Yoshida - 1981 - Fundamentals of Rice Crop Science

170 FUNDAMENTALS OF RICE CROP SCIENCE
free hydrogen sulfide can really be present in concentrations toxic to rice in the soil
solution of submerged soils.
The first approach is to calculate H 2 S concentrations based on the chemical
equilibria. As is understood from equation 3.31, the concentration of H 2S increases
with a decrease in pH:
p H 2 S = 2 pH – p Fe 2+ – 3.52. (3.36)
Similarly, the concentration of Fe 2+ also increases with a decrease in pH:
Combining these two equations:
p Fe 2+ = 2 pH – 10.8. (3.37)
p H 2 S = 2 pH – (2 pH – 10.8) – 3.52
= 10.8 – 3.52 = 7.28. (3.38)
[H 2S] = 10 –7.28 mol/liter = constant
= 5 × l0 –8 mol/liter = 0.0017 ppm. (3.39)
Thus, in the presence of sufficiently large amounts of ferrous iron such as in
latosolic soils, the concentration of H 2 S is independent of solution pH, and that
concentration is far below the critical toxic level (Ponnamperuma 1965). H 2S
toxicity, however, may occur in soils low in free iron oxides, such as degraded
paddy fields in Japan.
Tanaka et al (1968a) considered the FeCO 3 – FeS system in the presence of the
high partial pressure of carbon dioxide in submerged soils. High concentrations of
both ferrous iron and hydrogen sulfide can be present in the soil solution provided
carbon dioxide is produced faster than hydrogen sulfide. FeS is converted to
FeCO 3 by carbon dioxide and, as a consequence, free hydrogen sulfide comes into
solution. Thus, two theoretical considerations lead to entirely different conclusions.
The second approach is to measure hydrogen sulfide concentrations experimentally.
The most common method to measure free hydrogen sulfide in the laboratory
is to bubble the incubated soil with nitrogen (Suzuki and Shiga 1953, Yamane
Table 3.35. Free hydrogen sulfide (H 2 S) formation in a degraded paddy soil under
laboratory conditions. a
Amount of ferric H 2 S formed (mg S/100 g soil) in soil incubated for
hydroxide added
(g/100 g soil) 4 days 7 days 14 days 21 days 28 days
a Shiga (1962).
0 0.31 0.51 0.68 1.03 0.86
0.025 0.28 0.35 0.41 0.39 0.38
0.05 0.25 0.22 0.26 0.20 0.20
0.10 0.15 0.14 0.08 0.13 0.07
0.50 0.06 0.06 0.04 0.05 0.03
1.0 0.02 0.08 0.03 0.03 0.02
2.0 0.02 0.04 0.00 0.03 0.02