Geoscience Reference
In-Depth Information
Excess water related to presence of a permanent water table
. Many recent
alluvia of small streams or the large rivers such as the Volga or the
Amazon exhibit this condition.
Excess meteoric water
results when there is rainfall in excess compared
to the water needed for evapotranspiration, at least if the area is flat and
the substratum rather impermeable (crystalline rock or frozen horizon at
depth). This happens, for example, in Alaska. But in very cold regions
(Greenland, Antarctica), water most often exists as ice and the soil, if
there is one, can be dry, in a way...
Excess water of anthropic origin
. In many countries of the world, rice
is cultivated over large areas on artificially flooded and thus modified
soils.
Oxygenation by the pore water in soils is controlled by two factors: for
the one part, the diffusion of oxygen in water is about 10
-12
g cm
-2
s
-1
or 10,000 times slower than in the gaseous environment of the pores of
a dry soil; for the other part, the solubility of oxygen in water is limited,
although it depends on the temperature (Table 12.1).
Lack of oxygen
Table 12.1
Relation between solubility of oxygen and temperature of water.
Solubility, mg l
-1
Temperature, °C
0
14.6
10
11.3
25
8.26
A few fractions of a millimetre below the surface of a calm water
body, there generally is little oxygen because microorganisms consume
the gas as fast as it diffuses into the medium. This is why owners of
aquaria are strongly advised to install an oxygenating device to avoid
death of the goldfish! In the soil, therefore, anoxia is favoured by excess
water, by a texture and structure leading to low porosity and by strong
biological activity involving rapid consumption of the oxygen present
(related to the temperature of the medium).
Plant roots require mineral elements, water and also oxygen to
ensure respiration of their cells. A deficit of O
2
in the soil arrests root
growth, and causes reduction in absorption of nutrients and diminution
of photosynthesis. In the end, the plant may die (especially young
