Interference problems with phosphoantimonylmolybdenum colorimetric measurement of phosphorus in soil and plant materials

A critical evaluation of potential chemical interference on a molybdenum-based phosphorus ( P) colorimetric method that is used widely for soil, plant, and water research was conducted. A wide variety of elements and compounds [ aluminum ( Al), manganese ( Mn), iron ( Fe), potassium ( K), calcium ( Ca), magnesium ( Mg), sodium ( Na), nitrate ( NO3), and ethylenediaminetetraacetic acid ( EDTA)] commonly found in these materials or extracting solutions used on them were shown to cause interferences. The interferences were subtle but complex, with changes to the rate at which the color product was formed and its apparent final chemical composition. There were interactions between the type of element or compound involved and the concentration of P in the test solution. The effect of nitrate on the color product could not be distinguished from the effect of the cation ( K versus Na) with the nature of salt used because of the complexity of interacting factors. Increasing the time and temperature allowed the final absorbance for the color product to develop and produce more stable values, but the final absorbance was changed by the interfering element. Significant errors in P by the colorimetric method were observed in soil extracts and plant digests, showing that the resulting values were not accurate. Historic data using values measured by this method need re- evaluation. Alternative methods need examination to select for accurately measuring the P content to refine our knowledge of the role of this important element in soil, plant, and water studies.