“Potassium Paradox” Questions
“There were cases where soybean yields were significantly decreased with KCl applications, and one reason is that K depresses calcium and reduces uptake,” reported Mulvaney. “The chloride in KCl competes with nitrate uptake. You don’t want anything competing with N.”
The authors are quick to say that sandy or shallow soils and certain crops, such as alfalfa hay, sometimes benefit from K applications. However, they recommend using potassium sulfate (K2SO4) instead of KCl, noting that unlike chloride, sulfur does not depress nitrate uptake. With most soils and crops, and in particular grain production, the researchers argued that frequent K applications are not needed.
Fixen argued in turn that acting on the authors’ position could be economically damaging to farmers. He noted that an IPNI soil test summary of 700,000 samples from Iowa in 2010 showed 70 percent of samples testing below Iowa State University’s (ISU) new critical level of 200 ppm K and 49 percent testing below 160 ppm (by dry test). He added that an ISU data set showed that the soil test does work, though it has limitations, some of which are removed with the fi eld moist soil procedure.
“How does one draw the conclusion they draw when one of the most extensive K data sets in North America showed this kind of response?” asked Fixen, who noted the annual removal of K in harvested grain. “The K has to come from somewhere.”
Khan credited K mobility and abundance in the soil. He compared crop removal (in the case of grain) to removing a cup of salt from the ocean and expecting salinity to change. As K uplifts to the surface, the mass of K in the soil profile is essentially untouched, he argued.
He pointed to the famed Morrow plots in continuous corn for 137 years. “In 1955, a K test was 216 pounds per acre in a plot with no K ever added,” he said. “In 2005, it was 360.”
[Left to right] Saeed Khan, Richard Mulvaney and Timothy Ellsworth authored a study titled, “The Potassium Paradox: Implications for Soil Fertility, Crop Production and Human Health.”
TRUST BUT VERIFY
Where Fixen and the University of Illinois researchers do agree is the need for on-farm trials. Mulvaney added that ag retailers, agronomists and crop consultants have a vital role to play here.
“Help farmers establish split trials on their farms,” he said.
“Now is the time for farmers to utilize precision technology to check for a yield response to K. It can only improve management of K.”
Noting that it can take months for scientific review and reaction to an article, Fixen advised against throwing the baby out with the bathwater. “Our old soil tests are serving us pretty well, but as Ronald Reagan said, ‘trust, but verify,’” he said. “There is no better way of verifying than maintaining comparison strips using precision ag technology. If the strips show up, you know you have a problem. If they don’t, you aren’t shorting the crop. Replicate to eliminate other problems like drainage. It’s an easy thing to do.”
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