Improving Nitrogen Fertilizer Management in Surface-Irrigated Cotton

The land and canal infrastructure in central Arizona means that level-basin surface irrigation in raised beds is the predominant approach to cotton production. Nitrogen fertilizer is usually managed with preplant ground applications followed by “fertigations” dribbled into the canal. There is uncertainty about the efficacy and uniformity of N fertigations in surface irrigation systems. There are several ways to improve N management based on 4R principles. We investigated N application techniques (fertigation compared with knife injection). Soil-test based N management was compared with reflectance-based techniques, and two N sources (ammonium sulfate or urea ammonium nitrate). A N budget was constructed to account for all inputs and removals. Pre-plant soil nitrate (NO₃) was low in this study (23 lb NO₃-N/A). Nitrogen fertilizer applied was 132 lb N/A on the soil test-based N management treatments and 66 lb N/A on the reflectance-based treatments. Lint and seed yields were similar among all of the N-fertilized treatments, but significantly greater than the zero-N plots. Lint yield averaged 1,660 lb/A in the N-fertilized plots. Total N uptake at first open boll was positively related to N fertilizer rate but not influenced by N source. Amber NDVI (a graphical indicator used to analyze remote sensing measurements) showed N deficiency in zero-N plots before red NDVI.

Nitrogen uptake was very high from the unfertilized control plot (116 lb N/A). When this uptake is partitioned between 23 lb N/A of soil profile NO₃ and 16 lb N/A added in the irrigation water NO₃, then net N mineralization is estimated to provide 77 lb N/A. Recovery efficiency of N fertilizer was not affected by N treatment and ranged from 8 to 30%. While similar to values for furrow-irrigated cotton in West Texas, it is lower than that reported in other Arizona research (100 lb N/bale). The internal N use efficiency was greater than expected (40 lb N/bale), compared to previous Arizona data. The soil was sampled eight days after the first fertigation. Soil ammonium (NH₄) concentrations were low, but NO₃ concentrations were high in the subsoil. Inorganic soil N transects indicated that fertigation was as uniform as knifing N fertilizer. Nitrous oxide (N₂O) emissions were low in all N treatments during the 95-day measurement period following fertilization and fertigation. The soil-test based-fertigation treatment lost only 0.2% of added N fertilizer as N₂O, which is in the range of N₂O losses from drip-irrigated cotton in other regions, but elevated above the unfertilized control. AZ-08