In order to achieve high yield of maize farmers in many parts of Iran tend to apply nitrogen fertilizer (N) in excess of the crop requirements. Excessive N input in the beginning of growth season is the major cause of the poor fertilizer efficiency. Only less than 20% of the total N uptake by maize occurs before V8. Environmental and economic considerations require that nitrogen fertilizer should be applied only in quality and time which is strictly justified. Improving N management would eventually depend on the accuracy with which the N status of plant can be assessed. The usefulness of SPAD chlorophyll meter reading for plant N assessment is based on the direct proportionality between the leaf chlorophyll and the leaf N concentration. By using the critical value of SPAD it is possible to determine the best time for N application.

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