aridisols have a considerable content of calcium carbonate and can be distinguished only by the test described above.  

(viii) Salic horizons (Bz or Cz) are enriched with secondary salts more soluble than gypsum. A salic horizon is 15 cm or more in thickness and contains at least 20 g/kg salt. A gypsic horizon (By or Cy) is enriched of secondary CaSO4, is > 15 cm thick, and has at least 50 g/kg more gypsum than the C horizon. High pH values (> 9) are associated with nutrient deficiencies or toxicities induced by high pH. Calcium is immobilized because high pH promotes the formation of carbonate from CO 2, and carbonated precipitates with Ca, as CaCO 3. A high pH also affects the sorption behavior of these cations in the soil.  

Most Aridisols show a low permeability because of the presence of accumulated or cemented layers. The nutrient status of often low, however, supplies of micronutrients are usually abundant, although they may not be available because of the high pH.      

 

Aridisols - Classification
An criterion of salinity is the electrical conductivity (EC) of the saturation extract. Soils are considered saline if their EC exceeds 4 dS/m. Usefuls measures of sodicity are the exchangeable sodium percentage (ESP) and the sodium adsorption ratio (SAR). The ESP is the exchangeable Na expressed as a percentage of the total exchangeable cations. The SAR is a modified ratio of Na to other major cations (Ca and Mg) in the saturation extract.  

ESP = 100 (exch. Na) / (exch. Na + exch. Ca + exch. Mg)
(the cation amounts are expressed in mols of charge (gram equivalents).  

SAR = (Na) / √ (Ca * Mg) /2
(the cations are expressed in mols of charge (gram equivalents) per liter.  

Three classes of salt-affected soils are recognized and defined in terms of electrical conductivity and exchangeable sodium percentage:

  • Saline: Has a saturation extract conductivity of 4 mmhos/cm or greater and has a low exchangeable sodium percentage.
  • Sodic: Has an exchangeable sodium percentage of 15% or greater but has a low salt content.
  • Saline-sodic: Has both the salt concentration to qualify as saline and sufficient exchangeable sodium to qualify as sodic.
  The requirements to classify for an Aridisol are:
  • an aridic soil moisture regime
  • an ochric or anthropic epipedon, and
  • one or more of the following subsurface horizons within 100 cm of the soil surface: argillic, cambic, natric, salic, gypsic, petrogypsic, calcic, petrocalcic, or duripan.
    

The Aridisols are composed of 7 suborders distinguished by (i) soil temperature regime, and (c) occurrence of particular diagnostic horizons:

Cryids: Cryic soil temperature regime, MAT higher than 0 oC but less than 8oC.

Salids: Salic horizon that has its upper boundary within 100 cm of the surface.

Durids: Duripan that has its upper boundary within 100 cm of the surface.

Gypsids: Gypsic of petrogypsic horizon that has its upper boundary within 100 cm of the surface and lacks an overlying petrocalcic horizon.  

Argids: Argillic or natric horizon that has its upper boundary and does not have petrocalcic horizon within 100 cm of the surface.  

Calcids: Calcic or petrocalcic horizon that has its upper boundary within 100 cm of the surface.  

Cambids: Other Aridisols    

 

Aridisols - Distinguishing Characteristics
Soils with a dominance of attributes not specifically associated with arid-zone soil forming processes are assigned to other pertinent Orders even though their hydrologic regime is the same as that used for Aridisols. In such instances, the prefix 'Torr' or 'Torri' is used to identify these soils. This prefix refers to the Torric soil moisture regime which is identical to the Aridic soil moisture regime and is defined as:

  • Dry in all parts more than half the time that the soil temperature at a depth of 50 cm is above 5oC
  • Never moist in some or all parts for as long as 90 consecutive days when the soil temperature at a depth of 50 cm is at or above 8 oC
Other soil orders such as the Entisols and Mollisols use the prefixes 'Torric', 'Ustic', and 'Xeric' to classify soils developed in regions with dry climate. Many Aridisols are closely associated with the occurence of Entisols.