Modification of the AB-DTPA Soil Test to Correct BLUE-P Analysis.

The phospho-molybdenum blue color intensity and stability, in relation to phosphorus concentration in solution and color, are greatly influenced by the presence of large amounts of ammonium.  Our objective was to study the stability with time of the Soltanpour and Schwab blue-P calibration graph.  Standard solutions containing 0.00, 2.50, 5.00, 10.00, 20.00 and 40.00 mg P L^-1 were made in AB-DTPA extracting solution.  A calibration graph with an acidity of 0.18 M H₂SO₄ was prepared by placing a 0.25-ml aliquot of standard solution in a 2.5 cm matching spectrometric tube.  Then, 0.25-ml of 5 M NaOH was added and the reaction was allowed to occur for 15 min.  Then, 10.0-ml of pure water and 2.0-ml of color developing reagent were added.  Absorbance readings were obtained at 882 nm, at 10, 20, 30, 40, 60, 90 and 120 min. after the addition of the color developing reagent.  Results indicated that concentrated 5M NaOH greatly enhanced the stability of the phospho-molybdenum blue color with time for P concentrations up to 0.80 mg L^-1 in the AB-DTPA  extracting solution.