Characteristics of soil acidity of paddy soils in relation to rice growth

Abstract

Results of studies on soil acidity of paddy soils obtained in China are summarized. Soil pH may increase from an original value of 4.5–5.0 to 6.0–7.0 upon submergence due to the consumption of protons during reduction processes. The magnitude of increase is related to the amount of organic matter, because it is organic reducing substances that induces the reduction of the oxidized forms of inorganic compounds, notably ferric oxides. Easily-decomposable organic matter may also retard this increase temporarily during its intensive decomposition. Many species of rice plants can grow well at soil pH 5.5. The pH of the rhizosphere of rice is lower than that of the bulk soil. Perhaps the most important direct effect of soil pH on rice growth is the effect on the chemical equilibria of sulfides and ferrous iron, both of which are toxic to rice when present in large amounts. The pH₂S and pS^2- values are related to pH nearly linearly. The distribution among water-soluble, exchangeable and complexed (with the organic matter of the solid phase) ferrous iron is strongly pH-dependent. In the water-soluble portion of iron, the distribution between the ionic form and the chelated form is also linearly related to pH.