Yoshida - 1981 - Fundamentals of Rice Crop Science

MINERAL NUTRITION OF RICE 171
and Sato 1961) or argon (Connell and Patrick 1969). The evolved hydrogen
sulfide can be collected in an appropriate solution and measured by colorimetry or
by iodometry. In Table 3.35 free hydrogen sulfide is expressed in milligrams of
sulfur per 100 g soil. Assuming that the water content of this system is 200%
(which is an overestimation), the estimated concentration of hydrogen sulfide
(mg/liter) in the soil solution will be obtained by multiplying the figures in the table
by a factor of 5. Free hydrogen sulfide concentrations would then range from 0 to
5.2 ppm, which are quite high compared with the critical concentration of 0.07
ppm. The displacement technique may overestimate the concentration because it
pushes the direction of the equilibrium in favor of increasing free hydrogen sulfide.
Table 3.35 also indicates that additions of ferric hydroxide markedly decrease the
concentration of free hydrogen sulfide in a degraded paddy soil.
The total sulfide concentration can be measured by directly introducing a
reacting agent into the soil solution. The concentrations of total sulfide measured
by this method ranged from 0.29 to 0.37 ppm in the presence of 300 ppm of ferrous
iron at pH 6.5 for a latosolic soil (Tanaka et a1 1968a), and from 1.1 to 1.5 ppm in
the presence of 100–200 ppm of ferrous iron at pH 6.2 for an Akiochi soil in
Korea (Park and Tanaka 1968).
The most recent technique is to simultaneously measure solution pH and sulfide
ions (S 2– ) with an ion-selective electrode. The concentration of free hydrogen
sulfide is calculated by the following formula:
pH 2 S = 2 pH + pS 2- – 20.9. (3.40)
This technique can be used for in-situ measurement of free hydrogen sulfide in the
field (Allam et a1 1972). When this method was applied to rice fields in Louisiana,
USA, the concentrations of free hydrogen sulfide ranged from 5 x 10 -5 to 0.64
ppm with a mean concentration of 0.104 ppm for 53 sites; higher than 0.1 ppm was
found in 16 out of 53 sites. These measured concentrations of free hydrogen sulfide
were 3 to 10,000 times higher than expected from the theoretical calculation.
3.15.3. Varietal differences in tolerance for hydrogen sulfide toxicity
Rice shows some variation in Akiochi resistance (Baba 1955. Yamaguchi et al
1958, Shiratori et a1 1960). Growth differences in Akiochi soils are related to the
root’s capacity for oxidizing hydrogen sulfide and to the degree of starch accumulation
in the basal culm. The resistance to straighthead is also related to the
oxygen-release capacity of varieties. In a study by Joshi et al (1975), the oxygenrelease
capacity of 26 varieties ranged from 0.19 to 1.25 µl/min. Thus, the
tolerance of rice varieties for hydrogen sulfide toxicity appears to relate to the
oxygen-release capacity or to the roots’ oxidizing power.
3.16. ORGANIC ACID
3.16.1. Occurrence of toxicity
Organic acid toxicity may occur in organic soils and poorly drained soils, and