Tyler Nigon, Carl Rosen and David Mulla, Department of Soil Water & Climate, University of Minnesota, St. Paul, MN
Potato (Solanum tuberosum, L.) yield and quality are highly dependent on the availability of nitrogen (N) during the crop’s critical growth stages. One technique that can be used to optimize N availability to the crop is to apply split applications of post-emergence N fertilizer. However, the optimum timing and rate of split applications can vary from year to year because of variation in the weather. To overcome this variation, growers can base post-emergence N fertilizer applications on plant measurements, e.g., petiole nitrate-N (NO3-N) concentration, because they are more responsive to N stress and are easier to obtain than soil fertility measurements. A field experiment with five N treatments ranging from 34-270 kg N ha-1 was conducted in 2010 and 2011 at the Sand Plain Research Farm in Becker, MN to evaluate the effectiveness of using different plant measurements to monitor in-season N stress for two potato varieties (Russet Burbank and Alpine Russet). Measurements were taken on multiple dates throughout the season and included: petiole, leaflet, and entire leaf samples analyzed for both NO3-N and total N concentration; chlorophyll meter readings (SPAD); and spectral indices such as NDVI, NDI2, and SR8 calculated from narrowband reflectance obtained with a Cropscan multispectral radiometer. Of the tissue samples, leaflet N and entire leaf N had the best relationship with Grade A tuber yield, especially at the mid- to late growth stages (r2 = 0.21-0.67). However, NDVI and NDI2 had a better relationship with Grade A tuber yield than any of the tissue samples at all growth stages (r2 = 0.40-0.87). These data suggest that ground based canopy-level spectral data (NDVI or NDI2) are better suited to make in-season N fertilizer recommendations than plant tissue samples because they are affected by leaf area in addition to plant N status.