NO-TILL CORN RESPONSE TO NITROGEN RATE AND TIMING IN THE MIDDLE ATLANTIC COASTAL-PLAIN

Improved N use efficiency for no-till corn (Zea mays L.) production on sandy soils may be enhanced with split applications of N fertilizer immediately prior to periods of highest N uptake. A field study was conducted in 1988 and 1989 at the Eastern Shore Agricultural Experiment Station in Painter, VA, on a Bojac loamy sand (coarse-loamy, mixed, thermic Typic Hapludult) to investigate split N applications near silking. Nitrogen treatments (as 30% N-urea ammonium nitrate solution) of 0, 36, 72, 108, and 144 lb N/acre at the 5-leaf growth stage (ENR) were factorially arranged with 0, 25, 50, 75, and 100 lb N/acre applied at the 16-leaf stage (LNR) in a randomized complete block design with three replications. Corn cultivar Pioneer Brand 3320 was planted no-till into a killed winter rye (Secale cereale L.) cover crop onto the same site each spring. At the 14-leaf stage, samples of the leaf immediately below the whorl were collected from each plot and analyzed for N, P, K, Ca and Mg. Yield response was attributed less to timing of N applications than to the total N rate; however, at least 36 lb N/acre were required at the 5-leaf stage to prevent N deprivation and reduction of yield potential. While LNR treatments increased yield at all ENR treatments, the greatest yield response to late N application was obtained between 36 and 72 lb N/acre applied early. At equal N rates, N use efficiency was not significantly affected when fertilizer N applications were split between the 5-leaf and 16-leaf growth stages. Under the high rainfall conditions which increased yield and nitrate leaching potentials, recommended 16-leaf stage N rates for 14-leaf N concentrations of 1.7 to 2.0% ranged from 85 to 92 lb/acre. Calibration of 16-leaf stage fertilizer N requirements with 14-leaf stage critical N values were not successfully developed for leaf N concentrations greater than 2% because yields were not maximized despite total N applications considerably higher than routinely recommended. Diagnosis and Recommendation Integrated System (DRIS) norms developed in Georgia did not accurately diagnose corn N status in Virginia; hence, local calibration of DRIS norms is necessary.