Recommendations regarding P and K fertilization, as well as critical soil and tissue concentrations for optimal growth of alfalfa (Medicago sativa L.) differ throughout the literature.  The objectives of this study were to investigate how long-term P and K fertilization affect soil test and tissue P and K levels, and alfalfa growth and how this information can be used to predict alfalfa agronomic performance.  A factorial combination of four P treatments (0, 50, 100, and 150 lb P₂O₅ per acre) and five K treatments (0, 100, 200, 300, and 400 lb K₂O per acre) was replicated four times.  Beginning in 1998, herbage and soil samples were obtained after each forage harvest in May, June, July, and September.  Tissue and soil samples were analyzed for P and K concentrations.  Cluster analysis was used in the statistical analysis because inherent plot-scale variation in soil P and K levels confounded analysis of treatment effects using ANOVA and regression.  Cluster and variance reduction analysis applied to mass/shoot and shoots/ft² identified 6 clusters with large differences in yield, yield components, and plant persistence.  Results indicate that balanced fertility is important for maximizing yield and persistence of alfalfa.  Low P and/or low K fertilization generally led to small but sometimes many shoots per area.  High mass per shoot was clearly more important than a large number of shoots per area to obtain high yield.  Highest fertilizer application rate did not always result in highest yield and greatest persistence.  Although low fertilizer rates can result in high yielding stands for the first four years, moderate to high fertilizer rates are required to maintain high yields beyond four years.  This information will aid in the evaluation of existing alfalfa stands and the development of improved alfalfa fertilizer recommendations.