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Peter Motavalli, Kelly Nelson, and Manjula Nathan University of Missouri

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

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Peter Motavalli, Kelly Nelson, and Manjula Nathan University of Missouri

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  1. SOYBEAN RESPONSE TO PRE-PLANT AND FOLIAR-APPLIED POTASSIUM Peter Motavalli, Kelly Nelson, and Manjula Nathan University of Missouri Email: motavallip@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).

  4. 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.

  5. 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.

  6. MATERIALS AND METHODS: • Two-year field trial initiated in 2001 in a cooperator’s 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).

  7. 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.

  8. 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

  9. 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

  10. 2001 RESULTS - GREENLEY Pre-plant K Check

  11. 2002 RESULTS - GREENLEY Check Pre-plant K

  12. 60 LSD (0.05) 45 30 15 0 Preplant Applied Foliar Applied 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

  13. 60 45 Soybean yield 30 (bu/acre) 15 0 2002 RESULTS - YIELD 500 lbs K/acre 250 LSD 125 (0.05) 32 32 16 16 8 8 32 16 8 S0 S0 0 S0 R3- R4 V4 R1-R2 Preplant Applied Foliar Applied

  14. 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.

  15. Ammonium acetate K (lbs/acre) 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)

  16. 100 80 Relative 2 Y = -0.0038X + 1.92X - 154 yield (if X>253 then Y = 89) (%) 60 2 R = 0.64***, n = 32 40 20 100 150 200 250 300 350 RELATIONSHIP WITH SOIL TEST K Ammonium Acetate Soil test K (lbs/acre)

  17. 100 80 Relative yield (%) 60 40 20 RELATIONSHIP WITH SOIL TEST K Sodium Tetraphenylboron 2 Y = -0.00087X + 1.08X - 253 (if X>621 then Y = 82) 2 R = 0.52***, n = 32 250 500 750 1000 1250 Soil test K (lbs/acre)

  18. 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.

  19. 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

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