II. DESERT SOILS
37
dust; erosion and deposition. Desert lands thus present a high degree of
variable soil patterns which make them most difficult to describe, define
and classify. This makes it necessary to establish, in the classification
scheme, a separate category of very young soils having little if any morphological differentiation into horizons.
Soils belonging to the order of Entisols appear universally under all climatic conditions and are prominent in deserts. Those that are found in
arid and semiarid regions have been called Lithosols, Regosols, and alluvial
soils and lack diagnostic horizons other than ochric epipedons. They occur
on recent geomorphic surfaces, either on steep slopes that are undergoing
active erosion or on fans and flood plains where recently eroded materials
are deposited. The three suborders important to desert areas are Fluvents
(recent alluvial stream deposition material), Orthents (Lithosols of thin,
colluvial-alluvial or in-place material, with decreasing organic matter with
depth), and Psamments (Regosols, Ergs, dunes, sand fields). Neither barren sand dunes nor salt flats of desert playas are considered to be soil
unless they have some vegetation, even if it be ephemeral.
Since many of the characteristic desert surfaces are exposed subsurface
horizons or derived from subsurface horizons or layers, their further discussion will be postponed until characteristic subsurfaces have been given
attention. Excellent reviews of physical desert surfaces are given in publications of Clements et al. (1957), and the Office of Arid Lands Studies,
University of Arizona (Dregne, 1967).
III. Characteristic Desert Subsurface Horizons
Subsurface horizons are those that form below the surface and are most
often thought of as the B horizon, although some subsurface horizons are
exceptions and may be considered as part of the upper A horizon.
A. ARGILLIC
An argillic horizon is a subsurface layer in which silica clay minerals
have concentrated to a significant extent. In some instances erosive forces
have left soils truncated and the clay layer exposed. The argillic layer
forms in the B horizon below the upper A horizon as in situ clay, or it
may occur as a "migrated" clay layer, a remnant or relic of a more humid
climate. The amount of clay in the B horizon necessary for it to classify
as an argillic horizon varies with the total clay content of the A horizon,
but the B always will be appreciably higher.
37
dust; erosion and deposition. Desert lands thus present a high degree of
variable soil patterns which make them most difficult to describe, define
and classify. This makes it necessary to establish, in the classification
scheme, a separate category of very young soils having little if any morphological differentiation into horizons.
Soils belonging to the order of Entisols appear universally under all climatic conditions and are prominent in deserts. Those that are found in
arid and semiarid regions have been called Lithosols, Regosols, and alluvial
soils and lack diagnostic horizons other than ochric epipedons. They occur
on recent geomorphic surfaces, either on steep slopes that are undergoing
active erosion or on fans and flood plains where recently eroded materials
are deposited. The three suborders important to desert areas are Fluvents
(recent alluvial stream deposition material), Orthents (Lithosols of thin,
colluvial-alluvial or in-place material, with decreasing organic matter with
depth), and Psamments (Regosols, Ergs, dunes, sand fields). Neither barren sand dunes nor salt flats of desert playas are considered to be soil
unless they have some vegetation, even if it be ephemeral.
Since many of the characteristic desert surfaces are exposed subsurface
horizons or derived from subsurface horizons or layers, their further discussion will be postponed until characteristic subsurfaces have been given
attention. Excellent reviews of physical desert surfaces are given in publications of Clements et al. (1957), and the Office of Arid Lands Studies,
University of Arizona (Dregne, 1967).
III. Characteristic Desert Subsurface Horizons
Subsurface horizons are those that form below the surface and are most
often thought of as the B horizon, although some subsurface horizons are
exceptions and may be considered as part of the upper A horizon.
A. ARGILLIC
An argillic horizon is a subsurface layer in which silica clay minerals
have concentrated to a significant extent. In some instances erosive forces
have left soils truncated and the clay layer exposed. The argillic layer
forms in the B horizon below the upper A horizon as in situ clay, or it
may occur as a "migrated" clay layer, a remnant or relic of a more humid
climate. The amount of clay in the B horizon necessary for it to classify
as an argillic horizon varies with the total clay content of the A horizon,
but the B always will be appreciably higher.