Nitrogen Response in Cotton as Affected by Tillage System and Irrigation Level

More than 0.5 million ha of irrigated cotton (Gossypium hirsutum L.) are grown in the Southern High Plains of Texas. Conservation tillage cotton in terminated wheat (Triticum aestivum L.) has been shown to improve water use efficiency and reduce wind erosion. However, limited N fertilizer response research has been done in this system. The objective of this 3‐yr field study at Lubbock, TX was to characterize the response to N fertilizer (0, 28, 56, 84, or 112 kg N ha−1) at varying irrigation levels [0, 25, 50, or 75 % Evapotranspiration (ET) replacement] for conventional and conservation tillage cotton in an Acuff loam (fine loamy, mixed, superactive, thermic, Aridic Paleustoll). Additionally, we tested the chlorophyll meter as an indicator of in‐season N status of cotton and compared it to petiole NO3–N analysis. Cotton lint yields showed a quadratic response to irrigation level in 1996 and 1997, and a linear response in the drought year of 1998. Maximum lint yield varied from 71 to 97 % ET replacement. In 1997 and 1998, cotton lint yields responded to N at the 50 and 75% estimated ET replacement irrigation levels, but not at the 0 or 25% ET levels. Quadratic‐plateau models indicated that 19 to 38 kg N additional fertilizer ha−1 was needed to produce economically optimum lint yields near 1100 kg N ha−1 with conservation tillage than with conventional tillage. Chlorophyll meter and petiole NO3–N readings were positively related to N rate but were not affected by tillage system.