Evaluation of soil test procedures for determining rice-available silicon

Soil testing for silicon (Si) in the predominantly organic soils of the Eve rglades Agricultural Area has not been sufficiently investigated. The objec tive of this study was to identify a suitable Si extraction method for thes e soils for rice (Oryza sativa L.). Twenty-two surface soil samples were co llected from 10 farms. Of these 22, 10 samples were from fields that had re ceived Si fertilization at some time in the past, and 12 were from fields t hat had not received Si fertilization. Rice was grown for 80 days in each s oil in the greenhouse. Subsequently, the straw and immature panicles were h arvested, dried, weighed, and analyzed for Si. In the laboratory, seven dif ferent methods were used for extracting Si from the soils. The methods were : (1) 0.5 M acetic acid extract, 10:25 (soil:extract, v/v ratio), (2) citri c acid 1%, 1:10, (3) demineralized water, 1:10, (4) TRIS buffer pH 7.0, 1:1 0, (5) water-bath shaking (Batch method) 1:10 v/v ratio plus cation exchang e resin (Amberlite IRC-50 H type), (6) soil incubated with water for 1 week at 40 degreesC, 1:6 (Incubation method), and (7) soil incubated with water for 4 weeks at 30 degreesC, 1:4 (Extended Incubation method). The concentr ation of dissolved Si in soil extracts from all methods was determined colo rimetrically. Soil Si values were correlated with straw Si and panicle Si c ontent. Among the methods studied, extraction with 0.5 M acetic acid appeared most suitable for evaluating Si availability, followed by extraction with citric acid at 1%. These methods provided the best correlations with rice straw a nd panicle Si percentage (r(2) = 0.899***, r(2) = 0.768***, and P = 0.839** *, r(2) = 0.774***, respectively). These methods, being rapid and effective in extracting Si in comparison to other methods, appear to be the most sui table for routine soil testing for Si in the Everglades Agricultural Area i n South Florida.