An experiment was conducted in which spectral and agronomic measurements were collected from corn (Zea mays L.) canopies under four nitrogen treatment levels (0, 67, 134, and 202 kg/ha) at the Purdue Agronomy Farm, W. Lafayette, IN, on 11 dates during 1978 and 12 dates during 1979. Spectral measurements over the 0.4 to 2.4 µm wavelength region were acquired with a spectroradiometer and used to compute reflectance factor. Agronomic data collected included biomass, leaf area index, plant height, crop development stage, and percent soil cover. Data were analyzed to determine: (1) the relationship between the spectral responses of canopies and their agronomic characteristics and (2) the spectral separability of the four treatments.
Red reflectance was increased, while the near infrared reflectance was decreased for canopies under nitrogen deprivation. Spectral differences between treatments were seen throughout each growing season. The near infrared/red reflectance ratio increased spectral treatment differences over those shown by single band reflectance measure. The spectra of the four nitrogen treatments were significantly different on August dates; however, early and late in the season only two spectral classes were resolved. Of the spectral variables examined, the near infrared/red reflectance ration most effectively separated the treatments. Differences in spectral response between treatments were attributed to varying soil cover, leaf area index, and leaf pigmentation values, all of which changed with N treatment. The results further confirm the potential of remote sensing for monitoring the growth and condition of crops
