University of Minnesota Extension nutrient management specialist Dan Kaiser recently finished a four year study on potassium (K) fertilizer application for corn and soybeans. The study aimed to determine the optimal K rate for both crops, assess if spring application of potash (potassium chloride, KCl) negatively impacts soybean yield (potentially due to too much chloride), and evaluate the accuracy of current plant tissue and soil testing guidelines.
About the study
The study was conducted from 2022 through 2025 at locations around Minnesota with various soil types. There were six trial locations each year totaling 24 site-years of data.
Five K fertilizer rates were tested: 0, 40, 80, 120, and 160 lbs K2O per acre. Fall versus spring application timing was compared at each rate. Potassium was applied as potassium chloride (KCl), commonly known as potash.
Researchers targeted locations with low soil test K levels, specifically those testing at 150 ppm or less (ideally 100–120 ppm or less) to ensure the crops were more likely to respond to the fertilizer.
The study was funded by Minnesota’s fertilizer tonnage fee through the Agricultural Fertilizer Research and Education Council (AFREC).
Key takeaways from the study
1. K timing did not impact corn yield
Maximum corn yield was achieved with an application rate of 80–120 lbs K2O per acre across locations when applied either in the fall or the spring.
2. Soybean yield was slightly lower when K was applied in the spring compared to the fall
On average, spring application resulted in 1 bushel per acre less than fall application. Yield decreased when 80 lbs K2O or more was applied directly ahead of soybean planting in the spring. Spring-applied potash led to much higher chloride levels in soybean tissue—often double the levels of fall-applied K.
Source : umn.edu