Rehm: Farmers can adjust their fertilizer program downward without ‘falling off a cliff’

George Rehm, right, spoke to over 500 farmers and crop consultants about fertilizer rates at the Minnesota Crop Nutrient Management Conference earlier this month. In the photo, Rehm is shown at one of the Discovery Farms Minnesota sites.

Written by Jonathan Eisenthal

It’s well known: macronutrients phosphorous and potassium impact crop yields.

But the data are somewhat contradictory about how much of each will lead to what result, Prof. George Rehm told the packed audience at the 2015 Minnesota Crop Nutrient Management conference in Mankato.

Looking at studies — some four years, some eight years and longer — across a half dozen locations in Minnesota, the clear conclusions, according to Rehm, are that available phosphorous and potassium linger in the soil, and that  “soil test values in the medium range are satisfactory.” Rehm suggested that going to banded application, rather than broadcast application, can be an economical way to get good performance. For an explanation on both application methods, click here.

“If we have to cut back (on fertilizer) in 2015, as many farmers are talking about, we aren’t going to drop out of bed, we’re not going to fall off a cliff, the world’s not going to end, we’ll be alright,” Rehm assured. “Because these values don’t change very fast.”

Control plots where no nutrients were applied showed dramatic reductions in yield, but the performance of medium and high rates were at times contradictory, leading the researchers to find that soil characteristics and particular weather events in different years play a key role. In one example in Becker (central Minnesota), the plot where 60 pounds of potassium (K20) were applied each year over three years, K levels hovered between 60 and 70 parts per million, but the crop in 2014 yielded 220 bushels of corn, compared to the plot where they applied 100 pounds of K per acre, and soil values ranged between 80 and 85 ppm, yet the yield in 2014 was 218 bushels —essentially the same result.

Similar tests at the University of Minnesota Extension fields in Waseca (south central Minnesota), already higher testing for potassium, showed both a wider range of values, and a much higher difference in performance: on fields where they added 120 pounds of K per acre, they achieved a yield of 178 bushels in 2014, compared to only 158 bushels on fields where 60 pounds had been applied each year.

Rehm reflected on the economic picture for farmers: “In the current situation, when you look at cost and return with low commodity prices, If we want to be as economical as possible or try to reach a profit, there are only about three things you can change: one is fertilizer cost, two is seed cost and three is cash rent. Cash rent isn’t moving because a lot of farmers are willing to pay it. The seed industry has decided not to reduce the cost of the seed. The only other thing these folks can wiggle is with the fertilizer management. Try to put a lid on fertilizer costs or even reduce them if they can.”

A long term study at the West-Central Research and Outreach Center in Morris looked at ‘high testing soils’ and the rate of decline over an eight-year period when no phosphorous fertilizer was applied. The year-to-year rate of decline was never dramatic. On fields that started with soil values around 20 ppm, it took eight years to drop to 10 ppm.

The research confirms that soil pH impacts the availability of these nutrients to growing crops. At a Sibley County (south central Minnesota) test site, calcareous soils, which are more basic, offered a pound less phosphorous and 4.4 pounds less potassium per acre, than did more acidic soils at the same location.

The annual Minnesota Crop Nutrient Management Conference is organized each year by the Minnesota Agricultural Water Resources Center (MAWRC) in partnership with the Minnesota Department of Agriculture and co-sponsored by the Minnesota Corn Growers Association

Readers can see more from Rehm at

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