Soil testing to determine soil nutrient levels is essential for making good fertilizer decisions for next year's crops. George Rehm, soil scientist with the University of Minnesota Extension Service, cites some key numbers to keep in mind regarding fertilizer for corn and soybeans.
"Phosphate fertilizer is not likely to increase corn yields if the soil test for phosphorus is higher than 20 ppm on the Bray test or 16 ppm on the Olsen test," says Rehm. "Broadcast application of potash will probably increase corn yields if the soil test for potassium is less than 80 ppm."
For soybeans, Rehm says phosphate fertilizer will probably boost yields if the soil test for phosphorus is less than 10 ppm on the Bray test or 8 ppm on the Olsen test. Broadcasting potash is a good idea if the soil potassium test is less than 100 ppm.
"Take a close look at soil test results before making fertilizer decisions," says Rehm. "Soil test values have been increasing in Minnesota. Broadcast applications of phosphate and/or potash aren't always worthwhile. If they aren't needed, growers are better off to save their money."
County offices of the University of Minnesota Extension Service have soil sample bags and information sheets on soil testing.