Performance-based Assessment of Critical P and K Fertility Regimes for Alfalfa Using Cluster Analysis.

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 P and K levels, herbage 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, 25, 50, and 75 kg P ha^-1) and five K treatments (0, 100, 200, 300, and 400 kg K ha^-1) were replicated four times.  Beginning in 1998, herbage samples were obtained after each forage harvest in May, June, July, and September.  Soil samples were collected in April as well as after each harvest.  Samples were analyzed for P and K concentrations.  Cluster and variance reduction analysis applied to mass shoot^-1 and shoots m^-2 measured in each treatment plot identified 6 clusters with large differences in yield, yield components, and long-term persistence. Results show that balanced fertility is important for maximizing yield and persistence of alfalfa.  Highest fertilizer application did not always result in highest yield indicating that factors other than applied fertilizer affect plant performance.  Low fertilizer application occasionally resulted in high yielding stands for the first four years but moderate to high fertilizer rates were required to maintain high yields beyond four production years.  High yielding stands were associated with large mass shoot^-1 and intermediate number of shoots m^-2.  Taproot nitrogen (N) reserves were also highest in high performing plants.  The accuracy in predicting acceptable or unacceptable performance using herbage K and P concentrations and the ratio between these parameters was evaluated using binary logistic regression (BLR).  Combined, these parameters showed potential as a tool for categorizing alfalfa as having acceptable or unacceptable performance.