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This article in SSSAJ

  1. Vol. 55 No. 3, p. 892-895
    Received: July 23, 1990

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Determination of Low Concentrations of Phosphorus in Soil Extracts Using Malachite Green

  1. Tsutomu Ohno  and
  2. Larry M. Zibilske
  1. Dep. of Plant, Soil, and Environmental Sciences, Deering Hall, Univ. of Maine, Orono, ME 04469-0118



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 × 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.

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