Russet Burbank and Shepody potatoes were grown with the following four nitrogen treatments: 1) 90 kg ha-1 at planting; 2) 180 kg ha-1 at planting; 3) 90 kg ha-1 at planting followed by an additional 90 kg ha-1 sidedressed after tuber initiation; or 4) 90 kg ha-1 at planting followed by an additional 45 kg ha-1 sidedressing. When compared to the 90 kg ha-1 at-planting treatment, petiole NO3-N concentrations increased rapidly after sidedressing and were relatively constant through mid-season. Sidedressed N significantly increased total yields relative to the 90 kg N ha-1 at-planting treatment by an average of 5.0 t ha-1 in three of nine experiments. Three of the experiments, where yields did not significantly increase, were on sites which were not expected to respond to supplemental N based on petiole NO3-N testing. A red clover green manure crop was the previous crop for two of these experimental sites. Petiole NO3-N testing criteria were only partially effective in detecting sites where response to sidedressed N occurred. When compared to a single application of 180 kg N ha-1 at planting, split application of 90 kg N ha-1 at planting followed by a 90 kg N ha-1 sidedressing significantly reduced total yields in one of nine experiments and did not affect yields in the remaining eight experiments. Tuber uniformity was improved in three of nine experiments by the split-N treatment. Specific gravity was not significantly affected. Use of 45 kg N ha-1 at sidedressing resulted in similar yield as the 90 kg N ha-1 sidedressing, although yield of large-sized tubers was often decreased with the lower N rate. Use of reduced at-planting N rates followed by sidedressed N does not appear to increase yields of non-irrigated Russet Burbank and Shepody potatoes when compared to the at-planting N rates that are currently recommended. This management approach can maintain yields at levels comparable to at-planting N programs and does provide an opportunity to reduce N application rates on sites where soil N reserves and soil amendments may make a substantial N contribution to the potato crop. Sidedressed N application can frequently improve yields and tuber size when potatoes have been under-fertilized at planting; however, some inconsistency in response can be expected in regions that rely on unpredictable natural rainfall.
