Peter Motavalli, Kelly Nelson,

1
SOYBEAN RESPONSE TO PRE-PLANT AND FOLIAR-APPLIED
POTASSIUM
Peter Motavalli, Kelly Nelson, and Manjula
Nathan University of Missouri Email
motavallip_at_missouri.edu
2
INCREASING K DEFICIENCY
Increasing K deficiency in agronomic crops in
Missouri and other Midwestern states

• Decrease in soil K availability due to drought
  conditions and soil compaction.

• Reductions in applied K fertilizer and the
  frequency of soil testing by producers due to
  decreasing commodity prices.

• Higher corn yields and increasing soybean acreage
  in rotation with corn has increased K fertilizer
  requirements.

3
BACKGROUND RESEARCH

• Response to foliar N-P-K-S fertilizers applied to
  soybeans has been inconsistent with both observed
  increases and decreases in yields. Early emphasis
  was on targeting foliar fertilization during
  reproductive growth stages.

• One group found that foliar application at the R2
  to R7 growth stages at a 10-1-3-0.5 ratio was
  optimum (Garcia and Hanway, 1976).

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BACKGROUND RESEARCH

• A recent 3-year study in Iowa with 48 trials of
  foliar N-P-K fertilization of soybean at early
  vegetative growth stages (approx. V5) on
  different soil resources observed 7 sites with
  increased yields and 2 sites with decreased
  yields (Haq and Mallarino, 1998 and 2000).

• Yield response tended to occur in soils with high
  CEC, when soil test P was low and when rainfall
  in spring and midsummer was low.

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OBJECTIVES

• To examine improvements in crop yield and other
  agronomic benefits of applying pre-plant and
  foliar K fertilizer at different times during the
  growing season.

• To compare the use of different soil test K
  extractants (i.e. ammonium acetate and sodium
  tetraphenylboron) and their relationship with
  crop response.

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MATERIALS AND METHODS

• Two-year field trial initiated in 2001 in a
  cooperators farm field southeast of Greenley
  Agronomy Center on a claypan soil.

• No-till planted Asgrow 3701 Roundup Ready
  soybeans both years (7.5 inch rows at 200,000
  seeds/acre).

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MATERIALS AND METHODS

• 4 rates of pre-plant K fertilizer (0, 125, 250
  and 500 lbs K/acre as potassium sulfate)
  broadcast-applied.
• 3 additional K rates (8, 16 and 32 lbs K/acre as
  potassium sulfate) foliar-applied at 3 separate
  times (V4, R1-R2, and R3-R4 soybean growth
  stages).
• Had foliar checks of magnesium sulfate.
• Foliar application was 60 gallons/acre.

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INITIAL SOIL CHARACTERISTICS
pH
Bray-1 Exchangeable Year (0.01 M
CaCl2) OM P K Ca Mg


- - ------------- lbs/Acre
------------- 2001 7.2 3.6 30 148 5241
559 2002 7.1 2.9 43 143 7020 636
Desired Soil Test K Level 220 (5 x CEC) For
2001 298 lbs K/Acre For 2002 322 lbs K/Acre
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SOIL TEST K FROM TREATMENTS
Sampled June, 2001
Pre-plant Exchangeable K Applied
K

--- lbs/Acre --- 0 lbs
K/Acre 146 125 168 250 185 500 217
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2001 RESULTS - GREENLEY
Pre-plant K
Check
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2002 RESULTS - GREENLEY
Check
Pre-plant K
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2001 RESULTS - YIELD
500 lbs K/acre
250
125
32
32
16
8
32
16
16
Soybean
8
S0
0
S0
8
yield
S0
(bu/acre)
V4
R3-R4
R1-R2
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2002 RESULTS - YIELD
500 lbs K/acre
250
LSD
125
(0.05)
32
32
16
16
8
8
32
16
8
S0
0
S0
S0
R3- R4
Preplant
V4
R1-R2
Applied
Foliar
Applied
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SALT INJURY RATINGS

• Potassium fertilizers, such as muriate of potash
  (KCl), have a high salt index and therefore can
  cause salt injury when foliar-applied.
• Potassium sulfate has a lower salt index (46)
  compared to KCl (116).
• Leaf injury ratings in 2001 and 2002 showed
  minimal to no effects of foliar application of
  potassium sulfate at application rates used in
  this study.

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SOIL TEST K EXTRACTANTS
350
2001
300
2002
250
200
Y 0.26X 47.6
2
R
0.85, n 32
150
100
250
500
750
1000
1250
Sodium tetraphenylboron K
(lbs/acre)
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RELATIONSHIP WITH SOIL TEST K
Ammonium Acetate
Soil test K (lbs/acre)
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RELATIONSHIP WITH SOIL TEST K
Sodium Tetraphenylboron
2
Y -0.00087X
1.08X - 253
(if Xgt621 then Y 82)
2
R
0.52, n 32
250
500
750
1000
1250
Soil test K (lbs/acre)
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CONCLUSIONS

• Foliar K applications in soybeans may be a
  possible management tool to mitigate reduced
  yields caused by K deficiency.
• However, highest yields were obtained with
  pre-plant K fertilizer applications.
• Need additional testing of this approach at other
  locations and under different climatic conditions
  in Missouri to confirm these results and evaluate
  potential economic benefits.

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CONCLUSIONS

• Further testing to identify most practical foliar
  K source(s)

• Solubility
• Crop response
• Compatibility with post-emergence herbicides
• Effectiveness of K source/herbicide mix for weed
  control
• Cost effectiveness