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Using aerial
photography &
fertigation to fine-tune
N management
Peter Scharf
MU Agronomy Extension
Why fertigate?
Avoid N loss from leaching
Unable to apply at normal time due to
weather
Respond to N loss or deficiency
Fine-tune N management
Gives a yield boost that earlier
applications can’t achieve
Reason #1: Avoid N loss
from leaching
N uptake mostly from knee-high to early
grain fill
Later application reduces the window of
time between N application and uptake

Reduced time = reduced risk of N loss
N uptake timing: good
match with fertigation
Corn N uptake
% of uptake
100
0
V6
V12 VT
milk
Growth stage
Reason #1: Avoid N loss
from leaching
Especially on sandy soils
N loss can cause yield loss
Same N rate but applied later through
pivot = lower risk
This is the same concept as sidedress
N being lower risk than preplant
Losing N & yield (sandy
soil)
150 lb N timing
Preplant
2/3 PP, 1/3 12-leaf
1/3 PP, 2/3 12-leaf
8-leaf
12-leaf
4-way split
Source: University of Minnesota
Yield (3-year ave.)
147
159
167
179
185
184
2 of 3 years had heavy June rains
Situation #2: Unable to
apply N at normal time
2003 rainfall, April 1 - June 15
Situation #2: Unable to
apply N at normal time
This is not an uncommon situation in
Missouri
When it happens, what are the
considerations for amount and time of
fertigation?
Timing of fertigation when main
N application was missed
Corn is less sensitive to N timing than
you might think
28 experiments, mostly on-farm:
No yield loss if first N applied by chest high
 3% yield loss if first N applied from chest
high to 8 feet tall
 15% yield loss if first N applied at silking

yield response to N (bu/ac)
Timing of fertigation when main
N application was missed
Eight production corn fields, 1997
120
100
80
60
40
20
0
-20
plant
0
18”
5
36”
10
80”
15
time of 200 lb N application
20
Situation #2: Unable to
apply N at normal time
Soil is wet
You may be tempted to rush irrigation in
order to get N to the crop
This may not be necessary
Injection system needs enough capacity
to apply most of crop N need through
the pivot
Timing of fertigation when main
N application was missed
As soon as possible, but don’t rush to
irrigate on wet ground
Even if delayed until silking, fertigating
will be highly profitable if the main N
application was missed due to weather
No later than two weeks after tasseling
Situation #3: N was
applied, but it was lost
Northeast Missouri, 1998--wet May and June weather
Situation #3: N was
applied, but it was lost
2002—two experiments where applied N
was lost
Both near Brunswick/Miami
Soil N good on May 22 (but 9 inches of
rain since April 26)
9 more inches of rain, May 22 to June 15
40 and 60 bu/acre responses to added N
Diagnosing N loss
This is the hard part
Deciding if additional N is needed
 Prioritizing fields
 Deciding how much N to apply

Aerial view is best
Cover more ground faster than other ways
of doing diagnosis
 Better overall assessment

Diagnosing N loss from
an airplane
Photographs are better than memory
There are some pilots around who will do this
for a reasonable price
There are companies considering developing
an aerial photo/recommendation map service
This is not commercially available at present
A seat-of-the-pants interpretation of where to
put more N and where to put less will
probably be fairly accurate
Situation #4: Finetuning N management
Expect a yield response only when corn
has visible N stress
Gene Stevens: fertigation experiment
on Watkins farm
N stress measured with chlorophyll meter
 High N stress: 35 bu/acre response
 Low N stress: 10 bu/acre response
 No N stress: no response

Tools for fine-tuning N
management
Aerial photographs
Applicator-mounted
spectral radiometer
Aerial photos to finetune N management
Research at the University of Missouri
has related corn color from aerial
photos to:
Corn yield response to N
 Amount of N needed

Applying these relationships to photos
of pivot fields can guide fertigation
decisions
An example from project
Team Up!
Photo July 3, 2003
Atchison County
15%
lighter
25%
lighter
lighter,
needs N
darkest,
okay
Interpreting lighter
areas—watch out for:
Tassels


Tasseled corn appears lighter than corn that has
not yet tasseled
If only part of the field has tasseled, it will appear
lighter than the rest of the field

Rather than being the N-deficient areas, these areas are
farther along and may have the BEST nitrogen supply of
anywhere in the field
The influence of soil color (if the corn has not
reached full canopy)
The influence of corn size on how much soil you
see
An example from project
Team Up!
Photo July 3, 2003
Atchison County
50 N
100 N
Zero N
lighter,
needs N
darkest,
okay
An example from project
Team Up! Joe Henggeler
Image analysis predicts:
10 to
30 bu
30 to
60 bu
No response
calculates variable
fertigation $2000
better than uniform
lighter,
needs N
darkest,
okay
Fertigating corn:
summary
Reliable way to supply N

reduces risk of N (& yield) loss on sandy
soils
Good tool for managing unexpected N
loss
Potential for supplying needed N very
accurately with no overapplication