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Nitrogen (N) is the most limiting nutrient for crops growth and development. The optimal application of N significantly enhances crop metabolic process. On the other hand, overuse of nitrogenous fertilizer leads to excess N accumulation in crop tissues that increases the risk of health-related problems. Therefore, it is mandatory to develop a quantitative approach for monitoring the crop N status during growth period. The main objective of this study was to establish the critical nitrogen dilution curve as an effective method to diagnose nitrogen status of sweet basil (Ocimum basilicum L.) grown under controlled greenhouse conditions. Two experiments were carried out with seven N application rates in the research greenhouse of University of Tehran, Iran. Values of crop shoot biomass (t ha −1) and N concentration (%) were measured in each sampling date to develop the critical nitrogen concentration (N c) equation. The relationship between N c and shoot dry matter (DM) was described by a power function (N c = 4.80DM-0.38 , R 2 = 0.98). The validation results showed that the developed equation effectively distinguishes N-limiting from non-N-limiting treatments. The N nutrition index (NNI) ranged from 0.52 to 1.18 and 0.53 to 1.26 for the first and second experiment, respectively. It was concluded that the developed N dilution curve could provide useful insight into determination of basil N nutrition status and proper fertilizer management.

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