Comparison of Satellite and Aerial Imagery for Detecting Leaf Chlorophyll Content in Corn

In-season site-specific nitrogen application has the potential to improve the efficiency of nitrogen use and reduce environmental contamination. Identification of within-field crop nitrogen stress is essential for improved nitrogen management. Corn nitrogen stress is frequently associated with leaf chlorophyll content, which can be characterized by spectral reflectance measurements. Therefore, satellite remote sensing has potential for characterizing corn nitrogen deficiency on a whole-field scale. To evaluate the use of satellite imagery to detect nitrogen deficiency in corn during the growing season, this study compared spectral variables extracted from SPOT satellite imagery and digital aerial imagery, and investigated the relationship between spectral variables of the SPOT imagery and the measurement data of a hand-held chlorophyll meter (or SPAD meter). SPOT images and aerial images were acquired on one commercial field in 1999 and another one in 2000. The correlation coefficient (r) between the NDVIs derived from the SPOT image and the aerial image was 0.73 in 1999, and 0.54 in 2000, respectively. When the field had large spatial variability in crop development in 1999, the spectral variables from the SPOT image were strongly correlated with those from the aerial image. However, when the crop development was more uniform in 2000, the correlation between the SPOT image and the aerial image was not as good. The limited range of available digital counts that can be used to represent the reflectance from an individual farm field for the SPOT system limited its sensitivity in detecting crop stress. Nevertheless, the NDVIs from SPOT images were significantly correlated with SPAD data in both fields, with correlation coefficients of 0.90 in 1999 and 0.68 in 2000. The correlation between SPOT images and SPAD data was similar to that between aerial images and SPAD data, indicating that SPOT imagery may have potential for detecting chlorophyll levels and nitrogen stress in corn during the growing season.