Improving Nitrogen Fertilizer Management in Sprinkler-Irrigated Cotton

Aside from water, nitrogen (N) is the main limitation in cotton production in the western USA. Flood irrigation techniques are commonly used in Arizona cotton production, despite the relative inefficiency of these systems. With declining water resources there is increased interest in more efficient systems such as overhead sprinkler (center-pivot and linear-move) and subsurface drip (SSD) irrigation. But, N fertigation research for cotton in these systems is lacking.

This project was under linear sprinkler irrigation, but it was converted to SSD in 2016. The aim of this work is to improve and update N fertigation recommendations for 4-bale/A lint yield. The specific objectives are to: 1) compare soil test-based N fertilizer management with canopy reflectance-based approaches; 2) compare lint yields and N use efficiency (NUE) for full and deficit irrigation; and 3) construct N balances for SSD irrigated cotton.

Soil test-based N application was used to achieve on a 4 bale/A yield goal, requiring a total of 200 lb N/A. Soil NO₃-N (0-36 in. depth; 24 lb N) was accounted for, and it was assumed that irrigation would deliver 20 lb N/A during the season. Therefore, for the soil test-based N fertilizer approach 156 lb fertilizer N was used (i.e., 200-24-20=156). For sensor-based N management, Crop Circle ACS-470 units were used. Canopy reflectance was measured weekly from first square to first open boll. For this approach, the crop started out with half the total N rate of the soil test-based approach (i.e., 156/2=78 lb N/A). The NDRE index was used for in-season N rate determination. Two irrigation treatments were used, 100 and 75% ET. Nitrogen as UAN was fertigated in 24 separate doses between first square and mid bloom stage of growth.

The final reflectance-based N application rate was 141 lb/A, or 15 lb less than the soil test-based approach. Nitrogen fertilizer significantly increased lint yield over the control, but there was no difference in yield (average 1,385 lb lint) between the two N management approaches (100% ET). Excessive heat is thought to have reduced yield from the four bale (2,000 lb) target. Yield was significantly lower with 75% ET irrigation compared to 100%. High fertilizer N recovery efficiency (81%) was observed (soil test approach, 100% ET), thereby solidifying the hypothesis that NUE is improved with SSD fertigation. Nitrous oxide emissions were exceptionally low in all treatments, which is important given the growing emphasis on lowing greenhouse gas emissions in crop production.