Chlorophyll Meter Readings to Determine Nitrogen Need in Potatoes At Field Scale.

The need to determine adequate nitrogen (N) levels for potato crops in the Northwest during the growing season has required expensive petiole sample tests. The use of a SPAD Minolta chlorophyll meter by potato producers could reduce sampling costs and provide accurate in-season potato petiole N levels. The objective of this study is to compare the SPAD chlorophyll meter and petiole samples for accuracy in measuring N petiole levels for in-season N fertilization for potato crops. The study consisted of small plot studies in 2008, 2011, 2012, and on-farm studies in 2013. The small plot studies consisted of N treatments of 0, 56, 112, 168, 224, and 280 N kg ha-1 with three to four replications in a randomized complete block design were used to establish different N levels in the potato crop. Fertilizer was split applied with 56 kg N ha-1 applied at planting and remainder applied in June and July.  In 2013, the field scale trials had 20 sites within producer fields.  A single sample (of 20 petioles each) was taken from each field, which followed common practices for leaf petiole sampling).  Petiole nitrate (NO₃^-)samples, chlorophyll meter readings, and soil NO₃^- test were taken weekly for 8 weeks beginning in July through mid August.   Chlorophyll meter readings were compared to petiole NO₃^- and soil NO₃^- levels, which are current methods for determining in-season N fertilization need.  Potato plots were harvested by digging 3 meters of two adjacent rows and yields were determined based on total weight and weight by quality class.  Correlation between SPAD meter and petiole showed that a chlorophyll meter threshold of 47 or greater was equivalent to an 20,000 mg kg-1 petiole NO₃^- For mid-season, the petiole NO₃^- threshold drops to 15,000 mg kg-1 and the correlating chlorophyll reading was 41.  Similarly when the late season petiole NO₃^- threshold drops to 10,000 mg kg-1 the chlorophyll reading remains 41.  The similar chlorophyll meter reading between the mid and late season petiole NO₃^- levels is due to the lack of new vegetative growth and a lower NO₃^- concentration need in the petioles. Yield data did not correlate to N application, petiole chlorophyll meter readings probably due to large spatial variance and a large amount of residual N.  The data shows chlorophyll meter readings accurately indicate N responsiveness of the crop 80-100% of the time when compared to petiole samples. The 2013 field scale data will be needed to refine and support chlorophyll meter readings as a method to base in-season N fertilization decisions in potatoes.