Proximal nitrogen sensing by off-nadir and nadir measurements in winter wheat canopy

Proximal sensing opens a wide range of sensor applications with different viewing geometries in precision agriculture. Sensor-based diagnosis of in-season nitrogen (N) status in crops is mostly performed by non-destructive nadir measurements in proximal sensing. In this study, nadir and off-nadir handheld-based spectrometer measurements have been investigated in winter wheat (Triticum aestivum L.) across the growing season (2012 and 2014) to assess N uptake. Selected vegetation indices (VIs) and optimized VIs were related to N uptake. The results indicated good relationships between published VIs and N uptake (r2>0.5) while very good relationships between optimized VIs and N uptake (r2>0.9) were observed. When comparing the viewing geometry, the off-nadir measurements showed higher explained variability with optimized VIs (r2>0.927) and N uptake than nadir measurements (r2>0.912).