Improving Nitrogen Fertilizer Management in Surface-Irrigated Cotton

The first year of this project was devoted to comparing N sources for furrow-irrigated cotton, develop a tool for predicting N fertilizer response using canopy reflectance-based N management in surface-irrigated cotton, and constructing a N balance for surface-irrigated cotton (quantify total N uptake, recovery N use efficiency, NO₃ leaching, and denitrification losses).

High pre-plant soil NO₃ precluded yield response to N this year. We discovered that the field arrangement does not allow uniform irrigation of three replicates of 8 row plot-treatments with the 520 foot runs at 3 to 3.5 inches. We were not able to mix 5% ammonium sulfate solutions so that fertigation treatment was not applied. The in-season transect soil profile NH₄⁺ and NO₃^- data did not show the effect of knifed or fertigated N fertilizer against the high pre-plant soil NO₃^-.

We did collect valuable sets of canopy reflectance/four vegetative indices. The total N uptake data at first open boll resulted in valuable internal N use efficiency data (67 lb N/bale). The nitrous oxide emission data was high quality data, some of the first of its kind. Nitrous oxide emissions were barely detectable in all N treatments during the 56-day measurement period following fertilization and fertigation, with the exception of the fertigation treatment. That treatment lost 0.9% of fertigated N as N₂O, which is in the range of N₂O losses measured from drip-irrigated cotton in Texas.

Amber NDVI (amber being 590 nm) using 820 nm had less up or down noise than the other indices. Calculating NDVI with a red edge (i.e. 730 nm) of NIR reportedly has the advantage of not saturating at high leaf area levels. Amber NDVI using 820 nm had the highest correlation among the indices with in-season biomass and N uptake. AZ-08