Original Articles

Early detection of nitrogen deficiency in wheat at different times of N application by using spectral reflectance of crop canopy

Published in: South African Journal of Plant and Soil
Volume 23, issue 4, 2006 , pages: 225–229
DOI: 10.1080/02571862.2006.10634759
Author(s): P. ShakerDepartment of Soil Science, South Africa, P.S. FouchéDepartment of Soil Science, South Africa

Abstract

Physiological changes resulting from nitrogen limitations can be translated into clear spectral differences between treatments, demonstrating the relationship between leaf reflectance and leaf chlorophyll and nitrogen concentrations. A wheat trial was conducted in which nitrogen (N) was applied in the form of urea at four different rates (NO = 0 kg N ha−1, N1 = 60 kg N ha−1, N2 = 120 kg N ha−1, N3 = 180 kg N ha−1) with four different timings (T1 = at planting, T2 = 21 days after planting, T3 = 42 days after planting, T4 = 63 days after planting). An Ocean Optics S2000 Spectrometer was used to obtain a continuous reflectance spectrum of the vegetation canopy, covering wavelengths in both the visual and near-infrared domain of the spectrum (200–1100 nm). The reflectances at 550 and 679 nm were correlated with the N treatment levels and the yield. The results for the N treatment levels showed a very weak correlation for T3, and weak correlations for T1, T2 and T4 at 550 nm (green band). However, at 679 nm the observations were different. A strong correlation was obtained for T1 (R2 = 0.89), moderate correlations for T2 and T3, and a very weak correlation was observed for T4. The statistical analysis against yield resulted in a different correlation. At 550 nm there was a very strong correlation for T2 (R2 = 0.99) and moderate correlations for T1 (R2 = 0.79), T3 (R2 = 0.52) and T4 (R2 = 0.62). At 679 nm, T2 again showed a very strong correlation (R2 = 0.96), whilst a moderate correlation was observed for T1 (R2 = 0.52), and very weak correlations for T3 (R2 = 0.1) and T4 (R2 = 0.04). Both wavelengths (550 and 679 nm) proved to be good indicators of nitrogen stress in wheat for T2 (N application 21 days after planting). However, for the other three application dates the results were varying.

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