Precision nitrogen (N) management has a great potential to improve N use efficiency. It will require timely and efficient diagnosis of crop N status. Chlorophyll meter sensor is one of the commonly used diagnostic tools for crop nitrogen status, and different methods have been developed, including nitrogen sufficiency index, and ratio of chlorophyll meter values at different leaf positions. Up to date, no study has compared these two different methods. Therefore, the objective of this study is to evaluate these two different methods to identify a suitable method for diagnosing N status of summer maize in North China Plain. A N rate experiment was conducted in 2009 and 2010 at the Quzhou Experiment Station of China Agriculture University in Hebei Province of North China Plain. A randomized complete block design was used with five treatments and four replications. Chlorophyll meter values (CMV) were taken from the top first to third fully expanded leaves of 30 summer maize plants in each plot at growth stages V6, V12, VT in 2009 and V6, VT in 2010. On the same day, plant samples were collected for determination of shoot biomass and N concentration using the Kjeldahl method. The results indicated that the N sufficiency index (NSI) was significantly correlated to N nutrition index (NNI), and it is a suitable indicator of N status of summer maize. Using a critical NSI threshold value of 97%, we achieved an average accuracy of 86% in diagnosing N status of summer maize using the top 2 fully expanded leaf. This approach can eliminate the impact of growth stages, seasonal variation, variety, stress and other factors. The ratio of chlorophyll meter values is not a suitable method for summer maize N status diagnosis. More studies are needed to verify these results.
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