Improving Optical Sensor Calibration for Winter Wheat Nitrogen Management in on-Farm Conditions.

Several active optical sensor (AOS) systems are commercially available for optimizing variable nitrogen (N) fertilization in winter wheat (Triticum aestivum L.). However, the adoption of these systems is still low due to difficulties of calibrating AOS and selecting the most representative and suitable tramlines or sample strips within each field.
The objective of this study was to evaluate: 1) the sensitivity of sensor-based N-rate prescriptions for winter wheat to the selection of sample strips for AOS calibration in five wheat fields 2) if field parts with representative sample strips and lowest and highest vegetation indices (VIs) values used for AOS calibration are stable during a vegetation season over three winter wheat fields in Poland.
Results showed that sample-strip selection significantly affected sensor-based N prescriptions, which resulted in under- or over-applications of N. This issue may be resolved by collecting VI values of the whole-field during pesticide applications before sensor-based N applications. The VI values from the entire field then can be used to choose the most representative sample strips for successfully AOS calibration.
The location of the representative part of the field for AOS calibration differed significantly during the season. Higher nugget versus sill values of VIs variograms indicate large spatial variability, where low and high VI values often occur next to each other. This observation indicate the need to consider a full range of VI variability for a particular.