Long-term in-season grain sorghum and soybean response to tillage and nitrogen management

Information on tillage and N management is limited regarding in-season N uptake by grain sorghum [Sorghum bicolor (L.) Moench] and growth of soybean [Glycine max (L.) Merr.] on claypan soils in the eastern Great Plains. The objective of this study was to determine the 20-yr effects of tillage (conventional, reduced, and no-till) and N fertilization (no-N control; knifed anhydrous NH3; broadcast urea-ammonium nitrate solution, UAN; and broadcast dry urea) on grain sorghum N uptake and soybean biomass and their relationship to long-term yields of those crops grown on a claypan soil. Nitrogen uptake at the boot stage during the first 10-yr period was more than 20% greater with either conventional or reduced tillage compared with no-till, and often greater with anhydrous NH3 and urea N sources than with UAN solution or the no-N control. During the second 10-yr period, N uptake was generally greater with conventional and reduced tillage than with no-till at the 9-leaf stage, but this response did not persist to the boot stage. Anhydrous NH3 often resulted in greater N uptake at both the 9-leaf and boot stages compared with the other N sources. The relative yield increases with N uptake at the boot stage appeared due to increasing kernels head(-1) but diminished as uptake exceeded 70-80 kg N ha(-1). Although dry matter measured at the R2 growth stage of soybean rotationally grown in even-numbered years was about 10% greater with tilled options than with no-till, biomass was not related to yield increases.