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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 effi ciency requires development of new N fertilizer recommendations. We conducted a replicated 3-yr fi eld study on a Decatur silt loam (fi ne, kaolinitic, thermic Rhodic Paleudult) to determine the effects of N source (NHNO 3 [AN] and urea–NH 4 NO 3 [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 fi rst 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–NH 4 NO 3 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 NO 3 , and leaf N were not useful predictors of cotton N defi ciency or yield. The most effi cient 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. |
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