Determination of Phosphate in Selective Extractions for Soil Iron Oxides by the Molybdenum Blue Method in an Automated Continuance Flow Injection System

Phosphorus (P) mobility and bioavailability in soil depend on the pools in which it resides. The identification of such pools is essential for the full understanding of P behavior in soil. We evaluated the molybdenum-blue ascorbic-acid method in an automated continuous flow injection analyzer (MB-FIA) to analyze P in two selective soil extractions for soil iron oxide (i.e., the acid ammonium oxalate, AAO, and citrate-dithionite, CD, procedures). Uninterrupted development of the color reaction was obtained at citrate and oxalate concentrations less than 50 mM and 35 mM, corresponding to molybdate/citrate and molybdate/oxalate molar ratios of 2.4 and 3.4, respectively. High precision (% RSD, 1.7 +/- 1.3% for CD and 2.9 +/- 3.7% for AAO), accuracy, and recoveries (105.7 +/- 1.0% for CD and 99.8 +/- 3.8% for AAO) were obtained. This study demonstrates the utility of the MB-FIA system for the determination of AAO- and CD-extractable inorganic P.