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They generally lack diagnostic horizons other than ochric epipedons and argillic horizons. Ustalfs with high sodium content occur to a limited extent in the Willcox Playa vicinity. Aridisols are the developed soils of dry regions. They have light-colored surface layers ochric epipedons , generally have low amounts of organic matter and have at least one diagnostic subhorizon.

Calcium carbonate usually is in some or all parts of the soil. Some soils are high in soluble salts. The two suborders of Aridisols, Orthids and Argids, are widely distributed in arid and semiarid Arizona.


Orthids show little or no textural change with depth. Most are calcareous throughout and many have a distinct accumulation layer of carbonates calcic horizons. Some Orthids have hardpans cemented by carbonates petrocalcic horizons or by silica duripans. Soils with layers of gypsum accumulation gypsic horizons occur to a limited extent in northern and eastern Coconino, Apache and Navajo counties and in the San Simon Valley and possibly elsewhere in Arizona.

Argids have an accumulation of translocated clay in the subsurface layers argillic horizons. Argids usually are on older landscapes. Soil genesis research findings indicate that at least 10, years are required for Argids to form Soil Survey Staff, In fact, many soil scientists believe that argillic horizons in Aridisols developed in the past under a more humid climate. Argids may have other diagnostic horizons as well.

Calcic horizons are the most common, but a few Argids have petrocalcic horizons or duripans. Entisols are soils that show little or no evidence of horizon development. They may have thin surface horizons with some accumulation of organic matter, but they lack enough alteration of parent materials to form other horizons.


Entisols generally are in young landscapes where time has not been sufficient for soils to develop. Some Entisols may occur on older landscapes if they are composed of materials resistant to weathering, or if the climate has been too dry for appreciable soil formation. Entisols are widely distributed throughout all climates in Arizona. Fluvents, Orthents and Psamments are the three suborders of Entisols in the state. Fluvents formed in recently deposited alluvium in floodplains and near stream channels on alluvial fans or piedmont slopes. Fluvents lack horizon development because of flooding at fairly frequent intervals, leaving too little time for significant soil formation between alluvial depositions.

Fluvents are a significant proportion of the irrigated agricultural land in Arizona. Most Orthents in Arizona are shallow over rock, usually less than 50 cm 20 in. They typically occur on steep slopes where soil material is removed so fast that time is insufficient for significant horizon development. Other Orthents include deeper and older soils that lack horizon development because of dry environment.

Psamments are sandy Entisols. Soil textures are uniformly coarse, either sand or loamy sand. They include soils of stabilized sand dunes such as in Yuma, Navajo and Apache counties. Psamments also formed in sandy alluvium. Inceptisols are relatively young soils that lack horizons of illuvial clay accumulation argillic horizons.

They differ from Entisols because of weak to moderate profile horizonation.

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The horizons of Inceptisols result mostly from slight to moderate alteration of the parent material. These alterations may be expressed by soil structure development, carbonate removal and hydrolytic weathering to produce clay, form iron oxide minerals and accumulate organic matter. Inceptisols are limited in Arizona primarily to subhumid regions. Only the Ochrepts suborder of Inceptisols is officially recognized in Arizona, but soils of the Andepts suborder have been identified Hendricks and Davis, Ochrepts recently were recognized officially in the soil survey of central Coconino County Taylor, Ochrepts also are recognized in several of the national forests personal communication, Owen Carlton, Most Ochrepts identified in Arizona are shallow and characterized by weakly developed B horizons cambic horizons.

Because they occur mostly on relatively young geomorphic surfaces, time has limited horizon development. Andepts are associated with pyroclastic materials such as volcanic ash and cinders. Andepts characteristically have [page 69]. Mollisols have thick, dark-colored surface horizons mollic epipedons. Mollisols in Arizona occur at higher elevations under semiarid and subhumid climates in landscapes covered with grass or grass-tree mixtures, although in some areas they are mostly tree covered.

Aquolls, Borolls and Ustolls are the three suborders of Mollisols in Arizona. Limited Aquolls are in the Willcox Playa vicinity. These soils formed because of poor drainage and are generally high in sodium. Borolls are cool and cold Mollisols in Arizona's higher elevations that have a mean annual soil temperature of less than 8 C 47 F at a depth of 50 cm 20 in. Borolls include dark-colored forest and mountain meadow soils and are most common along the Mogollon Rim and in the higher mountain ranges throughout Arizona.

Some Borolls have argillic horizons. Others have formed from calcareous parent materials and have calcic horizons. Ustolls occur under warmer and drier conditions at elevations below Borolls.

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They are fairly widely distributed throughout Arizona under conditions a little cooler and more moist than those of Aridisols. Some Ustolls have argillic horizons. Others have calcic horizons and a few have petrocalcic horizons. Vertisols are clayey soils that have deep wide cracks at some time during the year and that are associated primarily with volcanic basalt rocks in Arizona.

They occur mostly in semiarid to subhumid regions in Yavapai, Coconino, Apache and Navajo counties and in the semiarid San Bernardino Valley of southeastern Cochise County. Vertisols generally are clayey in all horizons. The clay is composed dominantly of montmorillonite. This mineral causes the soil to undergo considerable shrinking and swelling during drying and wetting cycles. As a result, the soils crack and move both horizontally and vertically with changes in moisture content to produce a mixing action.

This action yields uniform textures with depth and causes stones and other rock fragments to move to the soil surface. The soil movement makes Vertisols troublesome for engineering uses and can seriously affect the growth of trees. Building foundations on Vertisols may crack and fences, power lines, highways and trees often become misaligned or variously tilted.

The Usterts suborder represents most Vertisols in Arizona. In addition there are a few Torrerts. Torrerts occur in a more arid climate than Usterts.

Soil families were established based on criteria significant to soil use, management and behavior. Although nine families are defined in Soil Taxonomy Soil Survey Staff, , only three, soil temperature, particle size and mineralogy, are discussed here. These three are the most widely used in soil family groupings.

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The other six are applied only to a limited number of soils. Soil Temperature Families. Four soil temperature families are recognized in Arizona: hyperthermic, thermic, mesic and frigid. They correspond to the four soil temperature regimes defined and discussed in the on climate.

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Soil Taxonomy Soil Survey Staff, also contains four other temperature families: isohyperthermic, isothermic, isomesic and isofrigid. These families experience less difference between mean winter and summer soil temperatures.

Additional Information

Soil temperature has a strong influence on plant growth. For example, commercial citrus production is limited primarily to hyperthermic soils and cotton production is restricted to hyperthermic and thermic soils. The boundary between the pinyon-juniper woodland and the montane coniferous forest zones in the Southwest generally follows the mesic-frigid boundary, although exceptions are known. Particle Size Families. Particle size refers to the grain size distribution of the whole soil and includes stones and gravel as well as sand, silt and clay particles.

All 11 particle size family classes defined in Soil Taxonomy Soil Survey Staff, occur in Arizona soils because of the wide range of soil-forming conditions. Their presence reflects the great diversity of soil parent materials and, to a lesser extent, that of climate in Arizona. Each particle size family is defined by properties that tend to group soils in relation to use and management.