Optical sensor based field element size and sensing strategy for nitrogen application

Fundamental field element sizes for the plant nitrogen spectral index (PNSI) were calculated for 12 transects at two locations in winter wheat. PNSI, the inverse of the NDVI, is a statistic calculated from the red and near-infrared radiance of wheat plants. Previously reported work had shown that PNSI was related to total N uptake, the amount of nitrogen in the vegetative portion of the plant. Based on semivariogram and mean difference analysis, the fundamental field element size for PNSI was shown to be between 0.70 and 4.6 m with 1.4 m being the field element length common to all transects at two locations, one with nitrogen applied in the fall and one with no nitrogen applied in the fall. Semivariograms, for three transects, of PNSI averaged over 1.5 m long field elements and total N uptake determined by dry combustion from wheat harvested from the same field elements were similar in shape and supported the conclusion that the fundamental field element length should be in the range determined from PNSI analysis. Attempts to predict the total N uptake in a field element from measurements of PNSI in preceding field elements yielded errors greater than 9% compared to directly sensing PNSI in the field element. This research suggests that variable rate technology which treats field elements greater than 1.96 m(2) will likely not optimize fertilizer N inputs while having the potential for misapplying fertilizers as a result of too coarse a grid.