Soybeans require a lot of potassium, about 170 lbs. to produce 70 bushels. The soil either has a sufficient reserve or it doesn’t. And recent evidence points to increasingly insufficient reserves.

Potassium levels have been dropping due to greater crop removal, while replacement rates haven’t compensated for this greater withdraw. The reality—soil test levels for potassium are increasingly below critical values.

Robert Mullen, agronomist with the Potash Corporation agrees soil test levels for potassium (K) are trending down and that investing in more potassium is a good decision. “The case we often make is that sound agronomic decisions usually translate into good economic decisions. When soil test levels are below the critical values applying potash makes economic sense.”

Mullen emphasized that according to the International Plant Nutrition Institute (IPNI) 30% of the soil tests conducted in Illinois for the corn and soybean rotation are below the critical level. In 2010 in Illinois, the median soil test value for K was 179 ppm (critical level is 125/250 and 180/360 ppm/lbs. per acre for low and high CEC soils, respectively).

“Even though commodity prices are low, we consider it very profitable to apply potash,” said Mullen. “Our sensitivity analysis shows that the expected yield increase when soil test levels are low will more than offset the cost of applying potash. In fact, we have a tool on our website that allows producers to test that statement if they like.”

The key in making a potash decision is the soil test level, said Mullen. “If near the critical level or above you can change what you apply and maybe cut back. However if the soil test level is below the critical level then it pays to apply.”

Most growers in Illinois apply dry phosphate and potash fertilizer in the fall and incorporate it. “Making fall applications is a good time to apply potash,” said Mullen. “Growers have time and the soil is usually in better condition. However if you didn’t make a fall application due to weather or economics, spring is another good option. The goal is to get it on the soil this spring and then lightly incorporate it to get it into the root zone since potash is not very mobile.”

“The point we are emphasizing today is that there isn’t a disincentive to applying potash,” said Mullen. “And in fields with high-yield potential there is even more justification for application.”

Today making a potash investment is more attractive and it is a good time to consider investing some of those fertilizer dollars into potash to reverse the downward trend.

Agronomist Dr. Daniel Davidson posts blogs on agronomy-related topics. Feel free to contact him at

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About the Author: Jason Haegele

Jason Haegele is the region agronomist for WinField United in Illinois and leads WinField United’s agronomy services team for the eastern United States. Employed by WinField United for four years, Haegele was previously a research scientist with DuPont Pioneer for two years. Haegele holds a bachelor’s degree in agronomy and ag engineering from Iowa State University, a master’s in crop production and physiology also from Iowa State, and a Ph.D. in crop sciences from the University of Illinois.