Nitrogen (N) fertilizer management in subsurface drip irrigation (SDI) systems for cotton (Gossypium hirsutum L.) can be very efficient when N is fertigated on a near daily time step.  Determining the amounts and timing of the N fertigation, however are questions that weekly canopy reflectance measurements may answer.  
The main objective of this 3-yr. study was to test two canopy reflectance strategies for adjusting urea ammonium nitrate (UAN) fertilizer in-season injections between squaring and early-mid bloom.  We also evaluated three fixed N rates; 50, 100, and 150 % of a soil test-based N recommendation for a 1400 kg lint ha-1 yield goal.
Based on the expected yield, the soil test-based N treatment rate was an N requirement of 140 kg N ha-1 minus 0- 0.6 m soil NO3-N, and minus an estimate of seasonal irrigation water NO3.   In the reflectance-based N strategy-1, UAN was injected starting at first square at 50 % of the rate of the soil test-based N treatment.   Normalized difference vegetative index (NDVI) was calculated from weekly canopy reflectance measurements at 1 m height. When NDVI in the reflectance strategy-1 plots fell significantly below NDVI in the soil test-based plots, the UAN injection rate was increased to match the injection rate of the soil test plots.  The reflectance-based N strategy-2 plots had an initial UAN injection rate equal to that of the soil test plots, and was raised to match the 150 % soil test plots based on NDVI.  Nitrogen rates for the reflectance-based strategy-1 averaged across 3 yr were 22 kg N ha-1, or 31 % less than the soil test treatment, without hurting lint or seed yields. Our newer, second reflectance N management strategy N rates were higher than the soil test N rates by 11 kg N ha-1 in 2007, without any yield benefit.  Quadratic regressions of seed vs. N rate indicated economically optimum N rates (EONRs) of 57 to 73 kg N ha-1.