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.

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