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.

This excessive chloride uptake is the primary suspect for the spring yield penalty. Fall application allows winter and spring moisture to leach the chloride deeper into the soil profile before the soybean roots reach it, while keeping the potassium available in the root zone.

"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."

3. Other findings

• K Base Saturation Ratio: While there was a relationship between K base saturation ratio and yield, it offered no advantage over standard June soil test K levels for predicting performance. Standard soil testing remains the most practical and effective way to manage potassium.
• Tissue Testing: Current leaf K concentration guidelines for corn ear leaf (1.8–3.0%) and soybean trifoliate (1.7–2.5%) were found to be accurate indicators of yield potential.

What this means for farmers

1. No need to change how you manage K for corn

The study’s findings align very closely with the current University of Minnesota fertilizer guidelines for corn, effectively validating them for modern production. The 80–120 lb range identified in the research could provide a narrower, more optimized target within those broader guidelines to ensure growers aren't over-applying while still hitting maximum yield. Corn growers can be flexible in their potassium fertilizer application timing.

2. Applying high rates of K ahead of soybean isn’t worth the risk

This research suggests that for soybeans, fall application is better to reduce the potential risk for yield reductions. If you must apply in the spring, keeping rates below 100 lbs of potash (KCl) to avoid yield drag is a good idea. However, if your soil tests show K is low or very low, apply the suggested rates. Yield decreases are more common when growers apply K to replace what is taken up by the crop, when soil tests are near critical levels and the need for high rates of K is low.

3. If your soil test calls for a high rate of potassium (more than 80 lbs K2O) for soybeans…

• Prioritize Fall Application: Get the potash down in the fall to mitigate chloride risk.
• Apply to the Previous Crop: Apply the majority of the K ahead of corn in the rotation. Corn does not suffer the same spring yield penalty from chloride, and the soybeans will benefit from the residual K the following year.