EVALUATION OF NITROGEN SOURCES FOR ALABAMA COTTON PRODUCTION SYSTEMS

Abstract

Cotton is the most-widely produced cash crop in Alabama, with an average of 415,000 acres per year planted during the 2011-2020 growing seasons. Nitrogen (N) is often the most limiting nutrient in cotton production systems and must be supplemented with fertilizer applications annually. Loss pathways such as leaching and volatilization can lead to significant losses of applied N fertilizer, particularly in coarse-textured, highly weathered soils of the Coastal Plain. Stabilized and controlled-released N fertilizers have been documented to reduce N losses and increase efficiency and profitability for farmers. However, these products lack evaluation in cotton (Gossypium hirsutum L.) production systems of the Coastal Plain. The objective of this experiment was to 1) evaluate N fertilizer source impact on N uptake and cotton yield and 2) evaluate urease inhibitors for their ability to reduce N volatilization. Field experiments were established at the Wiregrass Research Extension Center (WREC) in Headland, AL, and E.V. Smith Research Center (EVS) in Shorter, AL. Fertilizer treatments were organized in a randomized complete block design and included: 1) urea, 2) urea + NBPT, 3) urea + duromide/NBPT, 4) polymer-coated urea, 5) urea ammonium nitrate (UAN), 6) ammonium nitrate / ammonium sulfate blend, 7) urea / ammonium sulfate blend, 8) polymer-coated urea / ammonium sulfate blend, and 9) an untreated control. Data collected included leaf N, petiole N, soil N, cotton yield, and fiber quality. Leaf and petiole N were measured to assess uptake of N according to N source treatment. Leaf N for WREC 2021 showed treatment differences at early bloom, where 28-0-0-(5) had a greater leaf N concentration than all other treatments. Similarly, 28-0-0-(5) had a greater leaf N than all treatments at peak bloom for WREC in 2021 and was the only treatment with greater leaf N content than the control. However, no differences between treatments in leaf N were observed in 2020 at WREC or EVS. When evaluating petiole data, it was observed that 39-0-0 PC and 44-0-0 PC had greater petiole N content than all other treatments at WREC during first square. There were limited treatment differences in petiole N at early bloom and peak bloom at WREC or EVS. Soil N for both years and locations were variable and showed limited differences between treatments. Cotton lint yield for WREC showed 28-0-0-5 (1381 kg ha-1) as the greatest yielding treatment. At WREC, 28-0-0-(5), 46-0-0 +NBPT, 46-0-0 + NBPTD, 40-0-0, and 39-0-0 PC produced 400-600 kg ha-1 greater lint yield than the control. Other N source treatments including the 46-0-0, 44-0-0 PC, and 32-0-0, were not different than the control. Results indicate that urease inhibitor products 46-0-0 + NBPT (1216 kg ha-1) and 46-0-0 + NBPTD (1255 kg ha-1) were more likely to increase yields above the control treatment than base 46-0-0 (1185 kg ha-1). The polymer coated products did not provide differences compared to the untreated base products. However, these data suggest a possible reduction in production costs associated with polymer coated products since they require only one pass through the field. Laboratory incubation experiments were established to measure nitrogen volatilization for three conventional fertilizer sources: 1) urea, 2) UAN 3) homogenized urea + ammonium sulfate blend. Each fertilizer was evaluated with 1) NBPT 2) NBPT/Duromide (NBPTD) and 3) untreated control and replicated four times for the Coastal Plain (Shorter, Al) soil type. Ammonia volatilization studies were performed using a laboratory incubation method. Results from the Coastal Plain soils showed untreated urea having almost 40% of total N lost as NH3 compared to only 25% with urea was applied with NBPT products. NBPT and NBPTD applications significantly minimized NH3 losses by 10-15% within the first 4 days of treatment compared to the untreated urea. For urea, NBPT and NBPTD reduced N volatilization from 5% cumulative N loss to less than 1% three days after fertilizer application. At 4 days after application NBPT and NBPTD reduced cumulative N loss from 21% to less than 3%. For Amidas®, NBPT and NBPTD reduced nitrogen volatilization from 6% cumulative N loss to less than 1% cumulative N loss at day 4 compared to untreated Amidas®. The utilization of volatilization inhibitors proved to be effective at reducing volatilization up to four days, which urea would allow producers the time for a rain/irrigation event to occur.