ABSTRACT.etermination of optimal levels for nitrogen fertilizer application to agricultural crops is not a straightforward process and carries significant uncertainties. Traditionally, pre-plant nitrogen requirements have been estimated by utilizing soil samples and crop-yield levels from previous years. The estimated application rate is then applied uniformly to the field (Sawyer, 1994). Lack of soil homogeneity and difficulty in implementing effective sampling strategies can lead to misapplication of nitrogen. An under-application of nitrogen may diminish crop production, while over-application can lead to negative environmental impacts, including nitrogen leaching and groundwater contamination . Fertilizer in excess of plant needs may result in surface runoff and pollution of lakes and streams (Daughtry et al., 2000;Wood et al., 1993). In either case, the economic returns to the producer are reduced. Therefore, a method that would allow real-time, in-field detection of crop nitrogen needs could be useful in crop nitrogen management.Chlorophyll a and b and carotenoid concentrations correlate to the photosynthetic potential of a plant and give some indication of the physiological status of the plant (Danks et al., 1983;Gamon and Surfus, 1999;Young and Britton, 1990 mass per unit area or per plant (kg/ha or kg/plant). Chlorophyll concentration (C conc ) is defined as the chlorophyll mass per unit mass of dry plant material (mg/kg). C yld may be used to evaluate the overall photosynthetic capacity or productivity of the plant canopy. C conc may be an indicator of plant physiological status or level of stress (Blackburn, 1998).Chlorophyll a content is mainly determined by nitrogen availability (Moorby and Besford, 1983). Light reflectance by leaves in the visible region of the spectrum depends primarily on the concentration of chlorophylls and carotenoids. A deficiency in nutrients such as nitrogen decreases pigment formation and leaf color, which subsequently increases reflectivity due to reduced radiation absorption.The normalized difference vegetative index (NDVI) was proposed by Rouse et al. (1974) where I NIR = near-infrared irradiance I RED = red irradiance. NDVI has a range of -1 to +1, with bare soil surfaces having an NDVI of approximately 0 and heavy vegetative cover having an NDVI of near 1 (Thiam, 1998). Reflected red irradiance (I RED ) is strongly diminished through chlorophyll absorption, with peak chlorophyll absorption occurring at 647Ănm. Both red and NIR irradiance are strongly influenced by plant cover. Red irradiance (I RED ) decreases with plant cover, and NIR irradiance (I NIR ) increases. Given these relationships, NDVI from vegetated surfaces is heavily influenced by chlorophyll content of materials in the vegetation. NDVI was introduced as a measure of biomass and is successful because chlorophyll concentration in plants is relatively constant. NDVI has been correlated with such plant properties as leaf area index, fractional vegetation canopy, vegetative condition, biomass, nitrogen cont...