Corn Growth and Canopy Reflectance Properties as Affected by Urea-N Management Products.

Developing simple techniques that identify nitrogen stress and discriminate between the nitrogen sources can have major implication in crop fertilizer management. The aim of this study was to quantify the effect of different urea-N management products on corn growth and development and to examine whether canopy spectral reflectance can be used to detect differences between treatments. No-till field grown corn was subjected to the application of 0 N, and six urea-N management products applied at equal N amounts as urea, agrotain treated urea, polymer coated urea, nutrisphere treated urea, ESN urea and 28% UAN. Plant height, dry matter, SPAD reading, LAI and canopy reflectance were measured regularly over the course of the season. The normalized difference vegetation index (NDVI), enhanced vegetation index (EVI), modified chlorophyll absorption reflectance index (MCAI), photochemical reflectance index (PRI), mean reflectance of green (520-600 nm) and red (630-690 nm) spectrum based on Landsat Thematic Mapper bands (TM2-3), first derivatives of the reflectance in red edge (dR/dë) as well as several spectral indices and ratios were derived. The degree of response of different urea-N management products on the reflectance properties varied depending on the stages of plant growth. In general, between 60-70 days after emergence, NDVI, EVI, RNIR/RRed and PRI reached their maximum across all the treatments. Thereafter, it started to decline at slower (NDVI, EVI) or faster (RNIR/RRed, PRI) rate towards the end of the season. As expected, the 0 N treatment drastically increased the reflectance in the green and red regions centered around 555 nm and caused a marked shift in the red-edge reflectance (ë = 685-745) towards shorter wavelength of spectrum across the growing season. The dR/dë, RI (reflectance index [740-480)/(400-700)] provided the best measure of N stress as well as discriminated between the N sources.