Richard Keller 2013-05-06 04:38:19
There is growing concern that soils sufficient in potassium just a few years ago are now showing signs of needing potassium applications, but determining how much potassium is being investigated by soil scientists who are looking at changes in soil test lab procedures. There is a small controversy about using a dry sample soil test or moist sample soil test for determining the most accurate potassium level in a sample. The moist test was the standard used at Iowa State University’s soil test lab from the mid-1960s to the end of the 1980s, but it was a hold out to private soil test labs that were using a dry sample soil test process. That switch ultimately wasn’t the right thing to do, according to Antonio Mallarino, ISU Department of Agronomy soil fertility and plant nutrition specialist. Now new labs offering the moist soil test for potassium are coming online. Mallarino has cooperated with a private lab near Ames, Iowa. “There is no doubt that the moist soil potassium test is the most accurate from the research that we have done with Iowa soils,” said Mallarino. “What I did from 2001 to 2006 and then again during the last three years was field research and calibrations that essentially compared results and showed it is a better test.” He explained, “For potassium, because of the chemistry and the mineralogy, when you dry the sample you drastically change what is shown about the soil. The estimate of availability of potassium by drying the soil sample is deficient at predicting exactly what the plant sees. When the test is done without this drying, then it is closer to what the plant actually sees.” The ISU professor contends the test works well for Iowa soils and similar soil types found in neighboring states, but suggests that other soil scientists should evaluate the moist and dry soil test to prove things for themselves with their state’s soil types. Daniel Kaiser, University of Minnesota Extension soil fertility specialist, is doing his own evaluations and gathering data for soil types in Minnesota. He worked with Mallarino while doing his graduate studies at ISU. “What they have been seeing [in Iowa], in some situations, is that some poorly drained soils show some fairly large increases in the soil test levels when the dry soil test is used, but the soils are actually in a fairly low classification and shouldn’t test high,” said Kaiser. “Therefore, it is not a concern of over applying fertilizer but the potential of lost yield from under applying potassium.” There is no direct correlation between the dry and moist soil test lab numbers, noted both soils specialists. The results of the tests don’t match one to one; therefore, Mallarino has used his extensive years of field trials to draft new interpretations for fertilizer recommendations, and he is recognized as the only one currently with sufficient data for these recommendations. Accurate soil testing for potassium is becoming more of a concern as farmers continue to harvest larger and larger yields from their fields but might not be properly replacing the volume of potassium removed. “We are seeing soils in western Iowa that for 50 years farmers have said they don’t need potassium, but now we are seeing some soils that need potassium. It is because of the high-yield grain removal,” said Mallarino. Kaiser said, “We are starting to see some areas in the western part of the state [Minnesota] that have been high in native K start to drop a little bit … I think there are some areas that haven’t had a lot of potash applied and are starting to decline.” He added, “I’m having more concern now than in past years.”
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