What the research measured
Iowa State’s published probability of a yield response to K is 80% at Very Low soil-test K, 65% at Low, 25% at Optimum, 5% at High and about 1% at Very High. In 18 Iowa trials (2003–05) K raised yield at 7 sites, all testing Optimum, and responsive soybean sites gained linearly up to 106 lb K2O/ac. In 12 Iowa soybean trials (2018–19) a 45 lb K2O/ac fluid sidedress added 4–6 bu/ac where no K had gone on before the prior corn, 2–3 bu/ac after a 90-lb rate, and nothing significant after 135 lb or on high-testing plots. A 40-year northeast Iowa study found 144 lb K2O/ac per year began lowering soybean yield once soil-test K reached about 300 ppm.
Iowa State field programs: 18 one-year rate trials (2003–05), 12 soybean sidedress trials (2018–19, industry-funded) and a 40-year rate study (1979–2018), plus the Tri-State (Michigan State, Ohio State, Purdue) and Minnesota extension guidelines that set critical levels and rates.5 independent sources.What it is. Granular potassium chloride (0-0-62), broadcast and usually incorporated once for the two-year corn–soybean rotation, with fluid K sidedress tested as a rescue option. Iowa State interprets dry ammonium-acetate soil-test K as Very Low 0–125, Low 126–170 and Optimum 171–220 ppm; the Tri-State (Michigan, Ohio, Indiana) critical level is 75 + 2.5 × CEC ppm.
Study conditions and limitations
- Category matters more than product: no Iowa site testing High or Very High responded in 2003–05, and Iowa State recommends only removal-based maintenance K in the Optimum range.
- Residual K from the corn year carries over: without sidedress, 2019 soybean yield plateaued at 67.5 bu/ac with an estimated 126 lb K2O/ac broadcast before the prior corn, and sidedress added nothing significant on top of a 135-lb rate or on high-testing plots.
- More is not neutral for soybean: in the 40-year study, 144 lb K2O/ac per year lowered soybean yield relative to 72 lb since about 2004, which the authors attribute to salt and chloride sensitivity at about 300 ppm soil-test K or higher; corn was unaffected. Minnesota advises no more than 100 lb of KCl per acre applied directly to the soybean crop.
- Leaf K is not a yield signal: K raised leaf K concentration at 17 of 18 Iowa sites regardless of yield response, and 2018–19 leaf K kept rising at rates above those that maximized yield.
- Soil-test numbers are not interchangeable: Iowa’s Optimum range was 90–130 ppm in the 2007 paper and is 171–220 ppm on the dry test in the 2023 report (moist test 101–150 ppm), and the Tri-State critical level rises from 88 ppm at CEC 5 to 150 ppm at CEC 30.
- Soybean removes 1.40 lb K2O per bushel against 0.27 for corn grain (Tri-State), so the rotation’s K draw is concentrated in the soybean year.
Read the sources · 5
- Corn and Soybean Grain Yield and Concentration of Potassium in Plant Tissues and Soil as Affected by Potassium Fertilization ↗
Eighteen Iowa trials (2003–05) with 0 to 180 lb K2O/ac: K raised yield at seven sites, all testing Optimum, and at none testing High or Very High. Responsive soybean yield rose linearly to 106 lb K2O/ac and corn to 122. Iowa’s published response probabilities are 80% Very Low, 65% Low, 25% Optimum, 5% High, under 1% Very High.
- Evaluation of Sidedress Fluid Potassium Fertilizer Application for Soybean Production ↗
Twelve soybean trials (2018–19) following corn that had 0 to 135 lb K2O/ac: a 45 lb K2O/ac fluid sidedress added 5 and 4 bu/ac (2018) and 4 to 6 bu/ac (2019) after 0 or 45 lb before corn, 2 to 3 bu/ac after 90 lb, and nothing significant after 135 lb or on high-testing plots. University research funded by industry, not company-authored.
- Phosphorus and Potassium Fertilization for Corn and Soybean During 40 Years in Northeast Iowa - Final Study Report ↗
From 1979 to 2018 on Kenyon loam starting at 213 ppm K, unfertilized soil-test K fell to about 100 ppm after 25 years and it took 17 years before P or K raised yield. The 144 lb K2O/ac annual rate did not affect corn but lowered soybean yield relative to 72 lb once soil-test K reached about 300 ppm.
- Tri-state Fertilizer Recommendations for Corn, Soybeans, Wheat and Alfalfa ↗
Sets the critical soil-test K level at 75 + 2.5 × CEC ppm for all crops (88 ppm at CEC 5, 150 at CEC 30) and lists soybean removal at 1.40 lb K2O per bushel against 0.27 for corn grain. At CEC 10 and a 50 bu/ac yield potential, the soybean recommendation runs from 165 lb K2O/ac at 50 ppm to 90 lb in the maintenance range and 0 at 150 ppm.
- Fertilizing soybean in Minnesota ↗
Minnesota recommends 60 to 110 lb K2O/ac for a 60–69 bu/ac soybean yield goal at soil-test K of 0–150 ppm, 35 lb at 151–200 ppm and none above 200 ppm, says building soil-test K above 120 ppm is not recommended, and advises no more than 100 lb of KCl per acre directly on soybean because small yield reductions have followed higher rates.
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Record
- Sources checked
- October 12, 2026. Every figure shown is printed on the linked page; unit conversions are noted on the chart.
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