Evaluation of the nitrogen sufficiency index for use with high resolution, broadband aerial imagery in a commercial potato field

The nitrogen sufficiency index (NSI) can be used for in-season variable rate management of nitrogen (N) fertilizer to maintain productivity of potato (Solanum tuberosum, L.) while reducing leaching losses. The objective of this study was to evaluate the implications of using high spatial resolution broad-band imagery for determining N prescriptions at different growth stages. Aerial images were obtained for research plots, as well as for a commercial potato field (59 ha) near Becker, Minnesota on 30, 56 and 79 days after emergence (DAE) with a Redlake MS4100 multispectral camera. In research plots, experimental treatments included five N treatments with varying rates and timing of N fertilizer, and two potato varieties, Russet Burbank and Alpine Russet. Spectral indices investigated in this study adequately predicted N stress based on leaf N concentration (r ² values within dates ranged from 0.49 to 0.82). On 56 and 79 DAE, the Green Ratio Vegetation Index (GRVI) normalized by an NSI that used the recommended rate and timing from the research plots as a reference showed that most areas of the commercial field did not require supplemental N fertilizer (using an NSI over-sufficiency threshold of 120 %). Based on regional guidelines, N was over-applied to the commercial field, but in situations where N is applied more sparingly, a GRVI NSI threshold of 80 % should be used to identify areas that are most suitable for supplemental N fertilizer. A practical approach and the implications associated with using spectral data for in-season N management are proposed.