DETERMINATION OF LOW CONCENTRATIONS OF PHOSPHORUS IN SOIL EXTRACTS USING MALACHITE GREEN

A sensitive method is needed to analyze soil-water extracts that contain P at submicromolar concentrations. The proposed spectrophotometric method, based on the ionic association of malachite green with phosphomolybdate under acidic conditions, was evaluated on soil extracts. The molar absorptivity was 64641 +/- 743 L mol-1 cm-1. Absorbance of the reagent blank was 0.069 +/- 0.004. With 1-cm path length cells and limiting absorbance of 0.004, the detection limit calculated from the Beer-Lambert equation was 6.7 x 10(-8) M orthophosphate P. Recovery of added P in saturated paste extracts averaged 91% with a range of 87 to 94% and averaged 101% with a range of 93 to 109% for 1:1 soil/water extract. The method was successfully applied to saturated paste extracts from soils of varied organic matter content and pH. The proposed malachite green method allows for P determination in restricted volume samples and at concentrations too low for the standard Murphy-Riley method.