How to obtain highly accurate mid-season fertilizer N rates using the Ramped Calibration Strip 

The new Ramped Calibration Strip Applicator applies 16 different N rates (10 to 20 foot intervals), over 160 to 320 feet (actual rates and distances can be adjusted depending on the crop).  Companies interested in building variants of this applicator can adjust distances, N rates, intervals, etc., as they see fit.  The Ramped Calibration Strip is used to visually determine precise mid-season fertilizer N rates for wheat and corn production.  Although the use of hand-held GreenSeeker sensors are not required, the sensors offer the opportunity to sense the entire "RAMP" and thus accurately determine where the peak in NDVI exists over the range of N rates applied.  Without the sensor, farmers can simply walk from one end of the RAMP to the other and stop where they no longer see any differences.  Whether determined visually, or with a handheld sensor, the point where no differences no longer exist, is the TOPDRESS N Rate. 

This methodology is a variant of what has been developed using the N Rich Strip and the handheld sensing technology that relies on predicted yields and the Sensor Based Nitrogen Rate Calculator (http://www.soiltesting.okstate.edu/SBNRC/SBNRC.php)

Looking at the graph above, you can see exactly why this is the case.  By stopping at the point (recording distance in feet) where there are no longer visible or recorded differences in NDVI, you can plot a line directly down to "distance" that has a predetermined N rate associated with it.  That N rate (depending on how you set up the intervals to change the rate, and the actual rates chosen) is the TOPDRESS N Rate to be recommended.  For the 3 fields above where NDVI values were plotted with distance (green squares, red triangles, and blue circles), the recommended topdress N rates would have been 40, 80, and 100 lbs N/ac, respectively.  Why?  You have to remember that the Ramped Calibration Strip is applied "ON-TOP" of the farmer practice (whatever that may be).  Assuming that we can "catch up" and/or achieve maximum yields from the mid-season N application, and assuming that yield potentials were not severely restricted by excess early season N stress, the RAMP interpolated rate is how much you would need to apply on the rest of the field to achieve the same "visible" or "NDVI recorded" response. 

In corn, some would argue that this methodology is flawed, because you don't know whether or not this rate will "run out" later in the season.  This is to a certain extent correct, but if the farmer is wanting to avoid that potential "risk" the topdress N rate can be increased by whatever amount he/she deems appropriate.  However, the RAMP is an incredible starting point for determining the optimum mid-season N rate.  Why?  Because we have now evaluated enough corn and wheat data to know that in several long-term experiments (corn, NE, WI, MX; wheat, OK), there were years where the check plot (No N Fertilizer ever applied) produced near maximum yields.  A RAMP N Rich Strip in those fields would have told us "MID-SEASON" that there were no visible differences between the 0-N and all other plots within the RAMP receiving N.  Wouldn't farmers want to know that?  Wouldn't this be important to know?  If the check plots with no fertilizer N looked as good as the fertilized plots, where was their N coming from?  Over the years, we have learned that warm wet winters (winter wheat) and warm wet springs and early summer (corn) are conducive to increasing the amount of N mineralized from soil organic matter, and that N deposition in rainfall increases significantly in these years.  Because we have years where the demand for fertilizer N is less (and highly dependent on the environment), and other years where it is cool and dry and the demand for fertilizer N is greater.  The only way to determine how much the environment delivered for free is to have a Ramped Calibration Strip in each and every field. 

For those farmers/producers interested in using the sensors for determining their topdress N rates, all they have to do is mark the start and end of the Ramped Calibration Strip (preplant), and collect sensor data using the handheld GreenSeeker sensor walking at a constant speed over the length of the ramp.  Using the program below, they can automatically read that file, and it will compute the optimum N rate.  We recommend the use of the sensors simply because our eyes are not as sensitive in picking up these differences.

NOTE:  This approach (interpreting the Ramped Calibration Strip) is for determining field rate application (flat rates) and accounts for only temporal variability.  Simultaneous consideration of temporal and spatial variability requires on-the-go prediction of yield potential (YP0) and use of the response index for each 0.4m2 area (wheat), and every 2-3 plants in corn.