In‐Season Nitrogen Status Sensing in Irrigated Cotton

Nitrogen recommendations for Upland cotton ( Gossypium hirsutum L.) in the western USA are based on spring soil NO − 3 –N tests. In‐season monitoring of plant N status is another approach. Our primary objective was to test spectral reflectance and chlorophyll meter measurements as in‐season N decision aids for irrigated cotton, and to compare these with soil test‐based N management. The secondary objective was to determine the fate of 15 N as affected by N management and irrigation modes. Urea ammonium nitrate was applied with low energy precision (LEPA) center‐pivot, surface drip, and subsurface drip irrigation. Microplots received 3 atom% 15 N. Soil test N application was based on 0‐ to 60‐cm soil NO − 3 –N and 1400 kg lint ha −1 expected yield. Thirty‐four kilograms of N per hectare was applied when green vegetative index (GVI) or chlorophyll meter readings relative to well‐fertilized plots were <0.95. Lint yield responded to N at Lubbock in 2000 and 2001, but not at Ropesville. Nitrogen applied with in‐season monitoring in 2000 at both sites was 34 to 101 kg N ha −1 less than soil test N application of 134 kg ha −1 , with similar yields. In Lubbock, 2001 lint yields were near the expected yield, and in three of four cases, N applications with in‐season monitoring equaled soil test N applications of 101 kg ha −1 Nitrogen‐15 recovery in plants ranged from 19 to 38%, and was affected by N management in two of three site‐years, but not by irrigation. This study indicates that basing N applications on in‐season monitoring can reduce N applications in low yielding seasons and match the yield potential in high‐yielding seasons.