Jb. Solie et al., OPTICAL SENSOR-BASED FIELD ELEMENT SIZE AND SENSING STRATEGY FOR NITROGEN APPLICATION, Transactions of the ASAE, 39(6), 1996, pp. 1983-1992
Fundamental field element sizes for the plant nitrogen spectral index
(PNSI) were calculated for 12 transects at two locations in winter whe
at. PNSI, the inverse of the NDVI, is a statistic calculated from the
red and near-infrared radiance of wheat plants. Previously reported wo
rk had shown that PNSI was related to total N uptake, the amount of ni
trogen in the vegetative portion of the plant. Based on semivariogram
and mean difference analysis, the fundamental field element size for P
NSI 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 nitr
ogen 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 w
heat harvested from the same field elements were similar in shape and
supported the conclusion that the fundamental field element length sho
uld be in the range determined from PNSI analysis. Attempts to predict
the total N uptake in a field element from measurements of PNSI in pr
eceding field elements yielded errors greater than 9% compared to dire
ctly sensing PNSI in the field element. This research suggests that va
riable rate technology which treats field elements greater than 1.96 m
(2) will likely not optimize fertilizer N inputs while having the pote
ntial for misapplying fertilizers as a result of too coarse a grid.