Yoshida - 1981 - Fundamentals of Rice Crop Science

MINERAL NUTRITION OF RICE 169
Table 3.34. Criteria for predicting slag (calcium silicate) needed for a paddy field. a
SiO 2 content in rice Available SiO 2
Soil straw at harvest (on content in soil Rice-yield response to
class a dry-matter basis) (mg/100 g soil) slag application
(%)
I Less than 11 Less than 10.5 A profitable increase with
the highest probability
II 11–13 10.5–13 A profitable increase
expected in many cases
but not in others
III More than 13 More than 13 No profitable increase
expected
a lmaizumi and Yoshida (1958).
3.15.2. Hydrogen sulfide in soil solution
Sulfate is reduced to H 2 S in submerged soils (see Fig. 3.1, 3.30). The concentration
of free H 2 S, however, may be very low because of the formation of insoluble
sulfides, chiefly, FeS:
(3.31)
Ferrous sulfides are not toxic to rice; however, free hydrogen sulfide is toxic.
Depending on solution pH, H 2 S, HS - , and S 2 - can exist in different concentrations.
The relationships between these three species of free sulfides are given in the
following equations:
Equation 3.32 can be easily converted to:
(3.32)
(3.33)
(3.34)
The ratio of H 2 S to HS - varies with solution pH from 10 at pH 6.0 to 1 at pH 7.0.
Similarly, equation 3.33 is converted to:
(3.35)
It is obvious from equation 3.35 that the concentration of S 2- is negligible
compared with that of H 2 S within pH range from 4 to 8. Thus, H 2 S is the most
dominant species in the soil pH range usually encountered. HS - can be present in
the same quantity as H 2 S at pH 7.0; it is negligible at pHs lower than 6.0.
The occurrence of hydrogen sulfide toxicity in rice in flooded soils has been a
subject for controversy. There are two approaches by which to determine whether