Agriculture Reference
In-Depth Information
“RAMP” METHOD OF DETERMINING
MIDSEASON N RATE
the appropriate top-dress N rate. Where adequate
but not excessive preplant N is available, the ramp
interpolated rate provides a direct method to
determine how much midseason N should be
applied to achieve the maximum yields based on
growth response evidenced within the RCS.
This approach has now been extensively
adopted, and several farmers have ingeniously
developed their own versions of the RCS with
differing ranges in N rates, widths, and methods
of application, depending on their production
system, form of tillage, and crop being grown.
For each of these variants on the RCS, methodol-
ogy descriptions on how to build them are
reported by Raun (2008b).
For the winter wheat example in Color Plate
24 (taken near Feekes growth stage 5), the RCS
top-dress rate would have been near 80 kg N ha
−1
,
the point where total biomass was maximized.
Where adequate but not excessive preplant N is
available, the RCS interpolated rate (Fig. 10.3)
provides an applied method to determine how
much midseason N should be applied to achieve
the maximum yields based on growth response
evidenced within the RCS. This type of response
is typical and one that can be deciphered either
visually or using a hand-held Greenseeker NDVI
sensor. As visualized in Color Plate 24 and plotted
in Fig. 10.3, it is important to note that the
response curves will take on a wide range of forms,
and all are highly dependent on temporal vari-
The GreenSeeker NDVI sensor system has shown
promise to farmers in the developed world for
predicting yield and N responsiveness, and ulti-
mately for providing a refi ned midseason fertil-
izer N rate. However, simplifi ed technology may
be more affordable in the developing world or for
smaller-scale developed world farmers. As a
result, we developed the ramp calibration strip
approach that can be used to decipher midseason
fertilizer N rates, in much the same fashion, but
which does not require the more sophisticated
sensor currently used. Similar to the approach
described in the previous section, this methodol-
ogy assumes that waiting until midseason to apply
up to one-half of the required N fertilizer can
result in maximum or near-maximum yields. The
RCS applicator that we developed applies 16
incremental N rates at 3- to 6-m intervals over a
total distance of 45-90 m (number of rates, inter-
vals, and distances can be adjusted depending on
the crop and other conditions) (Raun et al., 2008).
Because the RCS is superimposed on the farmer
preplant N practice, producers can observe plant
responsiveness over the range of rates to deter-
mine the optimum top-dress N rate. Whether
determined visually or with simplifi ed active-
refl ectance-based sensors, the point where mid-
season visual growth differences no longer exist is
0.85
0.8
0.75
0.7
Fig. 10.3
Typical optical
sensor measurements from a
N-calibration ramp as a func-
tion of N application rate.
Curves from this graph,
adjusted for improvements in
N uptake effi ciencies realized
by mid-growing-season appli-
cation of N, can be used
to determine optimum N
application.
0.65
0.6
0.55
0.5
0
25
50
75
100
125
150
175
200
Application rate (kg N ha
-1
)
