Cotton Nitrogen Management in a High‐Residue Conservation System: Source, Rate, Method, and Timing

Abstract

More than 70% of cotton (Gossypium hirsutum L.) grown in the Tennessee Valley of northern Alabama is produced using conservation tillage systems with cereal cover crops. The resulting decreased N efficiency requires development of new N fertilizer recommendations. We conducted a replicated 3‐yr field study on a Decatur silt loam (fine, kaolinitic, thermic Rhodic Paleudult) to determine the effects of N source (NHNO3 [AN] and urea–NH4NO3 [320 g N kg−1, UAN]), N rates (0, 45, 90, 135, and 180 kg N ha−1), N application timing (all at‐planting or 50:50 split between planting and first match head square), and N application method (banded or broadcast) on cotton grown in high‐residue rye (Secale cereale L.) conservation systems. Generally, 67 to 80% more N was needed than average conventional N rate recommendations to reach optimal yields if N was split applied, while N applied at‐planting had yield responses with 169% of the recommended N rate. Urea–NH4NO3 applications resulted in greater yields when banded at‐planting (1045 kg lint ha−1), while AN was more effective when broadcast applied at‐planting or in split applications (1002 and 996 kg lint ha−1, respectively). Chlorophyll meter readings, petiole NO3, and leaf N were not useful predictors of cotton N deficiency or yield. The most efficient practice is to apply 88% more N (126 kg N ha−1 total) than the mean conventional N cotton recommendation as a broadcast split application using AN. We speculate that N requirements may be decreased with time as C and N pools reach a new equilibrium.