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SOIL FERTILITY & PLANT NUTRITION

Nitrogen Management for Subsurface Drip Irrigated Cotton: Ammonium Thiosulfate, Timing, and Canopy Reflectance

^a Sapphaire Technologies, Direct Marketing Centre of Excellence, Bank of America, Charlotte, NC
^b Plant and Soil Science Dep., Texas Tech Univ., Lubbock, TX 79409
^c Texas A&M Univ.–Texas AgriLife Res., Texas Agricultural Res. and Extension Center, 1102 E. FM 1294, Lubbock, TX 79403
^d Cotton Inc., 6399 Weston Parkway, Cary, NC 27513
^e Plant and Soil Science Dep., Texas Tech Univ., Campus Box 42122, Lubbock, TX 79409

^* Corresponding author (k-bronson{at}tamu.edu).

In subsurface drip irrigation (SDI) systems, water constraints to cotton (Gossypium hirsutum L.) production are greatly minimized and N management becomes the main priority. Injecting N fertilizer into SDI systems should, in theory, be as efficient as the irrigation delivery system itself. The objective of this study was to test duration of N injections, addition of the nitrification inhibitor ammonium thiosulfate (ATS), and using canopy spectral reflectance to guide N injections in a 2-yr study of SDI cotton in Lubbock, TX. For a 1400 kg ha^–1 expected lint yield, the N fertilizer rate was 170 kg N ha^–1 minus the 0- to 0.6-m soil NO₃–N and an estimate of irrigation water NO₃ (20 and 30 kg N ha^–1 for 2005 and 2006). We tested injection of urea–NH₄NO₃ (UAN) between first square and early bloom, and first square and peak bloom. There was no effect of timing or ATS on lint yield. Reflectance-based N management up to peak bloom resulted in savings of 28 and 17 kg UAN-N ha^–1 for 2005 and 2006, respectively, compared with soil-test-based N fertigation. Lint yields were similar between reflectance-based UAN management (1814 kg ha^–1) and UAN injected up to peak bloom based on soil test NO₃ (1880 kg ha^–1). Nitrogen fertilizer recovery at first open boll was not affected by treatment but was high (62–75%). Flux of N₂O + N₂ was low (800–2100 g N ha^–1) in both years and was not different between fertilized and unfertilized plots. Residual 0- to 1.2-m soil profile NO₃ after 2 yr was not affected by N fertigation treatments. Nitrogen fertilizer injections in SDI cotton are utilized very efficiently, and spectral-reflectance-based approaches have potential to reduce N fertilizer inputs.

Abbreviations: ATS, ammonium thiosulfate • DGPS, differential global positioning system • ET, evapotranspiration • GVI, green vegetative index • SDI, subsurface drip irrigation • UAN, urea–ammonium nitrate