Evaluation and Prediction of Silicon Bioavailability in Diverse Silicon Fertilizers

Silicon (Si) is a widely recognized beneficial element in plants. The application of Si fertilizers has become a rather common agronomic practice for sustainable agricultural production. However, due to the vast diversity of Si fertilizers, no official method is currently available for estimating the availability of Si in different Si fertilizers to direct their field application. In this study, we evaluated the bio-available Si concentration in eight Si fertilizers differing in Si form and pH by using different extraction methods including HCl, NaOH, Amberlite resin, 3-morpholinopropane-1-sulfonic acid (MOPS), ammonium acetate, and citric acid. To test the effectiveness of these methods, we investigated the relationships between bio-available Si and plant growth and Si accumulation by conducting a pot experiment with rice. The results indicate that the Si fertilizers applied increased shoot biomass and grain weight, but no single extraction method could be used to assess the bioavailability of Si in all the Si fertilizers tested. Accordingly, based on Si sources and pH, we classified the tested fertilizers into three categories, i.e. alkaline, acid and amorphous Si fertilizers, to find out their suitable extractant. The correlation analyses clearly show that 1) ammonium acetate could accurately predict Si availability and shoot biomass for alkaline Si fertilizers (Slag and Fulibang), 2) the amount of Si extracted by 0.2 M NaOH from amorphous Si fertilizers (Elkem A, Elkem B and Tangshan) correlated well with Si availability and shoot biomass and 3) the amount of Si extracted by 1.0 M NaOH and citric acid from acid fertilizers (Shanjia 1, Shanjia 2 and Shanying) could be used to assess Si availability and shoot biomass response, respectively. Furthermore, resin extraction was the best predictor of grain yield response for alkaline Si fertilizers (Slag and Fulibang) and acid fertilizers (Shanjia 1, Shanjia 2 and Shanying), whereas the amount of Si extracted by 0.5 M NaOH could satisfactorily estimate the response of grain yield for amorphous Si fertilizers.