Improving Nitrogen Management for Subsurface Drip-Irrigated Cotton in Arizona

Following water, N fertilizer is the main constraint to cotton production in the western USA.  Canal infrastructure in Arizona means basin, flood, and furrow irrigation are still the pre-dominant choices of irrigation methods.  Recovery efficiency of ground-based N applications in furrow-irrigated cotton ranged from 15 to 34 %.  With declining water resources and competition from growing urban areas there is renewed interested in subsurface drip irrigation systems.  However, N management research and recommendations in the far western US are lacking for drip irrigation.  Canopy reflectance-based N management in subsurface drip systems in Texas resulted in reduced N fertilizer use, without hurting lint yields.  We improved and updated an N fertilizer management recommendation for 4-bale/acres cotton based on a 36-inch NO₃-N soil test.  Additionally, we compared soil test-based N management for full and deficit irrigation and with canopy reflectance based N management (full irrigation only).  The study was conducted in 2016, in Maricopa, AZ on a Casa Grande sandy loam.  First open boll biomass yields were high at 13,000 lb/ac for soil test N, with 100 % irrigation. Final lint yields were lower than expected, due to many level two heat stress days in 2016 that led to early green boll shedding.   At 100 % irrigation, reflectance-based N management yields were similar to soil test-based N management (1400 lb lint/ac) with 18 lb N fertilizer savings.  Soil test N management with the water stress irrigation treatment only yielded 16 % less than the full irrigation soil test N treatment, although the irrigation was 25 % less (30 % less water, without the germinating irrigation).  Nitrous oxide emission were low and not affected by fertilizer.  Deep percolation of irrigation plus rain was only 1-3 % in this sub-surface drip system.  This compares with 14 to 21 % losses at this site with surface irrigation.