Ask an Agronomist
How can I use potash effectively in soybeans?
May 22, 2013
Question: I have two questions for you. To get 100-bushel soybeans, Soybean College suggested approximately using 187 lbs/acre potash. I am interested in placing 200 lbs of 0-0-62 per acre and then 3 gallons of 0-0-21-13 liquid fertilizer at planting in 2x2 on 30-inch rows because I also have white mold issues. Do I need to be concerned about root injury? Also, in winter soybean meetings, the use of boron blended at 3.5lbs/acre in 0-0-62 potash was recommended by the Michigan soybean board if soil tests were below .8ppm (which mine are). Do you advise the same?
Answer: The first answer is you don’t need to be concerned, as burn injury shouldn’t be an issue. The second answer is tougher to give you as I don’t know which extraction method your Michigan resource used, and the soil test extraction for micronutrients can vary quite a bit.
You’ll need to line up that .8 ppm recommendation with the extraction method used by your source, such as the Mehlich 3 extraction method, a DPTH extraction method or an ammonium acetate extraction method. I ran samples under three different extraction methods here to see what I came up with, using the DTPA, HCL and Mehlich 3 extraction methods to check for zinc. Under the DTPA I got a reading of .8; under the Mehlich 3 method I got 2.44; and under HCL I got a 1.8. With most laboratories when you get results back, they’ll say high, medium or low, and the results will be calibrated to the extraction. So, if Michigan is using the HCL extraction method, then the 1.8 would be in a medium to high range; with a DTPA extraction you have a medium to low range; with Mehlich 3 it would be drastically low. I can’t tell you without knowing which extraction was used. Boron is one of those nutrients, that when I do a soil test, and the results are medium to low, the soil tests and tissue tests don’t always line up. In a dry year, I almost always see a deficit in the tissue test. If I saw .8 here in my area, then I would make a boron application. But remember if you over-apply boron, it can be toxic. When we talk about a 3.5 pound-per-acre application of boron, that’s a pretty good-sized application. We talk a lot more about 1 pound to 1.5 pound application of boron here. Personally I wouldn’t do this large boron application without some tissue testing to follow it up. There’s good data from Michigan, so check into that. Now, we have done starter plots with potassium and phosphorus on our beans down here and looking at different starter rates, but we haven’t been very successful yet. I can’t tell you that putting 3 gallon of 0-0-21-13 liquid fertilizer down will be successful. I can tell you good potassium levels help the soybean plants hold blooms better and improve yields. You might do some plots to see if you can move the needle. You also might be better off elevating your potash levels if potash availability is a concern. I want to emphasize that you need to analyze test results in a way that lines up with the laboratory results and how that information is provided
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Micronutrient Deficiency Detection
Step-by-step guide to identifying micronutrients that your soils might lack.