Improving Nitrogen Use Efficiency in Subsurface Drip-Irrigated Cotton

Irrigation in arid lands is crucial for cotton production.  Declining water availability in the American West continues to generate interest in subsurface drip irrigation (SDI).  Fertigation of liquid urea ammonium nitrate is an important advantage of SDI.  However, Nitrogen (N) fertilizer guidelines, specific to SDI cotton are few.  The objectives of this study on a Casa Grande sandy/sandy clay loam soil in Maricopa, AZ were to develop a pre-plant soil profile NO₃ test algorithm and a canopy reflectance approach to managing in-season N fertilizer for SDI cotton in AZ.  Treatments included soil test-based N management, reflectance-based N management, and zero-N with 100 % ET irrigation replacement with < 30 % soil water depletion.  A second irrigation level of 70 % ET replacement, included soil test-based N  and zero-N treatments.  Plots were 8, 40-inch rows wide and 330 feet long.  The five treatments were replicated three times.  In 2016, the N fertilizer treatments were 156 and 141 lb N/ac for soil test and reflectance, respectively.  In 2017, the N fertilizer treatments were 154 and 112 lb N/ac for soil test and reflectance, respectively.  Nitrogen recovery efficiency of N was high with 24 fertigations between first square and mid bloom, ranging from 59 to 81 % in 2016.  Lint and seed yields were reduced with the 70 % irrigation treatment compared to 100%.  On the other hand, cotton yields with reflectance-based N management saved 15 to 42 lb N/ac without hurting yields, compared to the soil test-based N treatment.