Evaluation of Nitrogen Source and Rate on Irrigated Potato Production in New and Old Potato Fields.

Potato growers often seek fields with no recent history of potato production to reduce disease pressure.  Crop nitrogen response may also differ between new and old fields.  We evaluated responses of Russet Burbank potatoes to nitrogen (N) source and rate in new and old potato fields near Park Rapids, MN, in 2013 and 2014.  Different fields were used each year.  In addition to 115 kg N ha^-1 applied to all plots at other times, N was applied at emergence as Environmentally Smart Nitrogen (ESN – a polymer-coated urea, or PCU) at rates of 0, 90, 135, 180, 225, and 270 kg N ha^-1, or as uncoated urea, ammonium sulfate, or a 1:1 blend of ESN and Duration (a slower-release PCU) at 135 kg N ha^-1.  Petiole NO₃-N increased with N application rate and decreased over time.  In late July 2013 and late June 2014, the old field exhibited a stronger response of petiole NO₃-N to N application rate than the new field, producing a significant rate*field interaction.  Treatments receiving urea or ammonium sulfate had relatively high petiole NO₃-N in late June 2013, but this was no longer the case by mid July. Yield did not respond to N application rates above 90 kg ha^-1 at emergence in 2013, or above 135 kg ha^-1 in 2014, in either field.  In 2014, marketable yields in the old field responded more to N application rate than those in the new field at rates below 135 kg N ha^-1 as ESN.  The proportion of yield represented by large tubers (over 170 g or 280 g) increased with emergence N application rate to at least 270 kg ha^-1 in 2013, but peaked or plateaued at lower rates (90 to 180 kg ha^-1) in 2014.  Nitrogen source only affected tuber yield or size in the 2014 old field, where ESN produced higher yield than uncoated urea.  Soil Verticillium propagules were detected in the old fields in both years, with the 2014 concentration well above the treatment threshold.  In the new fields, propagules were present in very low concentrations in 2013 and not detected in 2014.  These results suggest that utilizing a new field reduces disease pressure.  New fields may also have higher baseline soil N.