Yoshida - 1981 - Fundamentals of Rice Crop Science

MINERAL NUTRITION OF RICE 173
Aromatic organic acids are much more toxic to rice than alipathic organic acids
(Takijima 1963). However, the production of aromatic organic acids in submerged
soils is not well investigated.
Because organic acids are weak acids, dissociation of the acid is strongly
influenced by the solution pH:
(3.41)
A large fraction of the acid exists in the dissociated form at high pHs, while the
undissociated form of the acid prevails at low pHs. For example, the dissociation
of acetic acid is described as:
(3.42)
(3.43)
(3.44)
At pH 4.76, concentrations of the dissociated (CH 3 ·COO – ) and the undissociated
forms (CH 3 ·COOH) become equal.
The relative proportion of the dissociated and the undissociated forms at
different pHs can be easily obtained by converting the above formula to:
(3.45)
Thus, the ratio varies from 10 -1 at pH 3.76 to 1 at pH 4.76, to 10 at pH 5.76 and to
10 2 at pH 6.76.
The undissociated form of the acid at low pHs is absorbed by excised roots faster
than the dissociated form and appears to be toxic to rice (Tanaka and Navasero
1967). At pH 4, organic acids impair the respiratory tate of rice roots while at pH 7
they do not (Table 3.36). In submerged soils, the soil pH tends to become 6.5 to
6.7 within 3 weeks of submergence (Ponnamperuma 1965). Organic acid toxicity
is unlikely to occur at near-neutral pH.
Whether a toxic level of organic acids could accumulate in submerged soils is
still an unsettled question.
Mitsui et al (1959b) found the concentration of all organic acids was 4 × 10 -3 N
in the leachate from pot experiments. They speculated that this concentration
could be toxic to rice if all the acids were butyric acid, which is unlikely under most
conditions. There is also a suggestion that organic acids could be toxic in neutral
submerged soils because the pH of the rice rhizosphere might be much lower than
the pH of the rest of the soil (Chandrasekaran and Yoshida 1973).
The kinetics of organic acid formation in the soil solution led Takijima and
Sakuma (1961) to conclude that concentrations could be high only for a short time