Improving Nitrogen Fertilizer Management in Surface-Irrigated Cotton

We propose conducting research in central Arizona to compare the common regional practice of N fertigation with knifed N injections in surface-irrigated cotton (i.e., level furrow irrigation). Additionally, we will compare reflectance-based nitrogen fertilizer management with soil test-based management. Furrow and other surface irrigation methods are still the most common irrigation mode for cotton in Arizona and worldwide.

IPNI-2010-USA-AZ08

24 Mar 2015

2014 Annual Interpretive Summary


Nitrogen fertilizer is the main constraint to cotton production in the western USA, following adequate water. Research shows N recovery in furrow-irrigated cotton ranges from only 15 to 34 %. There is interest in center-pivot or linear-move overhead sprinkler irrigation systems for cotton, however N fertilizer recommendations are lacking. Weekly petiole nitrate (NO3) sampling and analysis is recommended to monitor in-season plant N status, but petiole sampling is laborious, can be slow, and nitrate concentrations are variable. Enhanced-efficiency N fertilizers like Agrotain Plus(R) have been shown to reduce N2O emissions in corn, but have not been widely tested in cotton. This work improved and updated N fertilizer management recommendation for 4 bale/A cotton based on a 36 in depth NO3-N soil test. We compared UAN alone to UAN with the N-loss inhibitor Agrotain Plus. We compared reflectance-based N fertilizer management with soil test-based management. The study was conducted in Maricopa, AZ on a Casa Grande sandy loam. Specifically, the research was designed to 1) compare soil test-based N fertilizer management with two canopy reflectance-based N management approaches in sprinkler-irrigated cotton, 2) compare UAN alone to UAN with Agrotain Plus, and 3) construct N balances; i.e., quantify total N uptake, recovery N use efficiency, nitrate leaching, and denitrification.

Nitrification of added fertilizer was very rapid. There were some significant effects of Agrotain Plus in maintaining ammonium (NH4) concentrations, but they were still low and the Agrotain effects were not consistent. The amber normalized difference vegetation index (NDVI) in the two reflectance-based treatments never fell below their respective references during the growing season. Therefore the two NDVI-based N treatments were not adjusted upwards. In fact, amber NDVI did not drop significantly below the N-fertilized treatments until peak bloom. In contrast to NDVI, the NDRE index showed zero-N plot deficiency at mid-bloom and petiole NO3 samples showed the same at first bloom.

Final lint yields were significantly lower for zero-N plots (1,460 lb lint/A) vs. the average of the N-fertilized plots (1,630 lb lint/A) or 3.5 bale/A). These yields were lower than the 4 bale/A yield goals. There was no effect of Agrotain Plus on biomass, N uptake, recovery efficiency, agronomic efficiency, or lint yields. Nitrogen uptake was greater than in previous surface irrigation studies, but biomass levels were similar. Agrotain Plus mitigated N2O emissions. However it was only statistically significant with the high, soil test-based treatment of 160 lb N/A and for the average of UAN with Agrotain Plus and UAN alone. Nitrous oxide emissions losses were highest early in the season before rapid N uptake occurred. As a percentage of N applied (adjusted for zero-N emissions), N2O emissions ranged from 0.1 % to 1.0%.