Determination of low concentrations of organic phosphorus in soil solution

Phosphorus (P) export from agricultural land is an important source of water-quality deterioration in many areas of the world. Part of the total phosphorus in the soil solution is represented by dissolved organic P at concentrations that can be as low as 1x10(-6) Mof P. The suitability of four digestion methods for the destruction of organic P and determination of orthophosphate at low concentrations and small volumes using the malachite green method has been evaluated. The acid digestion procedures evaluated were 1) sulfuric and perchloric acid, 2) sulfuric acid and potassium persulphate, 3) nitric acid, and 4) nitric and perchloric acid. As inositol hexaphosphate (IHP) represents one of the more resistant molecules to acid hydrolysis in soils, this compound has been chosen to assess the recovery assay of the recommended procedure. The digestion procedures were adapted for the malachite green spectrophotometric method, in order to obtain lower analytical limits for P determination. The sulfuric-perchloric acid digestion gave excellent recovery and reproducibility, and can therefore be used for determining organic P in solution at concentrations as low as 6.45x10(-7) M.