Written Tests: Constructed-Response and Selected-Response ...
On-Farm Response Tests for P & K
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On-Farm Response Tests for P & K
Peter ScharfUniversity of Missouri Plant Sciences
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Q: What is the main tool we use to make P & K decisions?
A: Soil testing
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Q: What does a soil test tell you?
A: Probability of yield response to P or K
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Q: Why not just measure yield response to P and K, and skip the soil test?
A: Great idea! We have the tools.
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The concept: 0 PK strips…
2 strips with no P or K applied
The rest of the field got variable-rate P & K based on grid soil samples
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…plus yield maps
Yield response = yield with P & K
Minus yield without P & K
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Outcome: low yield response, spatially clustered
• 3.4 bu/acre average yield response to P & K• $20/acre benefit• $63/acre spent on P & K + $8/acre app cost• -$50/acre return to P & K
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Is yield responsepredicted by soil test?
0 20 40 60-120
-90
-60
-30
0
30
60
90
120
f(x) = 0.480194796114226 x − 12.4795383773949R² = 0.0261449290542726
soil test P (ppm)
Yiel
d R
espo
nse,
bu/
acre
Not by soil test P!
100 120 140 160-120
-90
-60
-30
0
30
60
90
120
f(x) = 0.671120049155461 x − 81.8239742224424R² = 0.0822204456454692
soil test K (ppm)Yi
eld
Res
pons
e, b
u/ac
re
Not by soil test K either
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Yield response appears to be strongly related to soil map unit!
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• 7.5 bu/ac average yield response
• But average soil test P was only 6 ppm!
• Applied 25-120-100, spent $134/ac, made $45 back
• Net return = -$89 per acre
• Soil map unit again appeared to be related to yield response
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• Spent $90/ac on P & K (23-60-79 average rate + variable-rate application)
• 6 bushel/acre average yield response
• -$53/acre net• Soil map unit again
related to yield response!
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P & K strip trials: Summary
• Return to P & K was strongly negative in all 3 fields
• Soil test values– No predictive ability in 2 fields– Weak predictive ability in 3rd field (lowest testing)
• Made $ on PK only in certain map units: all 3 fields
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Why low yield response to P & K?Maybe subsoil P & K?
<1x
Subsoil Bray-1 P 6-36”
1-2x
2-3x
>4x
3-4x
Subsoil K 6-36”
3-4x
2-3x
>4x
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How to make $ with this approach• Try strips with 0 P & K• If yield loss is small or zero, switch to only applying P & K on one or two
strips• Apply same strips every year• Carefully monitor yield response to P & K• When an economical yield response is seen, go back to replacement
applications– Maybe double application the first year
• Start replacement applications on responsive soil map units first• Savings are temporary—withdrawing from the soil ‘bank’• But if economical yield response is not seen for 5-10 years, the saved
capital can be put into something more profitable– Example: 1000 acres, 7 years, $80/ac/yr = $560,000
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Thanks for your attention!
Questions?