Georgia has an international reputation for the production of short-day, sweet onions, commonly known as Vidalia onions. They are primarily cultivated in the southeastern region of the state, where the favorable climate and naturally low sulfur soils allow growers to produce onions with low pungency levels that garner high returns. Georgia farmers follow precise cultivation practices to ensure high-quality onions; nevertheless, significant challenges persist in optimizing nitrogen (N) fertilization of this crop. In the present dissertation, we conducted four studies that will help provide the information necessary to update N fertilizer recommendations for conventional and organic onions cultivated in Georgia. Results from these studies indicate that N fertilizer requirements for conventional onions will vary depending on rainfall levels but may be reduced from current recommendations. In years with high precipitation, N rates of 135 lb/acre N or more may be necessary to achieve sufficiently high yields of onions, while in drier years, as little as 75 lb/acre N may sustain commercially acceptable crop yields. Nitrogen fertilizer applications at the transplant stage are inefficient, with limited uptake by the onion plant. In contrast, N fertilizers applied during bulb initiation and bulb swelling stages are more efficiently used. Further, the final N application for the season should be carefully managed at bulb initiation to reduce the pungency levels of onions and maintain yields. For organic onions, increased organic fertilizer rates are shown to enhance total yield; however, a mixed-source organic (MIX) fertilizer was more effective than the more commonly used pelleted-poultry litter (PPL) fertilizer, as field studies indicates a reduced less N uptake efficiency by onions fertilized with PPL. Results from a lab-based incubation study using two common Georgia soils indicated that PPL fertilizer had 0% to 22% net N mineralization (Net Nmin) after 120 days of incubation, whereas a MIX fertilizer had 26% to 59% Net Nmin, and a feather meal-based fertilizer provided 42% to 72% Net Nmin. Soil type and temperature had a lesser impact on N mineralization compared to fertilizer source. This suggested that choice of organic fertilizer source is crucial for N availability during the cropping season.