Improving Nitrogen Fertilizer Use Efficiency in Surface‐ and Overhead Sprinkler‐Irrigated Cotton in the Desert Southwest

Core Ideas Nitrogen use efficiency is low in surface‐irrigated cotton. Canopy reflectance can guide in‐season N management. Deep percolation of rain and irrigation is associated with nitrate leaching. Nitrogen fertilizer use efficiency (NUE) is low in surface‐irrigated cotton ( Gossypium hirsutum L.), especially when adding N to irrigation water. A NO 3 soil‐test algorithm was compared with canopy reflectance‐based N management with surface‐ overhead sprinkler‐irrigation in central Arizona. The surface irrigation studies also compared fertigation of N fertilizer with knifing‐in of N and the addition of a urease and nitrification inhibitor (Agrotain Plus, Koch Agronomic Services, Wichita, KS) to urea ammonium nitrate (UAN). Cotton lint and seed yields responded positively to N fertilizer in all four site‐years. Recovery efficiency (RE) of N at low N fertilizer rates (60 to 76 kg N ha –1 ) ranged from 21 to 61% with surface irrigation and from 81 to 97% with overhead sprinkler irrigation. Deep percolation below 1.8 m was 4 to 11% of applied surface irrigations and rain, but was undetectable in the overhead sprinkler. Leaching of NO 3 was apparently the largest N loss pathway in the surface‐irrigated system. Fertigating UAN into surface irrigation resulted in similar lint yields and RE as knifing UAN. Use of Agrotain Plus with UAN gave similar yields and RE as using UAN alone. Reflectance‐based N management using normalized difference vegetation index‐amber (NDVIA) saved 50% of N fertilizer of the full soil‐test based dose without a yield reduction in three of four site‐ years. Nitrogen fertilizer was over‐prescribed with the soil‐test‐based treatment. This may have been due to not accounting for N mineralization, which the reflectance method indirectly measures.