An Interview with Dr. Khan and Dr. Mulvaney – Authors of the Potassium Paradox

Having read the paper, "The Potassium Paradox" myself, I thought that it would be interesting to track down the researchers at the University of Illinois who wrote the paper and ask them a few questions I had about their research. After reaching out to Dr. Richard Mulvaney, one of the lead researchers for the article, he responded and agreed to have an interview and bring in Dr. Saeed Khan, the senior author and lead researcher on the project.

A major finding came from a field study that involved four years of biweekly sampling for K testing with or without air-drying. Test values fluctuated drastically, did not differentiate soil K buildup from depletion, and increased even in the complete absence of K fertilization. Explaining this increase, the U of I researchers pointed out that for a 200-bushel corn crop, about 46 pounds of potassium is removed in the grain, whereas the residues return 180 pounds of potassium to the soil—three times more than the next corn crop needs and all readily available. The uptake of K originates from the entire profile, and has the effect of enriching the surface 6 or 7 inches sampled for soil testing.

Khan emphasized the overwhelming abundance of soil K and its occurrence in mineral salts that accumulate in plant material and residues and are readily returned to the soil by rainfall, noting that soil test levels have increased over time where corn has been grown continuously since the Morrow Plots were established in 1876 at the University of Illinois. As he explained, “In 1955 the K test was 216 pounds per acre for the check plot where no potassium has ever been added. In 2005, it was 360.” Mulvaney noted that a similar trend has been seen throughout the world in numerous studies with soils under grain production.

Recognizing the inherent K-supplying power of soils due to the dynamic nature of soil K reserves and the critical role of crop residues in recycling K, the researchers came to understand why soil K test values continuously fluctuate over time and also across a field, which led them to realize that K testing is of no predictive value for managing K fertilization. They also began to seriously question why the intensive use of KCl (muriate of potash) has been promoted as a prerequisite for maximizing grain yield and quality. Their doubts were confirmed by an extensive survey of more than 2,100 yield response trials, 774 of which were under grain production in North America. The data revealed that KCl was 93 percent ineffective for increasing grain yield. Instead of yield gain, there were more instances of significant yield reduction.

The two researchers were unable to find any scientific justification for the fact that, on average for the past 40 years in Illinois, annual fertilizer K inputs have exceeded crop K removal by 66,000 tons, at a current cost of approximately $60 million per year. This investment will never be recovered by those who farm highly fertile soils that have only been cropped for the past 200 years, considering the widespread lack of crop K response observed in most parts of the world including India, where soils have been under cultivation for more than 5,000 years with four crops per year and no return of residues. In a survey of 8,800 Indian trials conducted between 1972 and 1974 with rice and wheat on different soil types, K responses were rare and did not exceed 30 pounds per acre. Rather, yield was depressed in several trials.

Nor were the researchers able to confirm the claim that KCl fertilization is essential for increasing the nutritional value of food and feed. To their surprise, they found that the qualitative effects were predominantly detrimental, based on a survey of more than 1,400 field trials reported in the scientific literature. As Khan explained, “Potassium depresses calcium and magnesium, which are beneficial minerals for any living system. This can lead to grass tetany or milk fever in livestock, but the problems don’t stop there. Low-calcium diets can also trigger human diseases such as osteoporosis, rickets, and colon cancer. Another major health concern arises from the chloride in KCl, which mobilizes cadmium in the soil and promotes accumulation of this heavy metal in potato and cereal grain. This contaminates many common foods we eat—bread, potatoes, potato chips, French fries—and some we drink, such as beer. I’m reminded of a recent clinical study that links cadmium intake to an increased risk of breast cancer.”

Khan and Mulvaney see no value in soil testing for exchangeable K and instead recommend that producers periodically carry out their own strip trials to evaluate whether K fertilization is needed. Based on published research cited in their paper, they prefer the use of potassium sulfate, not KCl.

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