Minnesota Research Reviews Fertilizer Timing Effects on Corn and Soybean Crop Yields
University of Minnesota Extension completed a four-year study examining how potassium fertilizer timing affects corn and soybean production across Minnesota. The research focused on fertilizer application timing, potassium rates, and crop responses to better guide nutrient management decisions for farmers.
The project was led by University of Minnesota Extension nutrient management specialist Dan Kaiser from 2022 to 2025. Research trials were conducted at several Minnesota locations using different soil types and potassium fertilizer rates ranging from 0 to 160 pounds of K2O per acre. Researchers also compared fall and spring applications of potash fertilizer, commonly known as potassium chloride.
Fields with low soil potassium levels were selected to better measure how crops responded to fertilizer treatments. Funding support for the project came from Minnesota’s Agricultural Fertilizer Research and Education Council.
Results from the study showed that fertilizer timing did not affect corn yields. Researchers found the highest corn yields occurred when potassium rates ranged between 80 and 120 pounds of K2O per acre, regardless of whether applications were made during spring or fall.
Soybeans responded differently to spring fertilizer applications. Researchers observed soybean yields averaged about one bushel per acre lower when higher potash rates were applied before planting in the spring. The study connected this reduction to higher chloride levels found in soybean plant tissue following spring applications. Researchers believe excess chloride uptake may negatively affect soybean growth.
"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."
Researchers suggest fall potash applications may provide safer conditions for soybean production because winter moisture helps move chloride deeper into the soil before soybean roots begin growing. The study also confirmed that current soil and tissue testing recommendations remain valuable tools for fertilizer management planning.
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