Comparing Reflectance-Based N Management Strategies for Irrigated Cotton.

Nitrogen fertilizer use (NUE) efficiency is usually low in irrigated cotton (Gossypium hirsutum L.) in the Western USA.  Management approaches that may improve NUE include knifing-in of urea ammonium nitrate fertilizer, the use of enhanced efficiency fertilizers (EEF) or ammonium-based N fertilizer, and the measurement of canopy reflectance to assess N status.  In a two-year field study, we compared fertigation of N fertilizer with knifing-in in surface-irrigated cotton in central Arizona. We also compared soil test- based N management (1700 kg lint/ha expected yield and credit for 1-m soil test NO₃) with reflectance-based management. A second, two-year N management study for sprinkler-irrigated cotton was also conducted.  Under the sprinkler two different reflectance-based N management studies were employed.  One used the soil NO₃ test algorithm (2000 kg lint/ac yield goal with credit for 1-m soil NO₃) for a reference and the second approach used 130 % of the soil test N rate as a well-fertilized reference.  In all studies, reflectance-based N treatments began at 50 % of the N rate of their respective references.  When amber NDVI in the reflectance-based treatments fell statistically below amber NDVI of the reference, then the subsequent N applications matched the reference N rates.  The first reflectance-based N strategy was successful in saving N without hurting lint yields.  The second approach did not call for additional N.