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Variable rate application technology based on spectral radiance has not previously been used for correcting in-season winter wheat nitrogen (N) deficiencies. Soil and yield mapping has been used to recommend variable amounts of applied fertilizer in crop production, however, both are restricted by the time required to obtain results and their utility is bound by the year in which they were generated. The objectives of this study were to determine the relationship between spectral radiance at specific wavelengths with wheat forage yield and forage N uptake, and to evaluate the potential use of spectral radiance measurements for correcting in-season wheat N deficiencies using sensor-based variable rate technology. Five studies were conducted, three in farmer fields where variable soil N deficiencies were present and two on Oklahoma Agricultural Experiment Station land. Spectral radiance readings for red and near-infrared (NIR) wavelengths were obtained in wheat between Feekes physiological stages 4 and 6 using photodiode-based sensors fitted with interference filters and interfaced to an embedded microcontroller. Correlation between a plant nitrogen spectral index (PNSI), a variation of the normalized-difference-vegetative-index (NDVI), and total N uptake in wheat forage was then established. Based on the PNSI readings, a variable 0 to 112 kg N ha1 topdress N rate was determined for 3 3 m plots and N as urea ammonium-nitrate (UAN) applied accordingly (variable rate). In addition to the variable rate treatment, a fixed rate and a check plot (no N applied) were evaluated in a randomized complete block experiment. The PNSI was highly correlated with estimates of wheat forage N uptake at all locations and stages of growth. Wheat grain yields increased significantly as a result of applying topdress N in both the fixed rate and variable rate treatments when compared to the check (no topdress N applied). However, no significant differences in wheat grain yield were found when comparing the fixed and variable rate treatments. Variable N rate treated plots (based on PNSI) resulted in a total N savings between 32 and 57 kg N ha1 when compared to the fixed topdress N rates. In addition to improving site-specific N use efficiency, this technology will likely decrease the risk that overfertilization poses to the environment.
Stone et al. (Mon,) studied this question.