Yoshida - 1981 - Fundamentals of Rice Crop Science

MINERAL NUTRITION OF RICE 175
which is soluble in water, and the iodide concentration in the soil solution
increases. The concentration of iodide in soil solution was found to be as high as
0.7–3 ppm when iodine toxicity symptoms were observed. This concentration is
far greater than the 0.05 ppm normally found in river and sea waters. In water
culture experiments, iodine toxicity symptoms were induced at a concentration of
about 1 ppm (Watanabe and Tensho 1970).
3.17.3. Plant’s response to iodine toxicity
Visible symptoms of iodine toxicity begin to appear a few weeks after transplanting,
and they are similar to those of zinc deficiency and iron toxicity (see Chapter
4). The alkali soluble iodine content of the affected plants is about 30–40 ppm;
healthy plants usually contain less than 10 ppm. Clear varietal differences in
tolerance for iodine toxicity have been recorded (Watanabe and Tensho 1970).
3.18. SALINITY
3.18.1. Occurrence of salinity
Salinity is defined as the presence of excessive concentrations of soluble salts in
the soil. Major ionic species of salts are sodium, calcium, magnesium, chloride,
and sulfate. Among those, sodium chloride is predominant. Salinity occurs in two
distinctly different regions: coastal regions and arid and semiarid regions. In the
coastal regions, salinity is induced by inundation of sea water; the salinity is often
associated with low soil pH. In the arid and semiarid regions, salinity occurs
mostly in canal-irrigated areas. Evapotranspiration is very high in the arid and
semiarid regions and, as a consequence, water movement is upward, resulting in
the accumulation of salts in the root zone. Such saline soils can be easily recognized
by the white salt crusts on the soil surface. This type of salinity is often
associated with high pH.
In relation to salinity, two soil types are recognized in the arid and semiarid
regions.. Saline soils normally refer to soils with pH lower than 8.5 and electric
conductivity of the saturation extract greater than 4 mmho/cm at 25°C. Sodic soils
are defined in terms of exchangeable sodium percentage (ESP). A soil may be
considered sodic if ESP is more than 6, and strongly sodic if the ESP is greater than
15 (Bhumbla and Abral 1978).
3.18.2. Salts in soil solution
Electric conductivity of either the saturation extract or soil solution collected from
the root zone is normally measured to quantify the degree of salinity. For rice
growing in flooded soils. the two conductivities can be considered comparable.
For upland crops and when soil moisture content is about field capacity or below,
electric conductivity of the soil solution will be about twice as great as that of the
saturation extract (Pearson 1959).
When salinity is moderate, electric conductivity of the saturation extract (ECe)
is related to osmotic potential by the following formula: