1. Third International, AGRONOMY CONGRESS Agriculture Diversification, Climate Change Management and Livelihoods
November 26-30, 2012, New Delhi, India
Hyperspectral remote sensing- A valuable tool for site specific nitrogen management
Suchit K. Rai, A.K.Rai, Sunil Kumar S.K.Das and Y.S.Saharawat
Indian Grassland and Fodder Research Institute,
Gwalior Road, Jhansi , India
Corresponding author

2. AIM
To determine spectral characteristics of fodder crops over the growing period
To develop functional relationship between canopy hyperspectral reflectance or reflectance ratio with canopy N and Chl concentrations

3. Introduction
Hyperspectral remote sensing provides a powerful tool for monitoring changes in the crop canopy over the growing season and crop developmental information that is time critical for site specific crop fertilizer management. Among the many foliar bio chemicals, chlorophyll (Chl) is an important indicator of photosynthesis rate and overall nutritional status as it is directly proportional to nitrogen (N).

4. Materials and Method
A field study was conducted during the kharif (July to December) seasons of 2006 to 2010 on sandy loam soil at C. R. Farm, IGFRI, Jhansi (25oC 27’N, 78o 35’E, 271m amsl). Fodder maize (cv. African tall) was sown with two nitrogen levels ( N0 : no application of N during the growing season and N1: recommended dose of N ) with three replications in randomized block design
Periodical N and Chl as well as canopy hyperspectral reflectance measurements were made using a portable ASD Fieldspec spectroradiometer (Analytical Spectral Devices, Inc., Boulder, Co.) with a wavelength ranging from 325 to 1075 nm during the growing season. The reflectance differences (RD) at each wavelength and reflectance sensitivity (RS) to N fertilizer were computed in order to determine the effects of N fertilizer on canopy hyperspectral reflectance. A linear regression was performed using corresponding single reflectance or reflectance ratios with Chl concentration and N content.

5. RESULTS and DISCUSSION
Leaf nitrogen (N) concentration differed across sampling dates and treatments levels and varied from to 1.87 % over the growing period irrespective of treatments and it was highest between 61 days after sowing (DAS).
Chlorophyll concentration was highest (3.9 mg/g) at 75 DAS with crop treated with Nitrogen, this could be mainly due to split application of nitrogen at 50% flowering stage.
Averaged across sampling dates, values of leaf N of the recommended and without N treated crops were 0.72 and 1.44 percent, respectively.

6. averaged spectral reflectance curves across the sampling dates in the growing season, it becomes very apparent that the treatments in the study produced effects in the wavelength regions of 525 to 680 nm. Maximum reflectance of 17.4 and 13.4 percent at 563 nm was obtained, under N0 and N1 treatment respectively. Reflectance difference revealed that the canopy reflectance at 560 and 713 nm rapidly increased in the treatment N0

7. RS reinforced that the reflectance at 592 and 704 nm were sensitive to N applications as RS values were maximum (27-28%) at these two wavelength.
Two specific wavelengths where reflectance provided highest correlation with maize canopy N concentration were 643(r= -0.62, n=20) and 697 nm (r=-0.65, n=20).
However, using single reflectance values at any one of these two wavelengths could only explain 38.4 to 42.2 % canopy N variations. However, ratio of R 704/921 and R643/R936 gave maximum (r=-0.82 t0 85, n=18) correlation with N and Chl concentration, respectively for maize crop.

8. Conclusion
These results indicated that all the reflectance ratios decreased linearly as leaf N concentration increased. Estimation of leaf N and chlorophyll concentration using nondestructive canopy spectral reflectance measurements can be an alternative method for plant N diagnosis and N fertilizer recommendation than the traditional methods of plant.