- Diagnostic methods assaying leaf optical properties can aid rapid site-specific screening of crop nitrogen status. A set of calibration curves relating many 1.5-nm band reflectance ratios to cotton (Gossypium hirsutum L.) leaf N concentration was established from plants grown in sunlit growth chambers and at a range of nitrogen levels. Predicted and actual concentrations were compared by regression for a validation set of field-grown leaf samples from diverse genotypes. Only those ratios that combined a red-edge measure (700 or 716 nm) with a waveband of high reflectance in the very near infrared region (755-920 and 1000 nm) provided good precision (correlation) and accuracy (one-to-one relationship between predicted to actual values). Other indices that included a chlorophyll-based reflectance feature also had good precision but were less accurate than those obtained from the red-edge/very-near-infrared reflectance ratios.