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 15NH4‐15NO3 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 15N 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 15N‐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 15NO3 was significantly higher on the average than recovery for 15NH4. 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.
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Roberts et al. (1991) studied this question.