II. DESERT SOILS
73
vegetation of higher plants. In an Iraq desert soil, cultivated for many centuries and low in humus, the microbial activity was very low (Chandra
et al., 1962). Additions of moisture and organic matter, however, raised
the level of bacteria and fungi to an activity level comparable to a moderately fertile humid-region soil.
5. Groups
The abundancy ratios of microorganisms in six harsh California desert
surface soils followed the order of: aerobes plus actinomycetes >
anaerobes > facultative anaerobes > algae > fungi (Cameron and Blank,
1965). Microaerophilic bacteria were found to be abundant in all desert
soils even when the more common species were impoverished.
B. NITROGEN FIXATION AND TRANSFORMATION
Early workers in the central Asian area claim to have observed an
abundance of nitrogen-fixing Azotobacter and nitrifying bacteria (Kostychev, 1930; Samsonov et al, 1930) in arid soils. A relationship between
the abundance of Azotobacter and soil formation, plant cover, and moisture has been sugggested (Sushkina, 1949). In extremely arid soils of the
Sahara (Pochon et al, 1957), certain central Asian takyr soils (Lobova,
1960), and the Chilean Desert of the Atacama (Cameron et al, 1965),
neither Azotobacter, Clostridia, nor nitrifying bacteria were found. There
was very little difference in the ammonia-oxidizing capacity of the desert
and the more humid soils in Australia (Sims and Collins, 1960). Virgin
red desert and range soils of Arizona contain Azotobacter only occasionally (Martin, 1940), whereas irrigated soils are well supplied. Martin further reported that Azotobacter were absent when the salt content of the
soil solution exceeded about 3000 ppm of calcium or sodium. In contrast,
Utah desert soils held dry at a level of 2% salt still had viable Azotobacter
and ammonifiers after 20 years (Greaves and Jones, 1941). Nitrobacter
and Nitrosococcus, nitrifying bacteria, did not survive. The abundance of
Azotobacter in Egyptian soils along the Nile is claimed to be associated
with the applications of irrigation water from the river (Abdel-Malek and
Isham, 1963), which is well supplied with these organisms, although the
soils of the Central Niger Delta are reputed to be very low in Azotobacter
(Rubenchik, 1960).
C. ALGAE
The ability of algae to withstand adverse habitats of extreme moisture,
temperature, and salt conditions, as well as their capacity to fix carbon
and nitrogen, make them the primitive life that represents the forerunner
73
vegetation of higher plants. In an Iraq desert soil, cultivated for many centuries and low in humus, the microbial activity was very low (Chandra
et al., 1962). Additions of moisture and organic matter, however, raised
the level of bacteria and fungi to an activity level comparable to a moderately fertile humid-region soil.
5. Groups
The abundancy ratios of microorganisms in six harsh California desert
surface soils followed the order of: aerobes plus actinomycetes >
anaerobes > facultative anaerobes > algae > fungi (Cameron and Blank,
1965). Microaerophilic bacteria were found to be abundant in all desert
soils even when the more common species were impoverished.
B. NITROGEN FIXATION AND TRANSFORMATION
Early workers in the central Asian area claim to have observed an
abundance of nitrogen-fixing Azotobacter and nitrifying bacteria (Kostychev, 1930; Samsonov et al, 1930) in arid soils. A relationship between
the abundance of Azotobacter and soil formation, plant cover, and moisture has been sugggested (Sushkina, 1949). In extremely arid soils of the
Sahara (Pochon et al, 1957), certain central Asian takyr soils (Lobova,
1960), and the Chilean Desert of the Atacama (Cameron et al, 1965),
neither Azotobacter, Clostridia, nor nitrifying bacteria were found. There
was very little difference in the ammonia-oxidizing capacity of the desert
and the more humid soils in Australia (Sims and Collins, 1960). Virgin
red desert and range soils of Arizona contain Azotobacter only occasionally (Martin, 1940), whereas irrigated soils are well supplied. Martin further reported that Azotobacter were absent when the salt content of the
soil solution exceeded about 3000 ppm of calcium or sodium. In contrast,
Utah desert soils held dry at a level of 2% salt still had viable Azotobacter
and ammonifiers after 20 years (Greaves and Jones, 1941). Nitrobacter
and Nitrosococcus, nitrifying bacteria, did not survive. The abundance of
Azotobacter in Egyptian soils along the Nile is claimed to be associated
with the applications of irrigation water from the river (Abdel-Malek and
Isham, 1963), which is well supplied with these organisms, although the
soils of the Central Niger Delta are reputed to be very low in Azotobacter
(Rubenchik, 1960).
C. ALGAE
The ability of algae to withstand adverse habitats of extreme moisture,
temperature, and salt conditions, as well as their capacity to fix carbon
and nitrogen, make them the primitive life that represents the forerunner