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Aridisols
Aridisols
Summary:
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Vegetation: Present vegetation - species adapted to arid climate; former
vegetation - not specified
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Climate: Arid regions (cold and warm deserts); cryid or frigid - thermic
or hypothermic soil temperature regime
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Soil moisture regime: aridic, torric
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Major soil property: crusts, desert pavement, accumulation of material
such as clay, CaCO3, or salts
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Diagnostic horizons: cambic, argillic, calcic, petrocalcic, natric,
gypsic, petrogypsic, salic
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Epipedon: ochric, anthropic
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Major processes: weathering, silication, calcification, hardening,
salinization, solodization, deflation
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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:
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Saline: Has a saturation extract conductivity of 4 mmhos/cm or
greater and has a low exchangeable sodium percentage.
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Sodic: Has an exchangeable sodium percentage of 15% or greater but
has a low salt content.
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Saline-sodic: Has both the salt concentration to qualify as saline
and sufficient exchangeable sodium to qualify as sodic.
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The requirements to classify for an Aridisol are:
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an aridic soil moisture regime
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an ochric or anthropic epipedon, and
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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.
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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:
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Dry in all parts more than half the time that the soil temperature
at a depth of 50 cm is above 5oC
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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
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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.
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