|Can We Detect Nitrogen Deficiencies With Sensors?|
1. Early work using indirect measures to determine the N status of plants employed chlorophyll meters (SPAD 501 and 502) that measured transmittance at 430 and 750 nm.
2. Plant tissue N and chlorophyll meter measurements have been found to be highly correlated, but they are crop and growth stage specific.
3. Once a reliable calibration between chlorophyll measurements and N status of the plant was established, N fertilization regimes were developed.
4. Similar to forage protein analysis using NIR, total N in growing plant tissue can be reliably detected from NIR measurements in the 780 and 1050 nm ranges
|Early Approach to Sensing and Treating N Variability in Winter Wheat at Oklahoma State University|
|1. Photodiode detectors and
interference filters for red (671±6 nm) and NIR (780±6 nm) spectral bands with a 0.46 m
wide by 0.075 m length spatial resolution were used (Stone et al., 1996).
2. Spectral radiance measurements (Red and NIR) were collected from winter wheat at Feekes growth stages 4 through 10 and NDVI (normalized difference vegetative index) was determined (NIR-red)/(NIR+red) as per the work of Perry and Lautenschlager,1984, and Duncan et al., 1993.
3. Wet and dry biomass were recorded from each plot and forage subsamples analyzed for total N using dry combustion
4. NDVI measurements were then correlated with the product of dry biomass and percent N (total N uptake) over a range of locations.
5. Once the calibration curve for NDVI and forage N uptake was established, corresponding N rates were applied based on NDVI (low NDVI - high N rate, high NDVI - low N rate).
6. Although not required in this work, surface contour maps can be generated as are included below for pre and post N fertilization, using NDVI as the response variable. Also see Management Resolution
|6. Following the application
of variable N rates, validation of response with fixed rate treatments and checks (no
nutrients applied) was required. Theoretically, variable rate treatments should
result in a significant reduction in total fertilizer applied while also reducing the
variability in observed yield or grain N uptake. This was observed at several of the
sites evaluated including the Miller-2 site below (graph and picture). However, it
should be noted that this kind of response will only be observed when N is limiting and
when the variability due to all other factors are accounted for or eliminated.
|Sensor Based Variable N Application Equipment Developed at Oklahoma State University|
|In the spring of
1996, OSU completed their first sensor based variable N rate applicator. A
photograph of this prototype is included below.
|Other Interfering Agronomic Variables|
at OSU has focused on the development of a "complete system" which can reliably
replace soil testing for N while also applying variable N fertilizer on-the-go.
However, many interfering factors still exist and which will require substantial research
and development. Some of these are included below and that are presently included in
our research agenda.
|Global Need For Precision Agriculture|