II. DESERT SOILS
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which the product of the thickness in inches and the gypsum percent is
equal to or greater than 60% inches.
G. DURIPAN
Silica cements the duripan subsurface horizon. Such layers will not slake
in water or acid. Silica duripans often contain iron oxides and carbonates
as accessory cementing agents. In arid soils, duripans are usually platy
and often capped or coated with opal as well as a microcrystalline form
of silica. Sand grains may appear in duripans cemented together by silica.
IV. Desert Soil Suborders and Great Groups
In the scheme of soil classification the desert order is called Aridisols.
There are two suborders under Aridisols—Argids and Orthids.
A. ARGIDS
Argids are Aridisols with an argillic or silica clay horizon. They characteristically have an illuvial horizon in which silicate clays have accumulated to a significant degree. Although the clay layer is formed below the
surface of mineral soil, it later may be exposed by desert climatic erosion.
Argillic layers do not form readily under arid climates. More often they
are residual, presumably from glacial periods when rainfall was higher.
However, during interglacial or more recent times petrocalcic layers (laminated, indurated limy layer) and duripans (hardpans, cemented with silica
and/or aluminum silicate) may develop in the argillic horizons. Argids
include five great groups: Durargids, Haplargids, Nadurargids, Natrargids,
and Paleargids.
1. Durargids
Durargids are characterized by a platy or massive duripan or hardpan
that is indurated in some subhorizon. These soils are rich in glass, which
is the source of cementing silicate. The duripans thus may be cemented
in part by silica, which exhibits solubility in concentrated alkali, as well
as lime. In the United States deserts, duripans also may possess thin, alternate layers of iron and silica of high density in association with the
upper surface of lime accumulation layer. In this case iron probably solubilized as a result of a "perched" water table on top of the lime accumulation layer as the downward penetration of water was impeded during
periods when water was more plentiful.
39
which the product of the thickness in inches and the gypsum percent is
equal to or greater than 60% inches.
G. DURIPAN
Silica cements the duripan subsurface horizon. Such layers will not slake
in water or acid. Silica duripans often contain iron oxides and carbonates
as accessory cementing agents. In arid soils, duripans are usually platy
and often capped or coated with opal as well as a microcrystalline form
of silica. Sand grains may appear in duripans cemented together by silica.
IV. Desert Soil Suborders and Great Groups
In the scheme of soil classification the desert order is called Aridisols.
There are two suborders under Aridisols—Argids and Orthids.
A. ARGIDS
Argids are Aridisols with an argillic or silica clay horizon. They characteristically have an illuvial horizon in which silicate clays have accumulated to a significant degree. Although the clay layer is formed below the
surface of mineral soil, it later may be exposed by desert climatic erosion.
Argillic layers do not form readily under arid climates. More often they
are residual, presumably from glacial periods when rainfall was higher.
However, during interglacial or more recent times petrocalcic layers (laminated, indurated limy layer) and duripans (hardpans, cemented with silica
and/or aluminum silicate) may develop in the argillic horizons. Argids
include five great groups: Durargids, Haplargids, Nadurargids, Natrargids,
and Paleargids.
1. Durargids
Durargids are characterized by a platy or massive duripan or hardpan
that is indurated in some subhorizon. These soils are rich in glass, which
is the source of cementing silicate. The duripans thus may be cemented
in part by silica, which exhibits solubility in concentrated alkali, as well
as lime. In the United States deserts, duripans also may possess thin, alternate layers of iron and silica of high density in association with the
upper surface of lime accumulation layer. In this case iron probably solubilized as a result of a "perched" water table on top of the lime accumulation layer as the downward penetration of water was impeded during
periods when water was more plentiful.