Spring Potash May Reduce Soybean Yield

By Scout Nelson

University of Minnesota Extension recently completed a four-year research study examining how potassium fertilizer timing affects corn and soybean production. The project focused on determining the best potassium application rates, understanding soybean yield responses, and reviewing current soil and tissue testing recommendations.

The study was led by University of Minnesota Extension nutrient management specialist Dan Kaiser and took place from 2022 through 2025 at multiple locations across Minnesota. Researchers studied several soil types and tested potassium (K) fertilizer rates ranging from 0 to 160 lbs. K2O per acre. The research also compared fall and spring applications of potash, also known as potassium chloride (KCl).

Researchers selected fields with low potassium soil test levels to better measure crop responses to fertilizer applications. The project received funding support through Minnesota’s Agricultural Fertilizer Research and Education Council.

The results showed that corn yields were not affected by fertilizer timing. Maximum corn yields were achieved with potassium application rates between 80 and 120 lbs. K2O per acre, whether fertilizer was applied during the fall or spring seasons.

Soybean yields, however, responded differently. Researchers found soybean yields averaged about 1 bushel per acre lower when higher potash rates were applied in the spring before planting. The study linked this yield reduction to increased chloride levels found in soybean tissue after spring applications. Researchers believe excess chloride uptake may affect soybean growth and productivity.

"You can overfertilize soybean – and yield reductions have been pretty common across the southern part of Minnesota,” said Kaiser. “It’s maybe only a bushel, but with the money spent on fertilizer, losing a bushel is a double negative. That’s a big thing to watch out for."

The research suggests fall potash applications are generally safer for soybeans because winter moisture helps move chloride deeper into the soil before soybean roots develop. Researchers also confirmed that current soil and tissue testing guidelines remain reliable tools for fertilizer management decisions.

The study encourages growers to carefully manage spring potassium applications, especially when higher fertilizer rates are needed ahead of soybean planting.

If you have questions or comments, please email us at nutmgmt@umn.edu.

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