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a Texas A&M Univ. Texas Agricultural Experiment Station, RR 3, Box 219, Lubbock, TX 79403
b Soil-Plant Nutrient Res. Unit, USDA-ARS, P.O. Box E, Fort Collins, CO 80522
c Dep. Agric. & Applied Econ., Texas Tech Univ., Lubbock, TX 79409
* Corresponding author (k-bronson{at}tamu.edu).
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 15N 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% 15N. 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.
Abbreviations: ANA-MS, automated N analyzer-mass spectrometer • ET, evapotranspiration • GNDVI, green normalized difference vegetative index • GVI, green vegetative index • LEPA, low energy precision application irrigation • LSD, least significant difference • NDVI, normalized difference vegetative index • NIR, near infrared • RNDVI, red normalized difference vegetative index • RVI, red vegetative index
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