Determination of phosphorus and potassium in commercial inorganic fertilizers by inductively coupled plasma-optical emission spectrometry: single-laboratory validation.

A two-part single-laboratory validation study was conducted for determination of the P and K content in commercial fertilizer materials by inductively coupled plasma-optical emission spectrometry (ICP-OES). While several methods exist for determination of P and K in fertilizer products, the main focus of this study was on ICP-OES determination, which offers several unique advantages. Fertilizer samples with consensus P and K values from the Magruder and Association of Fertilizer and Phosphate Chemists (AFPC) check sample programs were selected for this study. Validation materials ranging from 4.4 to 52.4% P205 (1.7 to 22.7% P) and 3 to 62% K20 (2.5 to 51.5% K) were utilized. Because all P and K compounds contained in fertilizer materials are not "available" for plants to use, this study was conducted in two parts. Part A focused on ammonium citrate-disodium EDTA as the extraction solvent, as it estimates the pool of fertilizer P and K that is considered available to plants. Part B focused on hydrochloric acid as the digestion solvent, as it estimates the total P and K content of the fertilizer product. Selectivity studies indicated that this method can have a high bias for fertilizer products containing sources of phosphite or organic P compared to gravimetric or colorimetric methods that measure just orthophosphate. Provided the analytical challenges outlined in this study are addressed, this method offers the potential for a quick, accurate, and safe alternative for determining the P and K content of commercial inorganic fertilizer materials.