Organic vegetable producers in Georgia, USA, utilize a range of amendments to supply nitrogen (N) for crop production. However, differences in soil type, fertilizers and environmental conditions can result in variability in N mineralization rates among commonly utilized organic fertilizers in the region. In this study, the effects of temperature on N mineralization from three commercial organic fertilizers [feather meal (FM), pelleted poultry litter (PPL) and a mixed organic fertilizer (MIX)] in two soil types from Georgia, USA (Cecil sandy clay loam and Tifton loamy sand) were evaluated for 120 d. Net N mineralization (Net N_min) varied with soil type, fertilizer and temperature. After 120 d, Net N_min from the FM fertilizer ranged between 41% and 77% of total organic N applied, the MIX fertilizer ranged between 26% and 59% and the PPL fertilizer ranged between 0% and 22% across all soil types and temperatures. Incubation at higher temperatures (20 °C and 30 °C) impacted Net N_min of FM fertilizer in the Tifton series soil. Temperature and soil type had a relatively minor impact on the potentially mineralizable N of the PPL and MIX fertilizers after 120 d of incubation; however, both factors impacted the rate of fertilizer release shortly after application, which could impact the synchronicity of N availability and plant uptake. Temperature-related differences in the mineralization of organic fertilizers may not be large enough to influence a grower’s decisions regarding N fertilizer inputs for vegetable crop production in the two soils. However, organic fertilizer source will likely play a significant role in N availability during the cropping season.

Efficient nitrogen (N) fertilizer applications in onion (Allium cepa L.) can reduce input costs and improve fertilizer-use efficiency, while maintaining high yields and quality. Understanding the N requirements of onion at different growth stages is necessary to enhance fertilizer N-use efficiency (FNUE). In a two-year study (2021 and 2022), the FNUE of onions was determined at five stages of development (at transplant, vegetative growth, bulb initiation, bulb swelling and bulb maturation). The FNUE was estimated by substituting a conventional N fertilizer (ammonium nitrate) with a 5% enriched ¹⁵N ammonium nitrate at a rate of 22.4 kg·ha⁻¹ N, at one of five application times corresponding to a stage of development. All onions received a season total of 112 kg·ha⁻¹ N. Marketable yield of onions was significantly greater in 2022 compared to 2021 and FNUE was affected by application timing in both years. In 2021, the FNUE at transplant was 8.9%, increasing to 26.4% and 35.28% at vegetative growth and bulb initiation stages, respectively. At bulb swelling and bulb maturation stages, FNUE was greater than 95%. In 2022, the FNUE at transplant was 25.2%. This increased to 75.7% and 103% at vegetative growth and bulb initiation stages, respectively. Results suggest that the application of fertilizer N at transplant is inefficient due to limited plant uptake ability, while N applications during bulb initiation and swelling were the most efficient.