SOIL SILICON EXTRACTABILITY WITH SEVEN SELECTED EXTRACTANTS IN RELATION TO COLORIMETRIC AND ICP DETERMINATION

Abstract

The importance of silicon (Si) in nutrition is currently being recognized by its beneficial effects in many crops. At present, various procedures are being used to extract Si and to determine soil Si status. This study was undertaken to evaluate the relationships among various extractants and to compare inductively coupled plasma (ICP) Si measurement with the molybdenum blue colorimetric method for possible incorporation of Si determination into existing routine soil and water testing procedures. Seven extractants were evaluated on 30 Louisiana soils. These were: deionized (D.I.) water, 0.5 M acetic acid, 1 M sodium acetate buffer (pH 4.0), 0.5 M ammonium acetate (pH 4.8), 0.1 M hydrochloric acid, 0.5 M citric acid, and Mehlich III. Soil-extractable Si determination by ICP correlated highly with that by colorimetric analysis in HCl, citric acid, acetic acid, acetate buffer, and ammonium acetate extractions (R2 ≥ 0.972, P < 0.001). The correlation between ICP and colorimetric determinations was low for Mehlich III (R2 0.762) and for H2O (R2 = 0.576) extractions. The latter has significant implications for interpreting Si status in soil water extracts or surface water samples. The amount of extractable Si resulting from use of different extractants was in the order of Mehlich III > citric acid > HCl > acetic acid > acetate buffer > NH4OAc > D.I. water, as determined by colorimetry. Silicon extracted by different extractants was well correlated among citric acid, HCl, acetic acid, acetate buffer, and NH4OAc (R2 0.611, P < 0.001). Water and Mehlich III showed poor correlations with other extractants (R2 ≤ 0.430). The results suggest that these seven extractants characterize different pools of Si-supplying capacity of the soil: extractable by water, extractable by any of HCl, citric acid, acetic acid, acetate buffer, and NH4OAc, and extractable by Mehlich III.