Diagnosis tool for plant and crop N status in vegetative stage: Theory and practices for crop N management

The environmental constraints to agriculture imply that nitrogen (N) fertilizer management should be adjusted to crop N requirements determined by target yields. Nowadays for environmental and economical reasons target yield of farmers can be lower than the potential crop yields as permitted by soil and climatic conditions. So it is important to provide farmers crop N status diagnostic tools in order to decide the rate and the timing of N fertilizer applications. Theory on crop N uptake and allocation allows the determination of a diagnostic tool, the Nitrogen Nutrition Index, based on the determination of the critical N dilution curve for each crop species considered. During the vegetative growth period of all the crop species studied, including C3 and C4 species and monocots and dicots, plant N concentration decreases monotonically as crop grows because of (i) the ontogenetic decline in leaf area per unit of plant mass, and (ii) the remobilisation of N from shaded leaves at the bottom of the canopy to well illuminated leaves at the top. NNI appears then as an indicator well connected with the physiological regulation of N uptake at canopy level. So this indicator can be used as the basis for determination of crop N nutrition status, and then for decision making on the necessity of an N application for achieving target yield. Nevertheless despite its high physiological relevance, NNI cannot be used directly in farm conditions because its determination is very time consuming. So it is necessary to develop indirect methods for NNI estimation through more operational procedures. Several methods have been proposed in literature, such as nitrate concentration in sap or chlorophyll meter. But the calibration or validation of these methods with NNI have not been always made and, when they have been, they did not give univocal relationships, showing a strong dependence of the relationship with cultivar and environment, that limits considerably the relevance of such diagnostic tools in a large range of situations. Easier to use is the indirect estimation of crop NNI by remote sensing measurements. This method allows the estimation of both actual crop mass, through LAI estimation and crop N content, through crop chlorophyll content. The possibility to have repeated estimations of crop NNI during the period of vegetative growth would allow a dynamic diagnostic tool of crop N status. The coupling of indirect measurements of crop N status with dynamic models of crop growth and development should allow a very promising method for crop N diagnostics for decision tools in N fertilization.