Yoshida - 1981 - Fundamentals of Rice Crop Science

172 FUNDAMENTALS OF RICE CROP SCIENCE
when large quantities of fresh organic materials such as green manures and straw
are incorporated.
3.16.2. Organic acid in soil solution
The concentration of organic acids increases with flooding, reaches a peak, and
then decreases to practically nothing (Fig. 3.1, Section 3.31).
Organic acids such as formic, acetic, propionic, and butyric acids occur in
submerged soils. Acetic acid is generally the major organic acid produced
(Motomura 1962; Takai and Kamura 1966). Incorporation of organic matter such
as green manure, glucose, and straw promotes the production of organic acids in
submerged soils (Onodera 1929: Takijima and Sakuma 1961; Motomura 1961,
1962; Ishikawa 1962; Gotoh and Onikura 1971).
Soil temperature has a significant influence on the kinetics of organic acid
production. Low temperature leads to a strong accumulation of organic acids (Fig.
3.37; Yamane and Sato 1967; Cho and Ponnamperuma 1971), which suggests that
organic acid toxicity is more likely to occur at low temperatures. Low temperatures
may also aggravate the harmful effects of organic acids by retarding increases
in soil pH. As soil pH decreases, the proportion of injurious undissociated acid
increases, as explained in the next section.
3.16.3. Mode of toxicity
High concentrations of organic acids impair root elongation, respiration, and
nutrient uptake in rice.
Alipathic organic acids are toxic to rice at concentrations of l0 -2 to l0 -3 M,
depending on the kind of acid (Mitsui et al 1959a, Takijima 1963). For monobasic
acids, the toxicity increases with increasing molecular weights. The toxicity of
polybasic acids and hydroxy acids is much milder than that of the monobasic acids.
3.31. Influence of temperature on the kinetics of volatile organic acids (Ponnamperuma
1976).