II. DESERT SOILS
71
injury can be classified under three headings: (1) general osmotic effect
in water competition not related to a specific ion, (2) specific ion effect
on inhibition of nutrition, and (3) toxic effect, causing injury symptoms
due to excess ion accumulation in the plant. Desert plants vary considerably in their reactions or susceptibility to the salt effects listed. Toxic effect
of specific ions in excess in desert soils is an extensive topic all its own
and cannot be discussed intelligently in the limited space here. The reader
is referred to Bernstein (1962; 1964a,b), Hay ward and Bernstein (1958),
Bernstein and Hay ward (1958), and others referenced earlier. Suffice it
to point out that toxic concentrations of salts do appear in desert soils
more commonly than in humid soils and these excesses influence the distribution and density of desert vegetation in one way or another.
H. RADIOACTIVE DEBRIS
World-wide sampling of soil to evaluate radioactive fallout from nuclear
testing activities has been underway since 1956 in an organized study that
led to accurate quantitative data (Alexander, 1959). Concentration of fallout,
90
Sr, in soils was found to be directly related to precipitation (Meyer
et al, 1968). This verifies the long recognition that precipitation scavenges
tropospheric nuclear debris. Desert soils, therefore, are expected to have
the least
90
Sr deposition, all other factors being equal. This is what generally is found (Meyer et al, 1968). For example, the lowest
90
Sr fallout
on the continental United States has occurred in the arid Southwest
(20-30 mCi/km
2
).
VIII. Desert Soils and Microorganisms
A. MICROBIAL POPULATIONS
1. Nature
The microbial population in desert soils may vary more widely than in
any other climatic zone. Some harsh, barren deserts, such as the Atacama
Desert of Chile, have low populations of microorganisms, particularly the
more common grouping (Cameron et al, 1965). The low numbers are
associated with the extremely low and irregular rainfall and absence of
higher plant life. The microbial population in loose desert sands of the
Sahara (Killian, 1940; Killian and Fehér, 1936) are minimal compared
with most deserts. Cameron claims organisms can have little or no influence on the soil-forming process in extremely arid deserts, therefore little
soil profile development may be observed.
71
injury can be classified under three headings: (1) general osmotic effect
in water competition not related to a specific ion, (2) specific ion effect
on inhibition of nutrition, and (3) toxic effect, causing injury symptoms
due to excess ion accumulation in the plant. Desert plants vary considerably in their reactions or susceptibility to the salt effects listed. Toxic effect
of specific ions in excess in desert soils is an extensive topic all its own
and cannot be discussed intelligently in the limited space here. The reader
is referred to Bernstein (1962; 1964a,b), Hay ward and Bernstein (1958),
Bernstein and Hay ward (1958), and others referenced earlier. Suffice it
to point out that toxic concentrations of salts do appear in desert soils
more commonly than in humid soils and these excesses influence the distribution and density of desert vegetation in one way or another.
H. RADIOACTIVE DEBRIS
World-wide sampling of soil to evaluate radioactive fallout from nuclear
testing activities has been underway since 1956 in an organized study that
led to accurate quantitative data (Alexander, 1959). Concentration of fallout,
90
Sr, in soils was found to be directly related to precipitation (Meyer
et al, 1968). This verifies the long recognition that precipitation scavenges
tropospheric nuclear debris. Desert soils, therefore, are expected to have
the least
90
Sr deposition, all other factors being equal. This is what generally is found (Meyer et al, 1968). For example, the lowest
90
Sr fallout
on the continental United States has occurred in the arid Southwest
(20-30 mCi/km
2
).
VIII. Desert Soils and Microorganisms
A. MICROBIAL POPULATIONS
1. Nature
The microbial population in desert soils may vary more widely than in
any other climatic zone. Some harsh, barren deserts, such as the Atacama
Desert of Chile, have low populations of microorganisms, particularly the
more common grouping (Cameron et al, 1965). The low numbers are
associated with the extremely low and irregular rainfall and absence of
higher plant life. The microbial population in loose desert sands of the
Sahara (Killian, 1940; Killian and Fehér, 1936) are minimal compared
with most deserts. Cameron claims organisms can have little or no influence on the soil-forming process in extremely arid deserts, therefore little
soil profile development may be observed.