Yoshida - 1981 - Fundamentals of Rice Crop Science

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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
174 FUNDAMENTALS OF RICE CROP SCIENCE Table 3.36. Effect of solution pH and organic acid on respiratory rate of rice roots. a Respiratory rate Treatment (µl O 2 /mg per h) pH 4 pH 7 No organic acid added 6.52 4.27 Acetic acid (10 – 2 N) 1.49 3.63 Butyric acid (10 – 2 N) 1.45 3.73 a Tanaka and Navasero (1967) soon after submergence and that they would affect rice growth only slightly. Most work on organic acids has been done in the laboratory or the greenhouse, which conditions may differ from the field. Gotoh and Onikura (1971) measured organic acids in soil collected from rice fields supplied with 0–15 trice straw/ha. They took into account the effects of pH values on the proportion of undissociated forms of organic acids in the soil solution. They concluded that the organic acids produced by straw are unlikely to be toxic to rice in southern Japan, where high temperatures prevent their accumulation. Low soil pH, low soil temperature, high soil organic matter, and incorporation of fresh organic matter favor an accumulation of organic acids and their toxicity to rice. A combination of these factors may induce the toxicity of organic acids in rice under certain circumstances. Furthermore, it is possible for the concentration of organic acids to reach toxic levels around fragments of organic materials. 3.1 7. IODINE 3.17.1. Occurrence of toxicity A nutritional disorder of rice is known to occur in some volcanic ash soils in northern Japan when upland fields are converted to lowland fields. The disorder is most conspicuous in the first year after conversion. Known as reclamation Akagare disease or Akagare type III (Baba et al 1965), the disorder disappears spontaneously after a few years of rice cultivation. Recent investigations reveal that iodine toxicity is the direct cause of this disease (Tensho 1970, Watanabe and Tensho 1970). 3.17.2. Iodine in soil solution The chemical transformation of soil iodine is governed by the redox potential (Tensho 1970): (3.46) Iodine (I 2 ) is insoluble in water and is held by soil organic matter under upland conditions. Under lowland (reductive) conditions, iodine is converted to iodide,
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FOREWORD Rice is vital to more than
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CONTENTS Foreword Preface GROWTH AN
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3.2. Adaptation to Submerged Soils
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4.5. Corrective Measures for Nutrit
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1 .1. OUTLINE OF THE LIFE HISTORY T
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GROWTH AND DEVELOPMENT OF THE RICE
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Table 7.6. An example of high yield
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REFERENCES AGRICULTURAL POLICY STUD
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Yoshida - 1981 - Fundamentals of Rice Crop Science

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