Peter Motavalli, Kelly Nelson,Peter Motavalli, Kelly Nelson, and Manjula Nathanand Manjula Nathan

University of MissouriUniversity of MissouriEmail: motavallip@missouri.edu Email: motavallip@missouri.edu

SOYBEAN RESPONSE TO PRE-PLANT SOYBEAN RESPONSE TO PRE-PLANT AND FOLIAR-APPLIED POTASSIUM AND FOLIAR-APPLIED POTASSIUM

INCREASING K DEFICIENCYINCREASING K DEFICIENCY

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

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

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

Increasing K deficiency in agronomicIncreasing K deficiency in agronomiccrops in Missouri and other Midwestern states:crops in Missouri and other Midwestern states:

BACKGROUND RESEARCH:BACKGROUND RESEARCH:

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

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

BACKGROUND RESEARCH:BACKGROUND RESEARCH:

• A recent 3-year study in Iowa with 48 trials of foliar N-P-K fertilization of soybean at early vegetative 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 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).with decreased yields (Haq and Mallarino, 1998 and 2000).

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

OBJECTIVES:OBJECTIVES:

• To examine improvements in crop yield and other agronomic benefits of applying 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.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 To compare the use of different soil test K extractants (i.e. ammonium acetate and sodium tetraphenylboron) and their relationship with crop response. and sodium tetraphenylboron) and their relationship with crop response.

MATERIALS AND METHODSMATERIALS AND METHODS::

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

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

MATERIALS AND METHODSMATERIALS AND METHODS::

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

INITIAL SOIL CHARACTERISTICS:INITIAL SOIL CHARACTERISTICS:

pH pH Bray-1 ExchangeableBray-1 Exchangeable Year Year (0.01 (0.01 MM CaCl CaCl22)) OMOM P K Ca Mg P K Ca Mg

- % - ------------- lbs/Acre ------------- - % - ------------- lbs/Acre -------------

20012001 7.27.2 3.63.6 3030 148148 52415241 559559

20022002 7.17.1 2.92.9 4343 143143 70207020 636636

Desired Soil Test K Level = 220 + (5 x CEC)Desired Soil Test K Level = 220 + (5 x CEC)For 2001 = 298 lbs K/AcreFor 2001 = 298 lbs K/AcreFor 2002 = 322 lbs K/AcreFor 2002 = 322 lbs K/Acre

SOIL TEST K FROM TREATMENTS:SOIL TEST K FROM TREATMENTS:

Pre-plant Exchangeable Pre-plant Exchangeable K Applied KK Applied K

--- lbs/Acre ------ lbs/Acre ---

0 lbs K/Acre0 lbs K/Acre 146146

125125 168168

250250 185185

500500 217217

Sampled June, 2001Sampled June, 2001

2001 RESULTS - GREENLEY2001 RESULTS - GREENLEY

Pre-plant KPre-plant K CheckCheck

2002 RESULTS - GREENLEY2002 RESULTS - GREENLEY

Pre-plant KPre-plant KCheckCheck

SoybeanSoybeanyieldyield

(bu/acre)(bu/acre)

00

1515

3030

4545

6060

0

125

250500 lbs K/acre

PreplantPreplantAppliedApplied FoliarFoliar

Applied Applied

V4V4 R1-R2R1-R2 R3-R4R3-R4

S08

16

32

S0

816

32

S08

16 32

LSD (0.05)

2001 RESULTS - YIELD2001 RESULTS - YIELD

2002 RESULTS - YIELD2002 RESULTS - YIELD

SoybeanSoybeanyieldyield

(bu/acre)(bu/acre)

00

1515

3030

4545

6060

0

125250

500 lbs K/acre

PreplantPreplantAppliedApplied FoliarFoliar

Applied Applied

V4V4 R1-R2R1-R2 R3- R4R3- R4

S0

816 32

S0

8 1632

S0

8 16 32

LSD(0.05)

SALT INJURY RATINGSSALT 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 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).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.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.

SOIL TEST K EXTRACTANTSSOIL TEST K EXTRACTANTS

Sodium tetraphenylboron KSodium tetraphenylboron K (lbs/acre)(lbs/acre)

250250 500500 750750 10001000 12501250

Am

mo

niu

m a

ceta

te K

Am

mo

niu

m a

ceta

te K

(lb

s/ac

re)

(lb

s/ac

re)

100100

150150

200200

250250

300300

350350

Y = 0.26X + 47.6R

2 = 0.85***, n = 32

2001

2002

RELATIONSHIP WITH SOIL TEST KRELATIONSHIP WITH SOIL TEST K

Soil test K (lbs/acre)Soil test K (lbs/acre)

100100 150150 200200 250250 300300 350350

RelativeRelative yieldyield(%)(%)

2020

4040

6060

8080

100100

Y = -0.0038X2 + 1.92X - 154

(if X>253 then Y = 89)R

2 = 0.64***, n = 32

Ammonium Acetate

RELATIONSHIP WITH SOIL TEST KRELATIONSHIP WITH SOIL TEST K

RelativeRelative yieldyield(%)(%)

2020

4040

6060

8080

100100Sodium Tetraphenylboron

Soil test K (lbs/acre)Soil test K (lbs/acre)

250250 500500 750750 10001000 12501250

Y = -0.00087X2 + 1.08X - 253

(if X>621 then Y = 82)R

2 = 0.52***, n = 32

CONCLUSIONSCONCLUSIONS

• Foliar K applications in soybeans may be a possible management tool to mitigate reduced yields caused by K deficiency.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.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 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.economic benefits.

CONCLUSIONSCONCLUSIONS

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

SolubilitySolubilityCrop responseCrop responseCompatibility with post-emergenceCompatibility with post-emergence

herbicides herbicidesEffectiveness of K source/herbicide mixEffectiveness of K source/herbicide mix

for weed control for weed controlCost effectivenessCost effectiveness