Monitoring nitrogen status of potatoes using small unmanned aerial vehicles

Small unmanned aircraft vehicles (UAV) are potential remote sensing platforms for precision agriculture. However, to be useful for in-season management, nitrogen status needs to be estimated sufficiently early in the growing season. To determine when differences in nitrogen status of irrigated potatoes could be detected, an experiment was established in 2013 with a randomized block design with four N fertilization rates and three replicates. Over the growing season, a small parafoil-wing UAV was used to acquire color-infrared images with pixel sizes between 20 and 25 mm. Two normalized difference spectral indices were determined from image digital numbers, the normalized difference vegetation index (NDVI) and the green normalized difference vegetation index (GNDVI), which were then calibrated using reflectance-based NDVI and GNDVI. Unexpectedly, there were decreases in the NDVI and GNDVI calibrations with increased camera exposure time. After calibration, both NDVI and GNDVI were about equal to indices calculated using reflectances from high-altitude aerial photography and the WorldView-2 satellite. During tuber initiation and early tuber bulking, differences in measured leaf area index (LAI), chlorophyll meter values and spectral indices were only detectable at the lowest N fertilization rate. Later in the growing season, all N treatments could be distinguished in the imagery, but too late to mitigate yield losses from N deficiency. Linear relationships between plot GNDVI and NDVI were hypothesized to differ among N treatments because there would be less chlorophyll content per leaf area. Contrary to the hypothesis, there were no differences among fertilization rates on either of the two sampling dates. Compared with alternative technologies, small UAV platforms and sensors may not provide value to farmers for in-season nitrogen management.