Long‐Term Potassium Requirements of Nitrogen‐fertilized Smooth Bromegrass1

AbstractNitrogen fertilization of smooth bromegrass (Bromus inermis Leyss.) has been extensively studied, but relatively little is known about its K requirements. A field study was conducted on a Clarion loam (Typic Hapludoll) site in central Iowa to evaluate the long‐term K requirements of smooth bromegrass grown under high N fertility and intensive harvest management. Forage yield and percent K was observed over 4‐ and 3‐year periods, respectively. Changes in soil exchangeable K and pH at 0 to 7.5‐ 7.5‐ to 15‐, and 15‐ to 30‐cm soil depths were monitored during 1970–75 and 1970–73, respectively. Ammonium nitrate (33.5‐0‐0) was applied at rates of 0, 125, and 500 kg N/ha in 1970, and 0, 250, and 500 kg N/ha during 1971–74 (N0, N1, and N2 treatments, respectively). Applications were made as single levels in 1970. Rates were split in subsequent years, with half applied in early spring and the rest after the second harvest. Potassium treatments consisted of a check and 125 kg K/ha applied each year in early spring as Cl‐ and S‐ containing sources (KC1 vs. K2SO4 and K2SO4 • MgSO4).Nitrogen fertilization produced more of a linear than quadratic increase in bromegrass yield. Yields were not increased by K fertilization even when soil exchangeable K approached “minimal” levels. No yield differences among K sources were observed, suggesting that yields were not enhanced by either Mg or S contained in the K sources.Nitrogen fertilization reduced forage percent K concentrations. Potassium fertilization without N had no effect, whereas, NK fertilization increased forage percent K. Differences in forage percent K among K sources occurred only in 1972 with higher concentrations observed for KC1 compared with K2SO4 and K2SO4 • MgSO4.Fertilization with N reduced soil exchangeable K. Decreases were most evident for the 0‐ to 7.5‐cm soil layer, but were significant to a depth of 15 cm. Potassium fertilization without N increased K in the 0‐ to 7.5‐cm soil layer. Soil K decreased 47% when no K was applied, suggesting that a severely N deficient bromegrass sward, if harvested, requires some applied K to maintain soil K concentrations. When both N and K were applied, soil K decreased to 137 kg/ha in the 0‐ to 7.5‐cm soil layer. These trends observed over time suggest that bromegrass obtains most of its K from surface soil layers.Fertilization at N1 and N2 levels reduced pH in the 0‐ to 7.5‐cm soil layer from 6.3 to 6.0 and 6.2 to 5.3, respectively. Most of the pH change developed during the 1st year as evidenced by linear regression coefficients of —0.0017, —0.0020, and —0.0025 pH units/kg N/ha applied in 1971, 1972, and 1973, respectively. Fertilization with K produced little change in surface soil pH. No effect of N and K fertilization on soil pH was observed below the 7.5‐cm depth.