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
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
Oklahoma State University
recently released the first automated calibration stamp
applicator for improving in-season fertilizer N rates and
ultimate Nitrogen Use Efficiency.
Accurate determination of mid-season fertilizer N rates for
cereal production is complicated. Mid-season
fertilizer N is currently recommended using a wide range of
soil-test and soil-N mineralization procedures, with no
fundamental scientific agreement anywhere in the world on
the methodology. A conventional 2003 Honda FourTrax
Foreman ES 4-wheeler (433cc, 127cm wheel base, 116cm wide)
with a 3 m wide spray boom and a 1 m spacing between nozzles
was modified to deliver a range of fixed N rates as urea
ammonium nitrate (28%N) within a 9m2 grid.
Within each grid, 9 separate 1m2 areas exist,
whereby each of the 4 corners receive no fertilizer N.
Rates of 22, 45, 67, 90, and 112 kg N ha-1 occupy
the other 5, 1m2 areas within the 9m2
grid (termed as an N rate calibration stamp).
Traveling at 5 mph, consecutive 9m2 grids can be
applied continuously. The calibration stamps should be
applied preplant and superimposed on top of the farmer
practice. By mid-season, differences between the 1m2
N rate areas can be visualized and a field-specific
topdress N rate can be prescribed by choosing the lowest N
rate where no visual differences were observed between it
and the next highest rate. Using preplant or early-season
applied calibration stamps, topdress N rates can be
determined that precisely account for N mineralization, and
atmospheric N deposition from planting to the time
mid-season N is applied.
Click here for VIDEO of the Calibration Stamp!
Clint Mack prepares to apply calibration stamps in Western
Calibration Stamp in Western Oklahoma showing the 3 strips
with decreasing N rates.
Calibration Strips applied
just north of Experiment #222, fall 2004, read on March 18,
Dr. Marvin Stone, Dr. John
Solie, David Zavodny, and Kyle Freeman stand in front of the
very first CST applicator.
(above and below). Calibration stamps applied soon
after planting (left) and visual stamp differences
mid-season (right) that are used to prescribe accurate
mid-season N fertilizer rates.
Continuous calibration stamps (9m2) applied at
Perkins, OK on top of winter wheat at Feekes growth stage 4,
and superimposed schematic of actual N rates in kg N/ha
using urea ammonium nitrate applied as a foliar spray.
UAN with Blue Dye applied soon after wheat planting such
that calibration stamps can be visualized by
Kyle Freeman and Brian Arnall
adjust the air pressure for the spraying system that
requires 30 PSI. Next to his left hand is the
controller which can be operated with 3 switches (power,
purge, and spray). The controller automatically cycles
if the spray button is held down. Each cycle includes
a stamp with 9 squares (4 checks of 0, and one each of 20,
40, 60, 80, and 100 lbs N/ac). Each square measures 1
x 1 m, while the entire stamp measures 3 x 3 m.
Prior to putting out the calibration stamps, Kyle Freeman
purges the system with UAN.