Optimizing nitrogen rates for rainfed cotton on sandy loam soils of Florida

Nitrogen (N) fertilizer recommendations for rainfed cotton (Gossypium hirsutum L.) in Florida have remained static at 67 kg N ha⁻¹ since 1981. A study conducted at the West Florida Research and Education Center in Jay, FL, in 2022 and 2023, evaluated the response of cotton (cv. DP-2038) to six N rates (0, 50, 101, 151, 202, and 252 kg ha⁻¹), using a randomized complete block design with four replications. Total rainfall during growing season was 923 mm in 2022 and 710 mm in 2023. Results show petiole nitrate N concentration of 5050–9700 mg kg⁻¹ at bloom and 363–1138 mg kg⁻¹ at 4 weeks after bloom is sufficient for rainfed cotton in Florida. Aboveground biomass achieved with 252 kg N ha⁻¹ was 25%–92% higher than with other N rates. The greatest seed cotton (3761 kg ha⁻¹) and lint yield (1817 kg ha⁻¹) were recorded with 151 kg N ha⁻¹, but this was statistically similar (p > 0.05) to 252, 202, and 101 kg N ha⁻¹. Lint turnout decreased with increasing N rate, with maximum in the unfertilized control and minimum with 252 kg N ha⁻¹. Fertilizer N use efficiency (NUE) and internal NUE decreased by 76% and 53%, respectively, with an increase in N rate from 50 to 252 kg ha⁻¹. The best fit linear plateau model indicated an agronomic optimum rate of 127 kg N ha⁻¹. These results show the need to revise N recommendations for rainfed cotton in Florida to maximize yield and economic returns while promoting sustainable agricultural practices.