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 Calc ids — 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.