Near-infrared analysis of cotton leaves as a guide for nitrogen fertilization

Nitrate level in cotton (Gossypium hirsutum L.) leaf petioles is commonly used as a practical guide to N fertilization, especially in cases of critically low N levels. The optimization of N supply to irrigated cotton requires a reliable method for monitoring the plant's N status. The feasibility of using leaf N concentrations monitored by near-infrared analysis (NIRA) as a guide for N fertilization in irrigated cotton was examined in this study. Three field trials were conducted in three consecutive years. The NIRA data from fresh leaf disks were calibrated against laboratory N measurements expressed as g N kg^-1 dry matter (R² = 0.90). Validation of the calibration showed a high correlation between NIRA estimates and laboratory data from two field trials. Dawn leaf sampling was selected for routine monitoring of plant N status. Application of NIRA-guided N fertilization was examined in the third-year trial. The NIRA-guided treatment, with only 60 kg N ha^-1, resulted in only a 7.5% lower yield (statistically nonsignificant) relative to the commercial predetermined N treatment with 150 kg N ha^-1. Yield was linearly correlated with leaf N concentration. Cotton leaf N concentration was reliably monitored by NIRA. Only 30 s are required for each N determination; using 10 samples, about 7 min are needed per plot. Once its reliability under various conditions is established, this method could be used for on-the-spot decision-making regarding N-fertilizer application in irrigated cotton.