Although long-term field experiments provide a valuable resource to assess crop yield response to climate and fertilizer, few studies have included grain sorghum (Sorghum bicolor L.). The objectives of this study were to quantify the effects of 55 yr of annual nitrogen (N), phosphorus (P), and potassium (K) application on irrigated continuous sorghum grain yield, grain nutrient uptake, and economic optimum N rates. Six N rates (0, 45, 90, 134, 179, and 224 kg N ha−1) and three combinations of P and K (0 P with 0 K, 20 kg P ha−1 with 0 K, and 20 kg P ha−1 with 37 kg K ha−1) were applied annually from 1961 to 2015 to a Ulysses silt loam near Tribune, KS. Average maximum grain yield with N was 53% greater than with no N applied; however, application of 20 kg P ha−1 with N resulted in a 70% increase in average maximum grain yield. Potassium fertilization had no effect on grain yield. The N rate required for maximum profit at 20 kg P ha−1 averaged 137 kg N ha−1. At the economic optimum N rate, apparent fertilizer N recovery in grain was 25% with no P and increased to 42% with P. Apparent fertilizer P recovery at the economic optimum N rate was 51% with 20 kg P ha−1. Fifty-five yr of irrigated sorghum response to N and P fertilization demonstrated a strong positive interaction between N and P on grain yield, apparent N and P recovery, and profitability.
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