Methods to quantify available silicon (Si) in fertilizers and slags are not yet sufficiently trustworthy. In this study, Si extracted from several sources was analyzed, using the extractors: Na2CO3 + NH4NO3 in varying concentration, time of agitation and of reaction; water; HCl 0,5 mol dm-3; 50 g dm-3 of Na2CO3; 50 g dm-3 of citric acid; 0,5 mol dm-3 of acetic acid; acid cation exchange resin (Amberlite IRC-50, pK 6.1); and the leaching column method. A greenhouse experiment, where 125 kg ha-1 of total Si from 12 different sources was applied on irrigated rice, was also conducted. For the determination of Si, shaking duration was not essential, although 3 h of shaking entailed a statistically superior result. The concentrations of 10 + 16 g dm-3 and 30 + 48 g dm-3 of Na2CO3 + NH4NO3 proved to be the most promising for Si extraction. The smaller concentration (10 + 16 g dm-3) was therefore chosen to evaluate Si sources in relation to settling time. All Si sources increased solubility during the rest time period. The best correlation between Si uptake by the rice plants and Si detected in the various analyzed sources was found in the period between day 5 and 9. According to the results, extractor Na2CO3 + NH4NO3 evaluates Si in fertilizers adequately and can be used as method to determine the potential Si release in the soil and its availability for plants. The most efficient source for Si solubilization for rice was Rhodia, followed by Wollastonita, while the sources MB-4 and blast furnace slag provided less available Si. The acid extractors were more efficient at extracting Si from blast furnace slags and less efficient with Wollastonita. Water was the extractor that presented the lowest Si recovery rate. The best correlation between Si contents and uptake by the rice crop were achieved by the extractors resin Amberlite and Na2CO3 10 g dm-3 + NH4NO3 16 g dm-3, followed by the column-method.
silicon determination; rice; silicate sources
