66
WALLACE H. FULLER
There appears to be a rather close relationship between phosphorus
available to plants, as evaluated by the C 0 2 - H 2 0 extraction method, and
"active" calcium in a single soil at different depths. "Available" P tended
to decrease with depth. The finding that there was no correlation between
"active" calcium and C 0 2 - H 2 0 extractable P for the same depth among
different soils emphasizes that different soils possess inherently different
characteristics, making it improbable that a single threshold value for calcium for all soils below which there will be sufficient P for maximum plant
growth under irrigation can be established readily. The inorganic P content
of calcareous soils, for example, has been shown to decrease in solubility
in the presence of neutral salts as a result of the increasing soluble calcium
( O l s e n s al, 1961).
Organic P content of desert soils in Arizona is shown to decrease with
an increase in soil depth (see Table I ) . Very little organic P is found below
the root zone of plants.
The organic C to organic P ratio for the virgin desert soils is, for the
most part, below that reported for soils of humid regions (Pearson and
Simonson, 1940). Possibly the organic P is associated with a more highly
decomposed organic residue in desert soils.
D. POTASSIUM
Potassium is rarely deficient for plant growth in desert soils. Potassium
is found abundantly in micaceous minerals. Desert soils of North and South
American deserts contain micaceous minerals generally in good supply and
seldom require additions of potassium (Fuller and Ray, 1965). Only total
analyses figures for K in other world desert soils are readily available. Such
data indicate that desert soils usually are well supplied with plant nutrients
except nitrogen (Rozanov, 1951). Only in certain limited areas of
Australia is K found to be deficient for cultivated crops (Stephens and
Donald, 1958) and these do not include the extremely arid desert regions.
They state, however, "that the marginal potassium status of great areas
of Australian soils makes the increased use of potassium fertilizers inevitable." The literature on plant-available K in world deserts virtually is
nonexistent.
E . MlCRONUTRIENTS
Of all the necessary trace or microelements for plant growth, iron and
zinc are found to be the least available in desert soils, although plants in
large areas of the Australian desert respond to copper, and molybdenum
is deficient to plants on salodic soils. Copper deficiencies are reported to
WALLACE H. FULLER
There appears to be a rather close relationship between phosphorus
available to plants, as evaluated by the C 0 2 - H 2 0 extraction method, and
"active" calcium in a single soil at different depths. "Available" P tended
to decrease with depth. The finding that there was no correlation between
"active" calcium and C 0 2 - H 2 0 extractable P for the same depth among
different soils emphasizes that different soils possess inherently different
characteristics, making it improbable that a single threshold value for calcium for all soils below which there will be sufficient P for maximum plant
growth under irrigation can be established readily. The inorganic P content
of calcareous soils, for example, has been shown to decrease in solubility
in the presence of neutral salts as a result of the increasing soluble calcium
( O l s e n s al, 1961).
Organic P content of desert soils in Arizona is shown to decrease with
an increase in soil depth (see Table I ) . Very little organic P is found below
the root zone of plants.
The organic C to organic P ratio for the virgin desert soils is, for the
most part, below that reported for soils of humid regions (Pearson and
Simonson, 1940). Possibly the organic P is associated with a more highly
decomposed organic residue in desert soils.
D. POTASSIUM
Potassium is rarely deficient for plant growth in desert soils. Potassium
is found abundantly in micaceous minerals. Desert soils of North and South
American deserts contain micaceous minerals generally in good supply and
seldom require additions of potassium (Fuller and Ray, 1965). Only total
analyses figures for K in other world desert soils are readily available. Such
data indicate that desert soils usually are well supplied with plant nutrients
except nitrogen (Rozanov, 1951). Only in certain limited areas of
Australia is K found to be deficient for cultivated crops (Stephens and
Donald, 1958) and these do not include the extremely arid desert regions.
They state, however, "that the marginal potassium status of great areas
of Australian soils makes the increased use of potassium fertilizers inevitable." The literature on plant-available K in world deserts virtually is
nonexistent.
E . MlCRONUTRIENTS
Of all the necessary trace or microelements for plant growth, iron and
zinc are found to be the least available in desert soils, although plants in
large areas of the Australian desert respond to copper, and molybdenum
is deficient to plants on salodic soils. Copper deficiencies are reported to