Use of Nitrogen Calibration Ramps and Canopy Reflectance on Farmers' Irrigated Cotton Fields

Abstract

Nitrogen is the main nutrient limiting irrigated cotton {Cossypium hirsutum L.) production in tbe southwestern United States. Canopy spectral reflectance may assess tbe need for in-season N in irrigated cotton and guide N fertilizer applications. However, calibration of remote sensing indices sucb as normalized difference vegetative index (NDVI) to tbe crop's need for N fertilizer is difficult. Well-fertilized reference strips or plots reference NDVI data in tbe crop area of interest but can result in rank growth and reduced lint yields. Recently, Oklaboma State University developed a calibration procedure of using multiple, sequential, N rate calibration plots, or a ramp approach for wbeat {Triticum aestivum L.) and corn {Zea mays I.). We tested tbis approacb in irrigated cotton fields in Lubbock County, Texas, In 2008 and 2009. Tbe main objective of tbis researcb was to test a calibration ramp approacb to determining optimum in-season N fertilizer rates in irrigated cotton in West Texas. Near infrared, red, and amber reflectance was measured witb active spectroradiometers at 1 m above tbe canopy. Wide ranges in soil type and irrigation amounts influenced NDVI mucb more tban N fertilizer rate. Normalized difference vegetative index at mid-bloom and at peak bloom were positively related to N fertilizer rate in only one ramp in eacb year. These two ramp-years also bad significant N fertilizer rate response in lint yield. Ramps tbat did not bave mid-or peak bloom NDVI responses to N rate, likewise bad no lint yield response to N rate. In botb low irrigation-low N input and in bigb irrigation-bigb N input farms, in-season NDVI correctly predicted lint yield response to N fertilizer rate.Abbreviations: aNDVI, amber normalized difference vegetative index; DGPS, differential global positioning system; EONR, economically optimum nitrogen rate; NIR, near infrared; NDVI, normalized difference vegetative index; rNDVI, red normalized difference vegetative index; SDI, subsurface drip irrigation; UAN, urea ammonium nitrate