Nitrogen contamination in ground water of potato (Solanum tuberosum L.) producing areas has indicated a need for improved management of N and water, particularly on sandy soil. Therefore, a field experiment was conducted with the objective of following the recovery and partitioning of starter15NH4 and15NO3 into potato plant tops and tubers in conjunction with additional supplementarily applied NH4NO4. Potato plants treated with starter15NH4 or15NO3 tended to increase the percent recovery of starter15N in tubers sampled from one growth time to the next to reach nearly 40% recovery toward the end of the season. Whole plants reached peak recovery of around 50% of the starter15N near mid-season. From then on, there was a trend for loss of starter15N by senescence, defoliation or translocation to the roots. The percent recovery of starter15N was significantly higher at final tuber harvest (not whole plants) for the treatment with starter15NH4 at 112 kg ha−1 combined with 112 kg ha−1 of supplemental N as compared to the treatment with 112 kg ha−1 of starter15NH4 plus 224 kg ha−1 of supplemental N. This difference may have been a result of isotope dilution. Early in June the accumulation of starter15NO3 in whole plants was about five times as high as that from starter15NH4. Later there was no difference in percent recovery of these two forms of N. The temporary delay in starter15NH4 uptake was probably related to slow nitrification early in the season instead of preferential uptake of starter15NO3.