The silicon availability in paddy soils as predicated by isothermal adsorption curve

PURPOSE: Silicon (Si) is important for rice growth and development. Hence, it is urgent and necessary to evaluate the silicon availability of paddy soils. At present, the content of available silicon in soils is measured with 1 M acetic acid–sodium acetate (pH 4.0) buffer solution to estimate the silicon availability of paddy soils, whereas the method may not be used to evaluate silicon availability of paddy soils which are rich in calcium carbonate, or alkaline or applied with the slag. The sorption equation parameters for predicting the silicon availability of paddy soils are discussed. MATERIALS AND METHODS: Isothermal adsorption with silicon equilibrium solutions (SiO₂ content 0, 10, 20, 40, 60, 80, 100 mg L⁻¹) and a pot experiment with rice plants (CK and Si treatments) were carried out with 18 paddy soils (0–20 cm) collected from major rice-producing areas in Liaoning Province of Northeast China to quantify the relationships between Si sorption parameters and soil silicon supply availability. DISCUSSION AND RESULTS: The results showed that the equation y = bx − a could describe the Si sorption feature of 18 soils, and there were obvious differences in parameters a, b, and a/b of different soils. The y-axis intercept a of the isothermal adsorption curve linear equation indicates the content of exchangeable silicon in soils and can be used as an index representing soil silicon availability. The a value was significantly correlated with soil organic matter (OM), free iron oxide (Fed), activity of Fe oxides(Feₒ/Fed), and crystal/cement ratio ((Fed − Feₒ)/Feₒ), but not with pH. Parameters b and a/b were not significantly correlated with the soil physical and chemical properties. The relative silicon content (SiRE) and the relative yield of rice (YRE) increased with the a value, and the positive linear correlation between SiRE or YRE and a value reached significant p levels of 5% and 1% respectively. CONCLUSION: Isothermal adsorption equation parameter a can be used to predict the soil silicon availability, and silicon fertilizer could obviously improve the rice yield at a ≤ 126 mg kg⁻¹ (SiO₂).