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 uptakeReduced time = reduced risk of N loss

N uptake timing: good match with fertigation

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Growth stage

Corn N uptake

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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 Yield (3-year ave.) Preplant 147 2/3 PP, 1/3 12-leaf 159 1/3 PP, 2/3 12-leaf 167 8-leaf 179 12-leaf 185 4-way split 184

Source: University of Minnesota 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 high3% yield loss if first N applied from chest

high to 8 feet tall15% yield loss if first N applied at silking

Timing of fertigation when main N application was missed

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time of 200 lb N application

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Eight production corn fields, 1997

Situation #2: Unable to apply N at normal time

Soil is wetYou may be tempted to rush irrigation in order to get N to the cropThis 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 partDeciding if additional N is neededPrioritizing fieldsDeciding how much N to apply

Aerial view is bestCover more ground faster than other ways

of doing diagnosisBetter overall assessment

Diagnosing N loss from an airplane

Photographs are better than memoryThere are some pilots around who will do this for a reasonable priceThere are companies considering developing an aerial photo/recommendation map serviceThis is not commercially available at presentA seat-of-the-pants interpretation of where to put more N and where to put less will probably be fairly accurate

Situation #4: Fine-tuning N management

Expect a yield response only when corn has visible N stressGene Stevens: fertigation experiment on Watkins farmN stress measured with chlorophyll meterHigh N stress: 35 bu/acre responseLow N stress: 10 bu/acre responseNo N stress: no response

Tools for fine-tuning N management

Aerial photographs Applicator-mounted spectral radiometer

Aerial photos to fine-tune N management

Research at the University of Missouri has related corn color from aerial photos to:Corn yield response to NAmount of N needed

Applying these relationships to photos of pivot fields can guide fertigation decisions

An example from project Team Up!

darkest,okay

lighter,needs N

Photo July 3, 2003Atchison County

15% lighter

25%lighter

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!

darkest,okay

lighter,needs N

Photo July 3, 2003Atchison County

Zero N

50 N 100 N

An example from project Team Up!

darkest,okay

lighter,needs N

Joe Henggeler calculates variable fertigation $2000 better than uniform

No response

10 to 30 bu

30 to60 bu

Image analysis predicts:

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