Cotton Leaf and Canopy Reflectance Spectra Analysis for Nitrogen Estimation.

This study was conducted to identify the most responsive electromagnetic wavelength(s) to the variation exhibited by different N regimes applied to cotton. Cotton leaf and canopy reflectance were measured from two separate irrigated N experiments using a portable GER 1500 spectroradiometer (291 nm to 1130 nm). N regimes were divided into 3 rates: low (0 kg/ha), intermediate (67 and 53 kg/ha) and high (134 and 106kg/ha). Simple subtraction analysis showed minimum variations in leaf spectra between all N regimes in blue and red wavelength regions. Nonetheless, striking peaks in the leaf spectra between high and low N rates were observed in the green and NIR region, indicating subtle greenness levels and apparent leaf thickness exhibited by the leaves treated with varied N rates. Further analysis showed that the variations in the green wavelength region between different N regimes were more prominent and consistent throughout the season in the canopy spectra than the leaf spectra. This consistency suggests that the N stress is more perceptible in the canopy spectra due to the mobility characteristic of the nutrient itself. Contradictory, the NIR region of the canopy spectra between different N regimes show high inconsistencies with regards to the amount of canopy ground cover. This may suggest that the quantity of ground cover is not a primary factor in the NIR region of cotton canopy spectra. The overall findings of this study demonstrate promising development for a new index for N detection and estimation using remote sensing method.