II. DESERT SOILS
59
Iron leaves the crystal lattice of silicates readily under high-temperature
conditions during the short periods of wetting. Dehydrated iron hydroxide
formed upon drying at elevated temperatures appears as films or touches
enveloping the soil minerals and sand as well as isolated scales on minerals.
The amount of ferrugination depends on the iron content of the mineral
from which the soils develop. Within a given area or zone the ferruginous
coloration is more distinct in sandy or coarse-textured soils than in fine-textured soils.
3. Distribution
Rozanov (1951), who studied the chemical composition of desert soil
colloids, found a high absolute as well as relative content of "free" Fe 2 0 3 .
Iron oxides in the form of goethite were found in the colloidal fraction
of a number of desert samples. X-ray lines for goethite appeared in upper
as well as lower horizons of sierozem soils. Soils of arid Arizona also contain a higher absolute "free" Fe 2 O a content than the parent materials from
which they originated.
E. SOLUBLE SALTS
7. Characteristics
Aqueous extracts of desert soils generally contain more salt than extracts
of soils of humid climates. Depressed alluvial soils are particularly abundant in soluble salts. Their presence as white powder or black crusts has
been used to classify and identify the soils on which they lie. Highly soluble
sodium and potassium salts also may be found buried and disguised beneath the soil surface in soils derived from saline shales and chloridecontaining gypsiferous materials. Large gypsum mineral deposits are well
distributed in arid lands, while salt beds of nitrate and borax are found particularly in extremely arid climates such as parts of the Atacama and Death
Valley deserts. Desert basins, pans, and playas at one place or another,
have supplied sodium chloride (salt) for human and farm animal consumption. Salt springs bring briny waters from ground waters to the surface
where they either evaporate and salt accumulates or they join and pollute
the more potable surface streams and rivers.
Lack of adequate drainage has resulted in movement of salts upward
by capillary action to the surfaces of certain soils with consequential deterioration of desert land. Irrigation has accentuated this damaging effect
on land productivity. Soluble salts represent an economic problem when
they are allowed to accumulate on the surface and within the root zone
of plants.
59
Iron leaves the crystal lattice of silicates readily under high-temperature
conditions during the short periods of wetting. Dehydrated iron hydroxide
formed upon drying at elevated temperatures appears as films or touches
enveloping the soil minerals and sand as well as isolated scales on minerals.
The amount of ferrugination depends on the iron content of the mineral
from which the soils develop. Within a given area or zone the ferruginous
coloration is more distinct in sandy or coarse-textured soils than in fine-textured soils.
3. Distribution
Rozanov (1951), who studied the chemical composition of desert soil
colloids, found a high absolute as well as relative content of "free" Fe 2 0 3 .
Iron oxides in the form of goethite were found in the colloidal fraction
of a number of desert samples. X-ray lines for goethite appeared in upper
as well as lower horizons of sierozem soils. Soils of arid Arizona also contain a higher absolute "free" Fe 2 O a content than the parent materials from
which they originated.
E. SOLUBLE SALTS
7. Characteristics
Aqueous extracts of desert soils generally contain more salt than extracts
of soils of humid climates. Depressed alluvial soils are particularly abundant in soluble salts. Their presence as white powder or black crusts has
been used to classify and identify the soils on which they lie. Highly soluble
sodium and potassium salts also may be found buried and disguised beneath the soil surface in soils derived from saline shales and chloridecontaining gypsiferous materials. Large gypsum mineral deposits are well
distributed in arid lands, while salt beds of nitrate and borax are found particularly in extremely arid climates such as parts of the Atacama and Death
Valley deserts. Desert basins, pans, and playas at one place or another,
have supplied sodium chloride (salt) for human and farm animal consumption. Salt springs bring briny waters from ground waters to the surface
where they either evaporate and salt accumulates or they join and pollute
the more potable surface streams and rivers.
Lack of adequate drainage has resulted in movement of salts upward
by capillary action to the surfaces of certain soils with consequential deterioration of desert land. Irrigation has accentuated this damaging effect
on land productivity. Soluble salts represent an economic problem when
they are allowed to accumulate on the surface and within the root zone
of plants.