AUTOMATED CALIBRATION STAMP TECHNOLOGY FOR IMPROVED IN‐SEASON NITROGEN FERTILIZATION

Midseason fertilizer N is currently recommended using a wide range of differing soil‐test, plant‐test, and soil‐N mineralization procedures. The objective of this paper is to present an applied approach for determining midseason fertilizer N rates. A conventional 2003 Honda FourTrax Foreman ES four‐wheeler (433 cc, 127 cm wheel base, 116 cm wide) with a 3 m wide spray boom and a 1‐m spacing between nozzles was modified to deliver a range of fixed N rates as urea ammonium nitrate (28%N) within a 9 m 2 grid. Within each grid, nine separate 1 m 2 areas exist, whereby each of the four corners receive no fertilizer N. Rates of 22, 45, 67, 90, and 112 kg N ha −1 occupy the other five, 1 m 2 areas within the 9 m 2 grid (termed as an N rate calibration stamp). Traveling at 8 km h −1 (5 mph), consecutive 9 m 2 grids can be applied continuously. Calibration stamps should be applied preplant or soon thereafter and superimposed on top of the farmer practice. By midseason, differences between the 1 m 2 N rate areas can be visualized and a field‐specific topdress N rate prescribed by choosing the lowest N rate where no visual differences were observed between it and the highest rate. Calibration stamps applied preplant or soon after planting can assist in providing visual interpretation of N mineralization + atmospheric N deposition from planting to the time midseason N is applied, and improved determination of topdress N rates.