The long-term impact of phosphorus and potassium fertilization on alfalfa yield and yield components

Addition of P and K fertilizer can increase alfalfa (Medicago sativa L.) yield and stand persistence, but the yield components associated with P- and K-induced variation in agronomic performance are not clear. Our objectives were: (i) to determine the impact of P and K nutrition on productivity of a relatively old alfalfa stand; and (ii) determine which yield components are associated with changes in alfalfa forage yield. Treatments were a factorial combination of four P and five K rates replicated four times. Forage harvests occurred four times annually. Plant populations were determined in early December and late May each year. When compared to unfertilized plots, addition of P and K increased forage yield each year. Fertilization with P decreased plants m(-2) at all K application rates, but especially in plots fertilized with P, but not K. By comparison, plots fertilized with K, but not fertilized with P, had the higher plant population densities. Although regression analysis eventually revealed a positive association between forage yield and shoots m(-2) in 2003 and 2004, the greatest forage,yields were not obtained in plots with the greatest plant population densities, shoots plant(-1) or shoots m(-2). Regression and path analysis revealed that improved forage yield in P- and K-fertilized plots was consistently associated with greater mass shoot(-1).