Study challenges soil testing for potassium, fertilizer value

University of Illinois soils scientists question current approach to potassium management of productive soils in the Corn Belt.

In the chemical age of agriculture that began in the 1960s, potassium chloride, commonly referred to as potash, is widely used as a major fertilizer in the Corn Belt.

Three University of Illinois soil scientists have serious concerns with the current approach to potassium management that has been in place for the past five decades because their research has revealed that soil potassium testing is of no value for predicting soil potassium availability and that potassium chloride fertilization seldom pays.

University of Illinois researchers Saeed Khan, Richard Mulvaney and Timothy Ellsworth are the authors of "The Potassium Paradox: Implications for Soil Fertility, Crop Production & Human Health," which was posted Oct. 10 by Renewable Agriculture & Food Systems.

A major finding came from a field study that involved four years of biweekly sampling for potassium testing with or without air-drying. Test values fluctuated drastically, did not differentiate soil potassium buildup from depletion and increased even in the complete absence of potassium fertilization, an announcement from the university said.

Explaining this increase, Khan pointed out that for a 200-bu. corn crop, "about 46 lb. of potassium is removed in the grain, whereas the residues return 180 lb. of potassium to the soil — three times more than the next corn crop needs and all readily available."

Khan emphasized the overwhelming abundance of soil potassium, 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 potassium test was 216 lb. 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 potassium-supplying power of Corn Belt soils and the critical role of crop residues in recycling potassium, the researchers wondered why producers have been led to believe that intensive use of potassium chloride is a prerequisite for maximizing grain yield and quality. To better understand the economic value of this fertilizer, they undertook an extensive survey of more than 2,100 yield response trials, 774 of which were under grain production in North America. The results confirmed their suspicions because potassium chloride was 93% ineffective for increasing grain yield. Instead of yield gain, the researchers found more instances of significant yield reduction, the announcement said.

Khan and his colleagues pointed out that potassium chloride fertilization has long been promoted as a prerequisite for high nutritional value for 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."

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

The full paper is available as an open access article at

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