Evaluation of available silicon content and its determining factors of agricultural soils in Japan

Abstract

ABSTRACTTo evaluate the available silicon (Si) content in agricultural soils in Japan and to investigate the determining factors of this content, we collected 180 soil samples from the surface layer of paddies and upland fields in Japan and determined their available Si contents. A phosphate buffer (PB; 0.02 M, pH 6.9) or an acetate buffer (AB; 0.1 M, pH 4.0) was used to extract available Si from the soil samples, and the Si concentrations in the extracts were determined by inductively coupled plasma-atomic emissions spectroscopy (ICP-AES). The total Si content and selected physicochemical properties were also determined for the soil samples. The median values of the available Si contents by the PB and AB methods were 48.8 and 79.7 mg kg−1 and corresponded to 0.017% and 0.027% of the total Si content, respectively. The overall data showed log-normal distributions. The available Si content of the upland soils was significantly higher than that of the paddy soils by both the PB (p < 0.01) and AB methods (p < 0.05). The available Si contents by the PB and AB methods had a significant positive correlation (p < 0.01) and they had significant negative correlation with the total Si content (p < 0.01). The values of the available Si contents by the PB and AB methods correlated positively with the pH, total carbon (C) content, and dithionite-citrate bicarbonate extractable iron (Fed) and aluminum (Ald), acid oxalate extractable iron (Feo) and aluminum (Alo), Fed-Feo and Alo+1/2Feo values (p < 0.01). A multi-regression analysis indicated that pH, amorphous minerals and crystalline iron (Fe) oxides were the dominant determining factors of available Si in the soils, and these three variables explained approximately two thirds of the variation of available Si content in agricultural soils in Japan. In terms of soil type, Terrestrial Regosols, Dark Red soils and Andosols had relatively high available Si contents, whereas Sand-dune Regosols, Red soils and Gray Lowland soils had relatively low contents. In terms of region, the soils in the Kanto and Okinawa regions had relatively high available Si contents and those in the Kinki, Shikoku and Chugoku regions had relatively low contents. In conclusion, the available Si content and its determining factors for agricultural soils in Japan were quantitatively elucidated, and this will contribute to the establishment of rational soil management —including the application of silicate materials, taking into account the Si-supplying power of the relevant soils—for sustainable and productive agriculture in Japan.