Introduction to Aridic Soils
The central concept of Aridisols is that of soils that are too dry for mesophytic plants to
grow. They have either:
(1) an aridic moisture regime and an ochric or anthropic epipedon and one or more of the
following with an upper boundry within 100 cm of the soil surface: a calcic, cambic,
gypsic, natric, petrocalcic petrogypsic, or a salic horizon or a duripan or an argillic horizon,
or
(2)A salic horizon and saturation with water within 100 cm of the soil surface for one
month or more in normal years.
An aridic moisture regime is one that in
normal years has no water available for
plants for more than half the cumulative
time that the soil temperature at 50 cm
below the surface is >5° C. and has no
period as long as 90 consecutive days when
there is water available for plants while the
soil temperature at 50 cm is continuously
>8° C.
http://ag.arizona.edu/OALS/IALC/soils/a
ridsoils/images/1.html
Aridisols. Aridisols are indicated by "id". These are dry-land soils, and are the
most abundant soil type worldwide. Aridisols are light-colored, rich in
carbonates, and are typically vegetated with bunchgrasses and shrubs. These
soils are often quite productive when irrigated & fertilized. Plate 2 in the
textbook shows an Aridisol of the suborder Argids. The suborders of Aridisols
are:
Argids — argillic horizon
Calcids — calcic horizon
Cambids — cambic horizon
Cryids — very cold
Durids — duripan
Gypsids — gypsic horizon
Salids — salty
Salinization
Major characteristics : Ca & Na accumulation, little organic matter
Climate: hot dry climates
Soil order: Aridisol
Location: SW U. S.A.
Accumulation of salts is easily seen in this salinized soil.
Salinization occurs in warm and dry locations where soluble salts precipitate from water
and accumulate in the soil. Saline soils are common in desert and steppe climate. Salt may
also accumulate in soils from sea spray. The rapid evaporation of salt-rich water irrigation
has devastated thousands of acres of land.
Almost one third of the Earth's surface has an arid or semi-arid climate.
The largest arid zones include the Sahara and Sahel regions of North Africa, the
Kalahari Desert in Southern Africa, the Arabian Desert in the Near East, the Great
Australian Desert, and the deserts of North and South America.
The aridity of these regions presents formidable environmental obstacles to human
habitation.
Yet these same lands have the potential to become the next frontier for agriculture. In
fact, some soils of arid regions have considerable promise for development. All that is
required is a reliable water supply and proper management.
Soil Taxonomy
The Aridisol soil Order is one of twelfth Orders recognized in Soil Taxonomy.
(Originally ten Orders, the eleventh, Andisol, was added to distinguish at the highest
level those soils derived from volcanic-glass in 1989 and the twelfth, Gelisol was
added in 1998.) Before we examine the taxonomic hierarchy of the Aridisol Order,
we'll discuss the two main criteria used to classify Aridisols: the aridic soil moisture
regime and diagnostic soil horizons.
Five soil moisture regimes characterize the presence or absence of groundwater or
water held at a tension in the soil at which it is available to most plants. This graph
depicts the soil water balance in an aridic soil moisture regime. The orange area
represents the soil moisture deficit which occurs when evapotranspiration exceeds
precipitation. Under these conditions plants do not have water continuously available
except for brief periods.
Thus, all Aridisols require irrigation to support cultivated crops.
Introduction to Soil Horizons
Soil horizons which have unique morphologic, chemical and mineralogic properties
are called diagnostic horizons. These horizons form the foundation of soil
classification in Soil Taxonomy.
Diagnostic horizons formed at the soil surface are called epipedons. The light colored
surface horizon found in most Aridisols is called an ochric epipedon. The term
epipedon is derived from the Greek word epi meaning "over," and pedon meaning
"soil." Ochric is derived from the Greek word ochros meaning "pale."
Due to the low amount or absence of leaching associated with the aridic soil moisture
regime, Aridisols normally have one or more subsurface soil horizons in which
suspended or dissolved minerals have been deposited.
Nine diagnostic subsurface soil horizons are important in the classification of
Aridisols. These horizons have accumulations of silicate clays, sodium, calcium
carbonate, gypsum or soluble salts. These horizons can also be cemented by
carbonates, gypsum or silica.
The Diagnostic Horizons
Argillic horizons are horizons of accumulated translocated silicate clays. The term
argillic is derived from the Latin word argilla, meaning "clay."
When a thin slice of an argillic horizon is viewed under a polarizing microscope, the
presence of translocated clay is confirmed. The yellow areas in this thin section are
the translocated clay-skins or cutans.
Natric horizons are argillic horizons that have high amounts of exchangeable sodium.
They frequently have columnar soil structure. The tops of the columns are sometimes
bleached, as shown here due to clay removal. The term natric is derived from the
Latin word natrium meaning "sodium."
Calcic horizons have pronounced accumulations of calcium carbonate, or of calcium
and magnesium carbonate. They are normally very light in color and must meet
specific criteria of carbonate content and thickness.
Gypsic horizons have significant accumulations of gypsum. Gypsum, or calcium
sulfate, is a moderately soluble salt - more soluble than calcium carbonate but less
soluble than sodium chloride. Gypsum may restrict soil use, both for agricultural and
engineering purposes.
In this magnified view of a thin section of a gypsic horizon seen under cross polarized
light, white lenticular gypsum crystals fill much of the void system of the soil.
The thin white surface layer of this soil is part of a salic horizon. Salic horizons
contain accumulations of salts more soluble than gypsum. These horizons must have
soluble salt concentrations of at lease 2 percent. Salts concentrate in salic horizons by
capillary rise and evaporation from shallow groundwater tables. The term salic is
derived from the Latin work sal meaning "salt."
Note the cubic cleavage of the halite crystals in this scanning electron micrograph of a
salic horizon. The most common salts in salic horizons are the chlorides and sulfates
of sodium, calcium and magnesium.
Cambic horizons are horizons in which minerals have been altered, removed or
accumulated but not to the degree necessary to meet criteria for other diagnostic
horizons. The origin of the term cambic is from the Latin cambiare meaning "to
exchange."
Petrocalcic horizons are cemented calcic horizons that are impenetrable by plant roots.
When they occur close to the surface, they present major constraints for agricultural
use.
Petrocalcic horizons are rock hard and massive. The primary cementing agent is
calcium carbonate. The origin of the term petrocalcic is from the Greek word, petra,
meaning "rock." Other names for petrocalcic horizons are croute calcaire, calcrete and
caliche.
Petrogypsic horizons are similar to petrocalcic horizons except that the primary
constituent is gypsum. Petrogypsic horizons are common in North Africa and the
Middle East.
Duripans are soil horizons cemented by secondary silica. Like petrocalcic and
petrogypsic horizons, duripans present major constraints for agriculture because they
limit root growth and water movement.
Soils of Arid Regions Not in the Aridisol Order
Not all soils in arid regions classify in the Aridisol Order. Many have unique
characteristics which place them into other soil Orders. The aridic soil moisture
condition of these soils is denoted by the formative adjective "torr" or "torri," added to
the Suborder or Great Group name. These adjectives are derived from the Latin word
torridus, meaning hot and dry.
For instance, Oxisols are extremely weathered soils of tropical and subtropical regions.
They consist mostly of quartz, low activity clays, and iron and aluminum oxides. At
present, some Oxisols occur in arid regions due to global climatic changes. These
soils comprise the Torrox Suborder. (Note that in the arid regions covered by this
project, Oxisols are found only in a the small area of Hawaii that is arid.)
Without amendments most Oxisols have low productivity for cultivated plants
because their strongly weathered minerals release few plant nutrients. However, with
fertilization, many Oxisols are highly productive. The irrigated Torrox soils here are
growing sugar cane.
Soils with high amounts of swelling type clays that have deep wide cracks at some
time of the year classify in the Vertisol Order. A significant amount of material from
the upper part of the profile may fall into the cracks, giving rise to a partial inversion
of the soil.
Torrerts are the Vertisols with aridic soil moisture conditions. Unless irrigated they
have cracks that stay open throughout most of the year. The location of Torrerts in low
lying areas combined with their normally good fertility characteristics make them
important agricultural soils in some arid regions of the world.
Mineral soils that show little or no evidence of development of pedogenic horizons
classify as Entisols. Three Entisol Great Groups have aridic soil moisture conditions;
Torripsamments, Torrifluvents, and Torriorthents.
Torripsamments are the sandy Entisols. They are usually too droughty and infertile to
be made agriculturally productive without careful irrigation and fertilizer management.
The formative element "psamm" is derived from the Greek psammos meaning sand.
Torrifluvents are recently deposited soils of alluvial plains which are periodically
flooded. Stratification of alluvium, as seen here, is normal. The formative element
"fluv" is derived from the Latin fluvus meaning "river."
Torrifluvents make up a high proportion of irrigated soils in desert regions because
they are normally located close to water, have gentle slopes and deep, medium
textured profiles.
Torriorthents are the dry or salty Entisols of arid regions. They have aridic soil
moisture conditions or high soluble salt concentrations or both. Some Torriorthents
lack horizon development because they occur on steep active slopes or eroded parts of
the landscape. Others, like the Torriorthent illustrated here, are very young soils.
Irrigation of Aridic Soils
The purpose of classifying soils in Soil Taxonomy is to enable soil scientists to predict
the behavior of one soil, for which technical data are lacking, by examining its
relationship to similar soils that have been studied. We have looked at the
morphologic characteristics of aridic soils and seen how they classify in Soil
Taxonomy. We'll finish the program with a brief consideration of what can be
accomplished when these soils are irrigated and properly managed.
Availability of water is the key to the use of Aridisols for agriculture. History is rich
with references to the practice of irrigation from wells, tanks and canals. Even today,
irrigation devices are being used which are identical to those built hundreds, and in
some cases, thousands of years ago.
The potato harvest of this proud Pakistani farmer would not have been possible
without irrigation and sound soil management.
The most widely used method of conveying water to agricultural fields is surface
diversion. One method of surface diversion that has been practiced for thousands of
years is called water harvesting. In this system, runoff is collected in a catchment area
and channeled to agricultural fields at the base of a slope.
In this and other surface irrigation systems, water is generally distributed within fields
in furrows...
...or in basins.
Surface irrigation systems require gentle slopes. In most instances, some land leveling
is needed before surface irrigation can be practiced efficiently. Land leveling
techniques vary from soil movement with a tractor drawn scraper...
...to the latest laser leveling systems using heavy equipment.
Another common irrigation method uses sprinklers. Sprinkler irrigation is particularly
useful on coarse textured soils or on fields that are difficult to level. Sprinklers range
in size and complexity from this small hand-moved system...
...to much larger, mechanized central pivot units.
Saline Soils
Availability of water is not the only constraint on the use of Aridisols for agriculture.
Soil horizons produced by arid climatic conditions also affect the use of these soils.
Soil management must address these problem areas.
One of the most extensive problems facing irrigated agriculture in arid regions is the
presence of soluble salts in the soil in amounts which inhibit plant growth. This aerial
photograph shows salt accumulation in fields and its effect on crop growth.
Approximately one-third of developed agricultural lands in arid regions reflect some
degree of salt accumulation.
Salt affected soils are commonly classified for management purposes into three
groups; saline, sodic, and saline-sodic. Two criteria are used for this classification, the
electrical conductivity of the soil saturation extract (EC) and the exchangeable sodium
percentage (ESP). Electrical conductivity is directly related to soluble salt
concentration in soils. The exchangeable sodium percentage is a measure of the
amount of sodium on soil exchange sites.
Saline soils often have a surface salt crust as seen here. Only very salt tolerant plants
can grow in such soils. As soil salinity increases, soil water becomes less available to
plants because of higher osmotic pressures, even if soil moisture is plentiful.
Plants vary widely in their tolerance to salts. This graph correlates decreases in crop
yields with salinity levels. Notice how barley and sugar beets are much more tolerant
of high soil salinity levels than are cowpeas and beans. Today, plant scientists
continue to breed crop varieties that are increasingly tolerant to soluble salts.
Saline soils can be reclaimed for agricultural use by leaching. Leaching involves
applying large volumes of water to the soil to move soluble salts out of the rooting
zone. To be effective, internal soil drainage must be adequate so salts will not rise
again into the rooting zone.
Seed placement is also important in the management of saline soils. Under furrow
irrigation soluble salts are concentrated toward the crest of the bed. Therefore,
optimum seed placement is on the lower side of the bed, as seen here.