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Dep. of Agronomy, Purdue Univ., 915 West State St., West Lafayette, IN 47907-2054
* Corresponding author (jvolenec{at}purdue.edu).
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.
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