Influence of Phosphorus and Potassium on Alfalfa Yield and Yield Components

Phosphorus and K fertilization increases alfalfa (Medicago sativa L.) yield and stand persistence, but the changes in yield components as affected by P and K fertility level are not known. Our hypothesis is that P and (or) K fertilization will increase one or more alfalfa yield components, and those component responses may change with stand age. The objectives of this field study were to determine the impact of P and K fertilization on alfalfa forage yield and yield components during the initial 3 yr after establishment. Treatments were the factorial combinations of four P rates (0, 25, 50, and 75 kg P ha−1) and five K rates (0, 100, 200, 300, and 400 kg K ha−1) arranged in a randomized complete block design with four replications. Forage harvests occurred four times annually, and yield, mass shoot−1, and shoots area−1 were determined. Plant populations were determined in early December and late May each year. Incremental additions of P and K increased alfalfa yield in each year. Potassium fertilization did not influence plant population, while robust P-responsive alfalfa plants apparently crowded out smaller, less vigorous plants thus decreasing plants m−2 Stand assessments based on shoot counts, or aboveground plant counts may not accurately indicate alfalfa yield potential. Shoots plant−1 was not affected by application of either nutrient, while shoots m−2 generally declined with increased P and K fertilization. Improved forage yield of P- and K-fertilized plots was consistently associated with greater mass shoot−1 Because fertilizer-responsiveness is closely associated with greater mass shoot−1, cultivars possessing this trait may be relatively more productive under well-fertilized conditions.