Study challenges soil testing for K and the value of KCl
In the chemical age of agriculture that began in the 1960s, potassium chloride (KCl), the common salt often referred to as potash, is widely used as a major fertilizer in the Corn Belt without regard to the huge soil reserves that were once recognized for their fundamental importance to soil fertility. 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 K testing is of no value for predicting soil K availability and that KCl fertilization seldom pays.
U of I researchers Saeed Khan, Richard Mulvaney, and Timothy Ellsworth are the authors of "The potassium paradox: Implications for soil fertility, crop production and human health," which was posted on October 10th by Renewable Agriculture and Food Systems.
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, Khan 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."
Khan emphasized the overwhelming abundance of soil K, 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 Corn Belt soils and the critical role of crop residues in recycling K, the researchers wondered why producers have been led to believe that intensive use of KCl 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 KCl was 93 percent ineffective for increasing grain yield. Instead of yield gain, the researchers found more instances of significant yield reduction.
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