“Potassium Paradox” Questions
“Farmers have been led to believe that the soil K test was not perfect, but was the best we had and was rated up to 80 percent effective in the Illinois Agronomy Handbook,” said Mulvaney.
“In fact, it doesn’t work, and it never will. How do you manage what can’t be measured?”
NOT EVERYONE AGREES
Paul Fixen, senior vice president, International Plant Nutrition Institute (IPNI), doesn’t agree with some of the paper’s conclusions; however, he does agree that many of the questions raised are valid. Although the authors may not be hearing much official response, Fixen said the paper was a hot topic of discussion at a recent annual meeting of the American Society of Agronomy. He questions reference selection and interpretation by the researchers. However, he said he had no argument with much of the paper.
“There have been gaps in the science of assessing soil K plant availability and predicting needs,” admitted Fixen.
“Those questions have been on the radar screen of the scientific community for the past few years, and concerns have been intensifying for a number of reasons.”
Fixen cited the increasing cost of K as an input, as well as the impact of long-term reduced tillage and resulting stratification from what some call “nutrient uplift,” biocycling from the root profi le to the surface. Adding to the uncertainty and difficulty in tracking K and predicting response is that these changes may be affecting basic soil mineralogy.
“There are two dominant types of clays in Midwest soils: smectite, an expanding clay, and illite, a non-expanding clay that traps K between its layers,” explained Fixen. “What mineralogists are seeing is if you create a higher K environment, over time you may convert smectite clay to an illite-like clay. Soil test extracting solutions don’t detect K trapped by illite clays.”
He noted that so-called fixed or nonexchageable K is neither fixed permanently nor very nonexchangable. The pools of fixed and exchangeable K are more variable within a season than previously thought. This suggests that nutrient uplift may create a big reserve that slips back and forth between exchangeable and nonexchangeable pools.
“There is a lot of stuff we don’t understand very well,” said Fixen.
CONTROVERSIAL YIELD RESPONSE
Perhaps even more controversial and of greater concern for fullservice ag retailers selling KCl is the statement mentioned above on yield response. The 2,100 yield response trials, including 774 grain production trials in North America, showed KCl as 93 percent ineffective at increasing yield with yield reduction more likely.
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