instantaneous results and has been demonstrated as an effective tool to schedule N fertilization for rice (Oryza Nondestructive monitoring and diagnosis of plant N status is necsativa L.) on an as-needed basis (Turner and Jund, 1991; essary for precision N management. The present study was conducted to determine if canopy reflectance could be used to evaluate leaf Peng et al., 1993). However, there are two factors that N status in rice (Oryza sativa L.). Ground-based canopy spectral limit the use of SPAD meters for N fertilization. First, reflectance and N concentration and accumulation in leaves were a within-field reference (usually an adequately fertilized measured over the entire rice growing season under various treatments area or strip within the field) is required to accurately of N fertilization, irrigation, and plant population. Analyses were quantify N deficiencies. Second, the SPAD meter colmade on the relationships of seasonal canopy spectral reflectance, lects point measurements from a single leaf on a single ratio indices, and normalized difference indices to leaf N concentration plant. Consequently, many leaves from a number of plants and N accumulation in rice under different N treatments. The results must be sampled to obtain a representative average showed that at each sampling date, leaf N concentration was negatively value for a particular sampling date and to adequately asrelated to the reflectance at the green band (560 nm) while positively sess the spatial variability. In contrast, remote sensing related to ratio index, with the best correlation at jointing. However, the relationships between leaf N accumulation and reflectance at of canopy reflectance has the capability to sample a plant green band and ratio index were consistent across the whole growth population or community rather than individual plants period. The ratio of near infrared (NIR) to green (R 810 /R 560 ) was esand to rapidly assess the spatial variability of a crop field. pecially linearly related to total leaf N accumulation, independent of The possibility of predicting crop N status using can-N level and growth stage. Tests of the linear regression model with opy reflectance spectra has been examined for major agrodifferent field experiment data sets involving different plant densities, nomic crops (Thomas and Oerther, 1972; Shibayama N fertilization, and irrigation treatments exhibited good agreement and Akiyama, 1986; Fernandez et al., 1994; Blackmer between the predicted and observed values, with an estimation accuet al.racy of 96.69%, root mean square error of 0.7072, and relative error Shen et al., 2001). For a crop canopy, reflectance is low of Ϫ0.0052. These results indicate that the ratio index of NIR to green near the 480-and 680-nm region due to the strong ab-(R 810 /R 560 ) should be useful for nondestructive monitoring of N status in rice plants.
