Comparison of three soil test methods for estimating plant-available silicon

Silicon (Si) confers increased disease resistance and nutritional benefits for both sugarcane and rice. Sugarcane is the primary crop grown in the Everglades Agricultural Area (EAA) in south Florida, USA, and production inputs routinely include Si fertilization. Soil testing for Si is based on a 0.5 N acetic acid extraction procedure that was developed for rice grown on the organic and mineral soils found in the EAA. The objective of this study was to compare a Florida based acetic acid extraction protocol with the sodium acetate buffer method used in Japan and Korea, and the 0.01 M calcium chloride method used in Australia. The three procedures were used to extract Si from soil samples collected from 31 countries, collectively representing 137 mineral soils. The collectors were asked to sample Oxisols, Ultisols, and coarse textured soils. A subset of the soil collection was classified as deficient (requiring Si fertilization for rice and/or sugarcane), based on published critical soil-test Si values specific to each extraction procedure. The sodium acetate buffer extracted the greatest amounts of Si (0 to 509 mg kg(-1)), followed by acetic acid (I to 239 mg L-1) and calcium chloride (3 to 109 mg kg(-1)). Acetic acid and sodium acetate buffer soil-test Si values were fairly well correlated (r = 0.77) and both methods performed well across a wide range of soils. Results with calcium chloride were less well correlated with acetic acid (r = 0.73) and were poorly related to sodium acetate buffer (r = 0.57). When considering only the subset of soils testing at or below the critical value, the correlation between acetic acid and sodium acetate buffer extractions was improved (r = 0.84).