WASECA, Minn. – Despite the heavy snow falling outside, the room was full at the Southern Research and Outreach Center for the University of Minnesota Winter Crop Day. The morning began with an in-depth discussion of potassium and the Outreach Center’s long-term study into potassium fertilizer.
“When we talk about a very low soil test for potassium, our categories are 0-40 parts per million, a low is 41-80 ppm and you may find 41-80 ppm on your farms, but probably not very often unless you have course textured soil,” said Jeffrey Vetsch, researcher with the University of Minnesota SROC, during his presentation.
Soils with 81-120 ppm potassium would fall into the medium category. The next category up is 121-160 ppm, which is fairly common in southern Minnesota. It is also the area where fertility decisions get tougher, whether or not adding more potassium is worth it.
The very high category is 161 ppm and up.
The University has fertilizer recommendations for each category depending on yield goals. They don’t recommend any fertilizer for corn when soil tests are 160 plus and only 25-35 pounds of K2O per acre in the 121-160 range.
Vetsch’s presentation focused on the data from a long-term study that has been conducted in three locations for the last seven years.
“The sites were at Waseca a glacial till soil, Rochester on a silt loam, loess soil and Becker on irrigated sand,” he said. “It was a corn, corn, bean rotation through all of the three sites.”
Soil tests were taken on the plots each year in June and October. At the start of the trial, fall of 2011, the locations had very different potassium levels. Waseca was at 100 ppm, Becker at 70 ppm and Rochester at 130 ppm.
The first three years of the test was the build phase. Each site was broken up into three sections to receive no potassium fertilizer, a medium rate and a high rate of potassium.
“If the whole field was high, very high or over 200 ppm, we wouldn't get a response,” he said. “Soil test levels – low, medium, and high – were created during the build period at each of these three locations.”
Making some assumption for price of corn, $4 per bushel, price of beans, $10 per bushel and the cost of potassium fertilizer, $420 per ton, during the three years of build phase, 2012 to 2015, calculations were made to determine the economic return on potassium applications.
“At Waseca, Becker, extraordinarily profitable to apply those rates to these sites during that four-year period, $600 to $700 of total net return to those fertilizer applications,” he said.
At the Rochester site, which started with higher levels of potassium in the soil already, the applications were still profitable, around $120 to $200 for the four-years.
With the build phase complete and each site having three different levels of potassium, the calibration phase of the test could begin. Each section of the field was fertilized with 4 different rates, 0 pounds K2O, 60 pounds, 120 pounds and 180 pounds per acre to determine yield response and test the University’s fertilizer recommendations.
“We can fertilize these low, medium and high testing soils and get the same yield potential, as long as we are putting on the right rates, as we would get if we build them up to high levels and maintain them there, the yield potential is not changing,” he said.
The yield data also showed that the University’s guidelines for potassium fertilizer rates on corn are accurate for medium to fine textured soils. Soils with very high soil test levels of potassium (greater than 160 ppm) did not respond to potassium fertilizer.
“The University of Minnesota guidelines are greater than what is needed for that irrigated sand in Becker and should be adjusted,” said Vetsch. “Data from this study suggests that our University of Minnesota guideline rates for soybeans may need to be increased on medium to fine textured soils.”