Evaluation of Current AOAC Fertilizer Sample Preparation Requirements

The official AOAC fertilizer sample preparation requires that all dry mixtures be ground to pass a U.S. No. 40 sieve. With current fertilizers and mechanical grinders, these criteria may no longer be appropriate. Blended fertilizers were ground and sieved, and the fractions were analyzed separately to show potential variability in results. In general, potassium was heavily concentrated in the smaller particles of the ground sample, whereas phosphorus tended to be concentrated in the coarser particles. A representative set of fertilizers was subjected to 7 grinding treatments designed to produce samples with a wide distribution of particle size. N, P, and K were determined in the samples by traditional methods. Analysis of the results demonstrated that precise, accurate results could be obtained from samples that did not technically meet current sample preparation requirements. The relationship between fineness of grind and size of the analytical sample portion was examined. With proper sample grinding, sample sizes of ≤20 mg could give representative results for nitrogen analysis, with precision equal to or better than traditional Kjeldahl analysis using 1 g samples. This is of particular interest, because various combustion instruments now becoming popular for nitrogen analysis are limited in sample size.