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Plant nitrogen status and yield potential are important factors for optimizing field management in cotton production. An experiment was conducted in 2011 at Stoneville, Mississippi to develop a method to assess plant N status and yield potential in cotton. The experimental plots were laid out in a split plot design with two irrigation treatments as main unit and six nitrogen treatments as subunit in a 10-ha cotton field. Cotton plant height was measured in real time in situ using an experimental ultrasonic device coupled with a GPS (global positioning system) receiver. Soil samples were collected and analyzed for soil-N residual and soil texture before planting. Cotton leaf-blade samples were collected and analyzed for N content. Amount of water applied in irrigated treatment was recorded during the growing season. The plant height showed a quadratic relationship with yield, and this relationship was stronger in the non-irrigated plots (R2=0.60) than that in irrigated plots (R2=0.16). Irrigation had a significant effect on plant height, leaf N content, and yield. Excess application of N would not improve cotton yield.