Twenty Years of Cotton Nitrogen Management and Cycling Trials in Texas and Arizona: What Have We Learned?

Over 20 site-years of nitrogen fertilizer management/cycling trials were conducted in Lubbock Texas and Maricopa Arizona from 1998 to 2019. Furrow, overhead sprinkler (OSI), subsurface drip irrigation (SDI) were used. Soil profile nitrate (0-36 inches in Texas and 0-72 inches in Arizona) was sampled and tested for in all trials. Nitrogen-15 labeled fertilizer was used for two years in Texas and for two years in Arizona. Canopy reflectance was measured with active sensors in every trial. A pre-plant soil NO₃ test (0-24 inches in Texas, 0-36 inches in Arizona)-based N recommendation algorithm was successfully developed. A complementary approach of guiding in-season N applications with a vegetation index relative to a well-fertilized soil test-based plot saved N without hurting yields in both states. The old 1984 recommendations for petiole NO₃ deficiency levels were lowered by 1000 ppm in Arizona after analyzing eight recent site-years of data. New lessons were learned on the fate of fertilizer N in irrigated cotton in Texas and Arizona. Nitrate leaching can be significant in furrow irrigation but is negligible in OSI and SDI. Recovery efficiency by cotton was about 20, 40, and 90 % of applied/fertigated N fertilizer for furrow, OSI, and SDI respectively. Internal N use efficiency for all yield levels, irrigation modes, and both states was a remarkably consistent 43 lb N per bale. Net N mineralization was estimated as N uptake in zero-N plots minus credits for soil NO₃. Testing irrigation water for NO₃ concentrations is an important N credit. This information all feeds into the pre-plant soil NO₃ test-based N recommendation which also requires a yield goal. Nitrous oxide (greenhouse gas 300 x more potent than CO₂) emissions were about 1 % of added N in furrow and OSI, but the N₂O emission factor was near zero with SDI and fertigation.