V. NEW APPROACH TO WATER RELATIONS OF DESERT PLANTS
275
ever, becomes significant in those arid areas where marine sedimentary
rocks (Jurassic, Cretaceous, or Tertiary) are exposed, e.g., in the northern
part of the Sahara Desert. Often efflorescence of salt is seen at the limits
of certain layers in these rocks. This salt originated from the seawater and
became included into the rocks during sedimentation of the sea bed. During weathering these salts are exposed at the soil surface and the small
amounts of precipitation wash the salt into undrained depressions. There,
evaporation over an extended period of time causes a considerable accumulation of salt and thus salt pans are formed. The more elevated parts
of the landscape of those regions, therefore, are seldom brackish, whereas
salt pans are frequently found in the lower parts. In the Central Sahara,
which has no rainfall, displacement of salts does not occur and therefore
salt soils are not found. When the water moves underground, salt is deposited where the water comes to the surface and evaporates, then a salt
crust is formed (Fig. 33).
Saline soils can also develop from a previous sea bed or big lake, when
the basin dries out slowly, so that finally only one large or several small
salt lakes remain. Examples of this type of formation of salt soils are the
Caspian depression, the Great Basin with the Great Salt Lake in Utah
and the area around the salt lake, Tuz Gölü, in central Anatolia (Turkey).
Even when the salt content of the ground water is low, a brackish soil
can develop in arid areas with time, if the ground water level is high
enough to moisten the soil surface by capillarity and if water evaporates
continuously. This kind of capillarity-caused brackishness is often climatedependent. Good examples of this correlation are found in Eastern European lowlands, where the temperature increases and the precipitation
decreases from NNW to SSE. Thus, the climate changes from a humid one
to a subhumid, semiarid, and arid climate. Correspondingly the composition of the ground water changes also gradually and the following types
can be distinguished:
1. In extremely humid regions the groundwater is acid and free of
salts. Humus colloids give the groundwater a brown color. On wet
soils oligotrophic bogs develop. In less humid areas, the ground water is
less acid, since small amounts of Ca(HC0 3 ) 2 are dissolved in it and in
wet locations eutrophic bogs usually occur. No brackishness develops from
this type of ground water.
2. In subhumid areas the content of limestone in ground water is considerable, but no highly soluble sodium salts are present. On places where
the ground water level is high, CaC0 3 accumulates at the soil surface. Alkalitrophic bogs, which are characteristic for the transition zone between
forest and forest steppe, develop there.
275
ever, becomes significant in those arid areas where marine sedimentary
rocks (Jurassic, Cretaceous, or Tertiary) are exposed, e.g., in the northern
part of the Sahara Desert. Often efflorescence of salt is seen at the limits
of certain layers in these rocks. This salt originated from the seawater and
became included into the rocks during sedimentation of the sea bed. During weathering these salts are exposed at the soil surface and the small
amounts of precipitation wash the salt into undrained depressions. There,
evaporation over an extended period of time causes a considerable accumulation of salt and thus salt pans are formed. The more elevated parts
of the landscape of those regions, therefore, are seldom brackish, whereas
salt pans are frequently found in the lower parts. In the Central Sahara,
which has no rainfall, displacement of salts does not occur and therefore
salt soils are not found. When the water moves underground, salt is deposited where the water comes to the surface and evaporates, then a salt
crust is formed (Fig. 33).
Saline soils can also develop from a previous sea bed or big lake, when
the basin dries out slowly, so that finally only one large or several small
salt lakes remain. Examples of this type of formation of salt soils are the
Caspian depression, the Great Basin with the Great Salt Lake in Utah
and the area around the salt lake, Tuz Gölü, in central Anatolia (Turkey).
Even when the salt content of the ground water is low, a brackish soil
can develop in arid areas with time, if the ground water level is high
enough to moisten the soil surface by capillarity and if water evaporates
continuously. This kind of capillarity-caused brackishness is often climatedependent. Good examples of this correlation are found in Eastern European lowlands, where the temperature increases and the precipitation
decreases from NNW to SSE. Thus, the climate changes from a humid one
to a subhumid, semiarid, and arid climate. Correspondingly the composition of the ground water changes also gradually and the following types
can be distinguished:
1. In extremely humid regions the groundwater is acid and free of
salts. Humus colloids give the groundwater a brown color. On wet
soils oligotrophic bogs develop. In less humid areas, the ground water is
less acid, since small amounts of Ca(HC0 3 ) 2 are dissolved in it and in
wet locations eutrophic bogs usually occur. No brackishness develops from
this type of ground water.
2. In subhumid areas the content of limestone in ground water is considerable, but no highly soluble sodium salts are present. On places where
the ground water level is high, CaC0 3 accumulates at the soil surface. Alkalitrophic bogs, which are characteristic for the transition zone between
forest and forest steppe, develop there.