II. DESERT SOILS
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The mineral nitrogen in desert soils is almost exclusively in the form
of nitrates. The amounts are low, however, and vary from season to season
and with rainfall patterns. Very slowly hydrolizable or stable organic nitrogen compounds account for nearly 30% of total nitrogen in central Asian
sierozem soils (Rozanov, 1951). About 55% of humus substances containing nitrogen are rather easily hydrolizable, whereas in the more humid
northern prairie soils the amount of this form is much less.
The physicochemical properties of Sierozems do not favor the fixation
of humus in the desert soils of the U.S.S.R. (Kononova, 1961). The rapid
cycle of the formation-decomposition microbial process, in the short periods when soil moisture is available, accounts for the formation of substances of the fulvic acid type. The very rapid rate of decomposition of
plant residues in desert soils of Arizona is also reported (Fuller, 1965)
with C to N ratios ranging from 8-10 in the upper few centimeters as compared with ratios of 10-12 in the more humid soils of the Midwest Prairies.
G. SALT
1. Salt Accumulations
As a result of a combination of circumstances peculiar to arid and semiarid lands, salts may collect in desert soils in sufficient concentration to
adversely affect vegetative growth. In fact, areas may be found in all the
major deserts of the world where the land is barren and even microbial
life retarded. Such harsh areas are avoided for irrigation projects or require
reclamation management practices to make them agriculturally productive.
The most widely distributed problems are associated with soils having excessive soluble salts, high exchangeable sodium, or both. These soils are
called saline (salic), alkali (sodic), and saline-alkali, respectively.
2. Salt-Plant Relationship
Salts in desert soils influence plant growth in many ways (Bernstein,
1962). Moisture stress as a result of an increase in the osmotic pressure
of the soil solution by the presence of salt can reduce the uptake of water
by the plant. Thus less water may be available to the plant if the soil solution is saline than if it is not. Moisture stresses can be observed in a field
of alfalfa or cotton in salty spots, for example, in the middle of a hot summer day when the surrounding nonsaline soil areas show no wilting. Vegetation in virgin soils may be seen to drop its leaves earlier in saline than
nonsaline soils. Germinating seeds are sensitive to salinity and alkalinity
of the soil solution. With certain plants this is the most critical or sensitive
69
The mineral nitrogen in desert soils is almost exclusively in the form
of nitrates. The amounts are low, however, and vary from season to season
and with rainfall patterns. Very slowly hydrolizable or stable organic nitrogen compounds account for nearly 30% of total nitrogen in central Asian
sierozem soils (Rozanov, 1951). About 55% of humus substances containing nitrogen are rather easily hydrolizable, whereas in the more humid
northern prairie soils the amount of this form is much less.
The physicochemical properties of Sierozems do not favor the fixation
of humus in the desert soils of the U.S.S.R. (Kononova, 1961). The rapid
cycle of the formation-decomposition microbial process, in the short periods when soil moisture is available, accounts for the formation of substances of the fulvic acid type. The very rapid rate of decomposition of
plant residues in desert soils of Arizona is also reported (Fuller, 1965)
with C to N ratios ranging from 8-10 in the upper few centimeters as compared with ratios of 10-12 in the more humid soils of the Midwest Prairies.
G. SALT
1. Salt Accumulations
As a result of a combination of circumstances peculiar to arid and semiarid lands, salts may collect in desert soils in sufficient concentration to
adversely affect vegetative growth. In fact, areas may be found in all the
major deserts of the world where the land is barren and even microbial
life retarded. Such harsh areas are avoided for irrigation projects or require
reclamation management practices to make them agriculturally productive.
The most widely distributed problems are associated with soils having excessive soluble salts, high exchangeable sodium, or both. These soils are
called saline (salic), alkali (sodic), and saline-alkali, respectively.
2. Salt-Plant Relationship
Salts in desert soils influence plant growth in many ways (Bernstein,
1962). Moisture stress as a result of an increase in the osmotic pressure
of the soil solution by the presence of salt can reduce the uptake of water
by the plant. Thus less water may be available to the plant if the soil solution is saline than if it is not. Moisture stresses can be observed in a field
of alfalfa or cotton in salty spots, for example, in the middle of a hot summer day when the surrounding nonsaline soil areas show no wilting. Vegetation in virgin soils may be seen to drop its leaves earlier in saline than
nonsaline soils. Germinating seeds are sensitive to salinity and alkalinity
of the soil solution. With certain plants this is the most critical or sensitive