Sugarbeet canopy type and accumulation of plant nitrogen as delineated by aerial photography and global positioning systems
1998
Moraghan, J.T.
Considerable within-field variation in the N content and appearance of sugarbeet (Beta vulgaris L.) leaf canopies is found often at root harvest. Since this heterogeneity can affect subsequent soil N mineralization, a study was initiated to determine if the within-field variation could be identified and located by aerial photography and a global positioning system (GPS). An aerial color photograph was taken of a commercial sugarbeet field prior to harvest. Three reflectance patterns associated with different degrees of canopy greenness ("green" [G], "yel1ow-green" [YG] and "yellow" [Y]) were identified readily. Four areas, each approximately 0.8 ha in size, with these three canopy types were selected. The canopy subsites were located by use of a GPS unit. The G, YG, and Y canopy types contained 277, 138, and 85 kg N ha-1, respectively, in sugarbeet tops. The corresponding leaf N concentrations were 30.6, 20.1, and 15.8 g kg-1, respectively. Postharvest soil NO3-N levels in the upper 120 cm of soil were 57, 14, and 10 kg ha-1 for the G, YG, and Y canopy sites, respectively. Nitrate-N in sugarbeet tops returned to the soils was at least partly responsible for the higher soil at the G subsites. In contrast to dry matter yield, recoverable sugar yield was significantly less for roots associated with the G-canopy type than for roots associated with the YG-and Y-canopy sites. Aerial photography and GPS technology can increase precision of soil testing for NO3-N after a sugarbeet crop, help to identify areas of increased soil N mineralization for a subsequent crop, and decrease subsequent N fertilizer use if a variable rate N-fertilizer applicator is used.
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