Nitrogen rate optimization for irrigated cotton in Florida

Abstract For irrigated cotton ( Gossypium hirsutum L.) in Florida, the current nitrogen (N) fertilizer recommendation is 67 kg N ha −1 and has not changed in the last 40 years despite changes in cultural practices and development of new varieties. A study was conducted at three locations to re‐evaluate cotton [Delta Pine 2038 B3XF (DP 2038)] response to six N rates (0, 50, 101, 151, 202, and 252 kg ha −1 ), using a randomized complete block design with four replications on sandy soils. The objectives of this study were to quantify N rate effects on (1) growth, (2) in‐season petiole nitrate‐N (PNN), and (3) yield and N use efficiency, with the goal of N rate optimization. Results indicate that leaf area index was maximized at 101–151 kg N ha −1 . Application of 101 kg N ha −1 maintained PNN sufficiency throughout bloom. PNN between 7800 and 8692 mg kg −1 at bloom, and 1733 and 4500 mg kg −1 at 4 weeks after bloom can be considered sufficient for optimum yield. Statistically, no significant increase in biomass and lint yield was found beyond the application of 101 kg N ha −1 . A negative correlation was found between N applied and fertilizer N use efficiency ( r = −0.85), and internal N use efficiency ( r = −0.61). The best‐fit linear plateau model showed 113 kg N ha −1 as the agronomic and economic optimum N rate for irrigated cotton in Florida. Yield goal‐based analysis indicates that 50 kg N ha −1 (45 lbs N acre −1 ) is required to produce 2.5 bales of cotton ha −1 (∼1 bale acre −1 ; 1 bale = 218 kg lint), enabling site‐specific, yield‐targeted N application.