POTATO UPTAKE AND RECOVERY OF NITROGEN-15-ENRICHED AMMONIUM-NITRATE FROM PERIODIC APPLICATIONS

There is a trend in potato (Solanum tuberosum L.) production to make repeated additions of N fertilizer in sprinkler irrigation water for an extended part of the season. More information is needed on scheduling (rate and timing) these N supplements. A 3-yr study on the use of frequent supplements of (NH4)-N-15-(NO3)-N-15 by 'Russet Burbank' potato was conducted on Quincy loamy sand (mixed, mesic Xeric Torripsamment). The purpose of the experiment was to trace changes in the uptake and partitioning of N-15 in vines and tubers from starter and periodic additions totaling 336 kg N ha-1. Labeled N fertilizer was sprayed on a different set of replicated potato plots according to the application schedule. Plant tissue samples were taken and analyzed regularly for labeled N uptake. Whole potato plants over the season recovered a maximum of 61 to 67% of the N-15-enriched increments of fertilizer applied, as compared to a maximum of 42 to 54% partitioning of the labeled N in tubers at final harvest. Recovery of (NO3)-N-15 was significantly higher on the average than recovery for (NH4)-N-15. Furthermore, in many cases the average recovery was highest from labeled N applied during tuber bulking early in July, which suggests that this is a crucial time for maintaining adequate N fertility.