62
WALLACE H. FULLER
effectiveness of precipitation and contributes further to aridity and undesirable salt collection in depressions. Overgrazing has an added deterioration effect on desert soils, since animal hooves break up desert pavement
that absorbs the fall of raindrops and destroys its protection against runoff
and compaction.
High evaporation and transpiration losses of moisture from arid soils
further reduce effectiveness of soil moisture. On the other hand, dry sand
has a low coefficient of conductivity and upward movement is slow. In
general, a greater loss of moisture occurs from salty than nonsalty soils.
B. NITROGEN
Of the required plant nutrients, nitrogen is the most limiting for maximum growth in desert soils. Nitrogen is particularly needed to maximize
crop plants when desert soils are irrigated (Fuller and Ray, 1965). Arid
and semiarid lands are characterized by low levels of organic matter and
consequently low nitrogen reserves since the soil organic matter is the
nitrogen carrier.
Nitrogen has been shown to increase the growth of native vegetation
in Arizona range soils (Stroehlein et a/., 1968) under natural rainfall conditions. The effectiveness of nitrogen depends on the amount and distribution of rain at the time soil temperatures are most favorable for plant
growth. In an extension of this study the effect of nitrogen on desert grass
carried over for at least 2 years. Because of the sparse rainfall, downward
leaching of nitrogen is at a minimum compared with that of humid regions.
Arid grassland also produced higher yields and better quality forage in
California (Martin et al, 1957) when supplied nitrogen. The basic concepts of nitrogen reactions in calcareous soils under irrigated agriculture
are not the same as those for soils in humid climates (Fuller, 1963; Fuller
etaU 1950a,b).
Nitrogen relationships in desert soils, undisturbed by man, depend upon
a balance between gaseous loss of nitrogen due to chemical and bacterial
denitrification and N fixation by desert legumes, autotrophic algae, and
free-living soil bacteria. Some small quantities of nitrogen are added to
the soil through the sparse rainfall in deserts. The literature on quantitative
N fixation and loss in virgin desert soils is lacking.
C. PHOSPHORUS
There is little indication that phosphorus is a limiting plant nutrient for
native vegetation in desert soils. On the other hand, certain crops grown
in desert soils under intensive irrigation agriculture are known to respond
to phosphorus (Fuller, 1953). Legumes, generally, have shown the greatest response. There is a remarkably high proportion of legumes present
WALLACE H. FULLER
effectiveness of precipitation and contributes further to aridity and undesirable salt collection in depressions. Overgrazing has an added deterioration effect on desert soils, since animal hooves break up desert pavement
that absorbs the fall of raindrops and destroys its protection against runoff
and compaction.
High evaporation and transpiration losses of moisture from arid soils
further reduce effectiveness of soil moisture. On the other hand, dry sand
has a low coefficient of conductivity and upward movement is slow. In
general, a greater loss of moisture occurs from salty than nonsalty soils.
B. NITROGEN
Of the required plant nutrients, nitrogen is the most limiting for maximum growth in desert soils. Nitrogen is particularly needed to maximize
crop plants when desert soils are irrigated (Fuller and Ray, 1965). Arid
and semiarid lands are characterized by low levels of organic matter and
consequently low nitrogen reserves since the soil organic matter is the
nitrogen carrier.
Nitrogen has been shown to increase the growth of native vegetation
in Arizona range soils (Stroehlein et a/., 1968) under natural rainfall conditions. The effectiveness of nitrogen depends on the amount and distribution of rain at the time soil temperatures are most favorable for plant
growth. In an extension of this study the effect of nitrogen on desert grass
carried over for at least 2 years. Because of the sparse rainfall, downward
leaching of nitrogen is at a minimum compared with that of humid regions.
Arid grassland also produced higher yields and better quality forage in
California (Martin et al, 1957) when supplied nitrogen. The basic concepts of nitrogen reactions in calcareous soils under irrigated agriculture
are not the same as those for soils in humid climates (Fuller, 1963; Fuller
etaU 1950a,b).
Nitrogen relationships in desert soils, undisturbed by man, depend upon
a balance between gaseous loss of nitrogen due to chemical and bacterial
denitrification and N fixation by desert legumes, autotrophic algae, and
free-living soil bacteria. Some small quantities of nitrogen are added to
the soil through the sparse rainfall in deserts. The literature on quantitative
N fixation and loss in virgin desert soils is lacking.
C. PHOSPHORUS
There is little indication that phosphorus is a limiting plant nutrient for
native vegetation in desert soils. On the other hand, certain crops grown
in desert soils under intensive irrigation agriculture are known to respond
to phosphorus (Fuller, 1953). Legumes, generally, have shown the greatest response. There is a remarkably high proportion of legumes present