II. DESERT SOILS
43
(1962) reports a higher clay content in the Bt horizon of an old alluvial
soil, than in the parent material or A horizon. This young soil profile developed in a well-drained valley from parent material composed of granite,
rhyolite, and some gneiss, under arid climate, high temperatures, and
sparse vegetation. The clay was formed in situ. Clays form more slowly
in arid soils than in humid or wet soils.
Clay may be found deposited in depressed areas on valley floors, in
playas, and boisons ranging in thickness up to many meters (see Fig. 4).
These clay depressions may be relics of accumulations under sea or lake
conditions during more humid climates or alluvial "end" collections from
flash floods. As the climate became more arid, the water evaporated, leaving basin soils of finely divided colloidal material. Because of slow permeability, water continued to collect in these enclosed drainage basins and
evaporate into the air, leaving behind both clay and salts. Natric (sodium)
and salic (salty) soils (Solonetz, solodized-Solonetz, and Solonchak) may
develop under these conditions. The more soluble salt (sodium chloride)
is located at the center where evaporation takes place last. Sulfates most
often surround the chlorides and carbonates surround the sulfates where
they have precipitated underneath the chlorides and sulfates.
Clay plains appear to have their origin as a result of lacustrine deposiFig. 4. A flat clay basin, very slowly pervious to water (Willcox Playa, Arizona,
U.S.A.). Ray Manley Photo.
43
(1962) reports a higher clay content in the Bt horizon of an old alluvial
soil, than in the parent material or A horizon. This young soil profile developed in a well-drained valley from parent material composed of granite,
rhyolite, and some gneiss, under arid climate, high temperatures, and
sparse vegetation. The clay was formed in situ. Clays form more slowly
in arid soils than in humid or wet soils.
Clay may be found deposited in depressed areas on valley floors, in
playas, and boisons ranging in thickness up to many meters (see Fig. 4).
These clay depressions may be relics of accumulations under sea or lake
conditions during more humid climates or alluvial "end" collections from
flash floods. As the climate became more arid, the water evaporated, leaving basin soils of finely divided colloidal material. Because of slow permeability, water continued to collect in these enclosed drainage basins and
evaporate into the air, leaving behind both clay and salts. Natric (sodium)
and salic (salty) soils (Solonetz, solodized-Solonetz, and Solonchak) may
develop under these conditions. The more soluble salt (sodium chloride)
is located at the center where evaporation takes place last. Sulfates most
often surround the chlorides and carbonates surround the sulfates where
they have precipitated underneath the chlorides and sulfates.
Clay plains appear to have their origin as a result of lacustrine deposiFig. 4. A flat clay basin, very slowly pervious to water (Willcox Playa, Arizona,
U.S.A.). Ray Manley Photo.