Abstract
The estimation of prediction optimum rates of fertilizer involves fitting some type of statistical model to yield data when several rates of fertilizer are applied. The objective of the work, reported here, was to evaluate four models (square root, quadratic, quadratic- plus- plateau and linear - plus - plateau) commonly used for describing the response of alfalfa (Medicago sativa L., c.v Hassawi) yield to phosphorus fertilizer. The evaluation involved 10 soil sites and 7 rates of fertilizer application. The four models indicated similar maximum yields, but there was considerable disagreement among the models when predicting economic optimum rates of P fertilization. Optimum rate of fertilizer P (mean all sites) as indicated by the four statistical models ranged from 113 to 412 kg P/ha. The quadratic statistical model did not give a valid description of yield response and predicted optimum rates of fertilizer that were too high. The quadratic- plus-plateau model, best described the yield response was observed in this study. The reason for prefering one statistical model over other deserves more attention than it has received in the past.