II. DESERT SOILS
53
Removal of vegetation and farming to row—or grain—crops, results
in a deterioration of aggregation and stable structure. Accumulation of
sodium in cultivated soils as a result of irrigating with low-quality (sodic)
water disperses soil and contributes to a less stable structure. The transitory nature of desert soil aggregation is readily demonstrated by saturating
soil with water. Equipment and even animals have been known to sink
to serious depths on flooded land.
G. SOIL TEMPERATURE
Temperature is an important factor in the control of soil formation as
well as the control of the biology of the desert. The rate of chemical reactions, for example, is known to increase about twofold for every 10°C
rise in temperature. Within limits, biological activity in soils also is influenced by temperature in about the same magnitude. Physical changes of
expansion, differential expansion and contraction, are controlled by temperature. The effect of temperature is seriously modified, however, by
desiccation. In this respect desiccation under high temperatures and low
rainfall has a similar effect on the soil-forming processes as does freezing.
Thermal effects on soils are dependent upon the mineral composition, dry
density, as well as moisture content (de Vries, 1958).
Diurnal air temperatures generally are greater in arid than in humid
areas. Heat stored in the soil during the daytime, though, is lost to the
atmosphere at night more rapidly in arid than in humid areas. Convection
accounts for as much as half of the transfer from soil to air at night in
Arizona (Guild, 1950). In California when the air temperature was 47°C
at a level of 60 cm above the soil, the soil temperature was 62°C at 2.5
cm, 42°C at 7.5 cm, and 29°C at 60 cm below the surface (Smith, 1929).
At an air temperature of 48 °C, the range in soil temperature during this
same year was 63°C at the 2.5 cm depth and 17°C at 80 cm. Variations
of this magnitude are not unusual in other desert soils at other locations
around the world. Soils have a great insulating effect against temperature
changes.
VI. Accumulations in Desert Soils
A. CARBONATES
1. Characteristics
Lime, calcium carbonate, is characteristically found in desert soils. It
may be dispersed throughout the soil depth in powder dimension, in soft
53
Removal of vegetation and farming to row—or grain—crops, results
in a deterioration of aggregation and stable structure. Accumulation of
sodium in cultivated soils as a result of irrigating with low-quality (sodic)
water disperses soil and contributes to a less stable structure. The transitory nature of desert soil aggregation is readily demonstrated by saturating
soil with water. Equipment and even animals have been known to sink
to serious depths on flooded land.
G. SOIL TEMPERATURE
Temperature is an important factor in the control of soil formation as
well as the control of the biology of the desert. The rate of chemical reactions, for example, is known to increase about twofold for every 10°C
rise in temperature. Within limits, biological activity in soils also is influenced by temperature in about the same magnitude. Physical changes of
expansion, differential expansion and contraction, are controlled by temperature. The effect of temperature is seriously modified, however, by
desiccation. In this respect desiccation under high temperatures and low
rainfall has a similar effect on the soil-forming processes as does freezing.
Thermal effects on soils are dependent upon the mineral composition, dry
density, as well as moisture content (de Vries, 1958).
Diurnal air temperatures generally are greater in arid than in humid
areas. Heat stored in the soil during the daytime, though, is lost to the
atmosphere at night more rapidly in arid than in humid areas. Convection
accounts for as much as half of the transfer from soil to air at night in
Arizona (Guild, 1950). In California when the air temperature was 47°C
at a level of 60 cm above the soil, the soil temperature was 62°C at 2.5
cm, 42°C at 7.5 cm, and 29°C at 60 cm below the surface (Smith, 1929).
At an air temperature of 48 °C, the range in soil temperature during this
same year was 63°C at the 2.5 cm depth and 17°C at 80 cm. Variations
of this magnitude are not unusual in other desert soils at other locations
around the world. Soils have a great insulating effect against temperature
changes.
VI. Accumulations in Desert Soils
A. CARBONATES
1. Characteristics
Lime, calcium carbonate, is characteristically found in desert soils. It
may be dispersed throughout the soil depth in powder dimension, in soft