Colorimetric method for phosphorus measurement in ammonium oxalate soil extracts

Ammonium oxalate extractable phosphorus represents a potentially useful measure of the P associated with the amorphous iron and aluminum oxides in soils. However, because oxalate complexes molybdate, the colorimetric determination of phosphorus in ammonium oxalate extracts by the traditional molybdenum‐blue procedure is not possible. In this study, a method was developed whereby excessive molybdate was added to the extracts to complex the oxalate and eliminate the interference of oxalate with the formation of the phospho‐moblydate complexes necessary for color development. The method development and recommended procedure for colorimetrically measuring phosphorus in ammonium oxalate soil extracts are described.