Determination of direct available phosphate in fertilizers by a discrete analyzer: single-laboratory validation

To improve throughput during peak seasonal demand, a screening method for the determination of fertilizer-available phosphate using a discrete analyzer for semi-automation was validated in a single laboratory. The fertilizer materials were extracted using a neutral EDTA-ammonium citrate solution as detailed in AOAC Official Method 993.31. Phosphate was subsequently freed from the matrix and converted to orthophosphate using an alkaline persulfate digestion modified from a U.S. Geological Survey water method. Phosphorus was determined colorimetrically on a discrete analyzer. Twelve check samples from the Magruder Fertilizer Check Sample Program and Association of Fertilizer and Phosphate Chemists Fertilizer Check Program were used for method validation experiments. The proposed method is linear from 0.01 to 20 mg/L (ppm) phosphorus. Recovery for all materials averaged 101%, with a range of 99.2 to 103%. Bias for all materials averaged 0.59% with a range of -0.11 to 1.68%, with bias increasing at concentrations of available phosphate exceeding 40%. The LOD was calculated to be 0.001% available phosphate and the LOQ 0.002% available phosphate. The method was found fit for purpose as a screening method for available phosphate analysis in fertilizers.