Phosphorous (P) and potassium (K) fertilization has a great impact on alfalfa yield and persistence. The objectives of this study were to investigate how long-term P and K fertilizer application altered soil test P and K levels, tissue P and K concentrations, and relate these to variation in alfalfa herbage yield and persistence. A factorial combination of four P treatments (0, 25, 50, and 75 kg P per ha) and five K treatments (0, 100, 200, 300, and 400 kg K per ha) were replicated four times. Beginning in 1998, root samples were collected in May and December of each year, and soil and herbage samples were obtained after each forage harvest in May, June, July, and September. Samples were analyzed for P and K concentrations. Root tissues were also analyzed for starch and sugar concentrations. Relationships between soil test P and K levels, tissue P and K concentrations, and herbage yield and plant persistence are currently being evaluated. Acquiring a more complete understanding of alfalfa P and K uptake and critical tissue concentrations will assist us in optimizing fertilization strategies for maximum yield and plant persistence.
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