Tractor-Based Quadrilateral Spectral Reflectance Measurements to Detect Biomass and Total Aerial Nitrogen in Winter Wheat

Spectral measurements can be used to detect the canopy N status and allow locally adjusted N fertilizer application during the growing season. The present study investigates the potential of a tractor-based field spectrometer with a quadrilateral view optical setup to detect aboveground biomass dry weight and total aerial N in winter wheat (Triticum aestivum L.) in a 3-yr field experiment. The measuring time and the weather conditions were chosen, to be able to compare instrument performance under various sky conditions (sunny, cloudy, hazy) and at different solar zenith angles. New and known vegetations indices were tested in dense, high-yielding crop stand (average yields 6-10 Mg ha(-1)) receiving 0 to 210 kg N ha(-1) to detect aboveground biomass dry weight and total aerial N. Validations were performed on large calibration areas of 25 m(2) matching exactly the field of view from the sensor. The results obtained show that strong relationships (R-2 = 0.90) exist between reflectance indices and total aerial N from the end frittering at growth stage CS 29 to flowering at CS 71, whereas aboveground biomass dry weight could be detected with a R-2 of 0.75. A new simple index R-780/R-740 was developed with similar potential to the red edge inflection point (REIP). The optical setup allows for measurements to be highly independent of time of day and sky conditions. Consequently the system promises to contribute to improved N management in heterogeneous fields.