Timing of K application for corn and soybean production
Study author(s): Daniel Kaiser, University of Minnesota
Growing seasons: 2022, 2023, 2024, 2025
Minnesota location(s): Crookston, Rosemount, Lamberton, Rochester
Note: All tables and figures are available by downloading the reports below.
PROJECT Summary
- Potassium application rate and timing had inconsistent impacts on corn leaf and soybean trifoliate K concentration. Leaf and trifoliate K concentration was related to corn or soybean yield response. Calculated values for R1 sampling indicated that the current range for corn ear leaf (1.8-3.0%) and soybean trifoliate (1.7-2.5%) appears to be accurate. Corn grain yield was at maximum when V10 corn upper leaf K concentration was 0.81% or greater.
- Leaf Cl concentration increased greater with spring application of potash fertilizer. Fall application either did not increase or only slightly increased Cl concentration. Results show that increased Cl concentration in the leaf tissue did not result in decreased corn or soybean grain yield.
- Time of K application did not affect corn grain yield. Corn grain yield was maximized when 80-120 lbs K2O was applied across locations.
- Soybean grain yield was significantly lower when K was applied in spring. Across locations, soybean grain yield was not increased by K fertilizer but was 1 bushel per acre less when K rates were applied in spring. There was no difference in yield between fall and spring K applications when only 40 lbs K2O was applied. Yield decreased when 80 lbs K2O or greater were applied to soybean. Fall application is the best option if K is applied ahead of the soybean crop.
- Soybean seed protein and oil concentration were not impacted by K application rate or timing.
- There was a relationship between the K base saturation ratio and relative corn or soybean grain yield. However, this relationship was because there was also a relationship between soil test K in June and relative corn grain yield. There was no advantage of using K base saturation ratio to predict yield. The optimal K base saturation ratio was 1.0 to 1.5 for corn and soybean.
- Change in soil test K from application to a sampling in mid-June was similar regardless of K timing.
- At the sites studied, there was no increase in non-exchangeable K measured in June due to K fertilizer application. The K applied did not appear to be ‘fixed’ in non-exchangeable forms.