Spectral measurements of the total aerial N and biomass dry weight in maize using a quadrilateral-view optic

Heterogeneous crop stands require locally adapted nitrogen fertilizer application based on rapid and precise measurements of the local crop nitrogen status. In the present study, we validated a promising technique for the latter, namely a tractor-mounted field spectrometer with an oblique quadrilateral-view measuring optic, measuring solar radiation and canopy reflectance in four directions simultaneously. Dry matter yield (kg ha(-1)), total N content (g N g(-1) dry matter) and total aerial N (aboveground N-uptake) (kg N ha(-1)) in maize were determined in 10 m(2) calibration areas in 60 plots differing in their N treatment and seeding density three times in each of three ears under field conditions. Results show that the sensor used can reliably determine total aerial N ranging from as little as 5 kg N to 150 kg N ha(-1) with R-2-values >= 0.81 in 2002 and 2004, and with R-2-values ranging from >= 0.57 to 0.84 in 2003. Dry matter yields from as low as 0.3-4.2 t ha(-1) could be determined with R-2-values ranging from 0.67 to 0.91 in 2002 to 2004. The capacity to ascertain DM yield spectrally was drastically reduced in the higher yield range (>6 t ha(-1)) probably due to decreased sensitivity of the spectral signal. N-contents were generally not well determined. Taken together there is a good potential to determine reliably differences in total aerial N or DNI yield from the five leaf stages unfolded to the five node stage where typically nitrogen applications are carried out. Weather conditions and zenith angles of up to 30 degrees hardly influenced the results appreciably. We conclude that the tractor-based oblique quadrilateral-view optic reflectance sensor represents a suitable instrument to quickly and non-destructively determine crop nitrogen status and therefore allows for site-specific fertilizer application when coupled with a fertilizer algorithm and variable-rate applicator. (C) 2007 Elsevier B.V. All rights reserved.