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Agronomy Update

Confused About Your Soil Potassium Readings?

Published on Wednesday, October 21, 2015

Soil fertility tests can be a moving target since soil chemistry is constantly changing, the soil is a living organism, and crop removal is different each year. Because of these and other factors, results can be dynamic over time. Potassium soil test results this fall have been lower than expected, even with a proper history of recent K2O fertilization. In talking with several farmers and reputable Midwest soil testing lab scientists about these results, lower potassium readings have led to many questions this fall.

Keep in mind that potassium chloride (KCl) is a salt that moves with soil water. Just like table salt (NaCl), KCl will move in solution once dissolved in the soil. In silt loam or silty clay loam soils with CEC greater than 15, the potassium can be held by clay particles in the soil deeper than the typical 6 to 7 in. soil probe depth. This year, the excessive rainfall in May and June across much of the area has likely caused potassium to move deeper into the soil profile. Soil samples taken from 7 to 12 in. would likely show a higher potassium level than shallower depths. The good news is that next year’s crop will likely be able to access this potassium, use it for crop uptake, and return some of the potassium into the upper layer of the soil profile with crop residue. If you have managed your soils with good potassium levels recently (300 to 400 lb./A. and 3 to 4 percent base saturation), I would recommend calculating your potassium fertilization needs based on crop removal (yield).  

Crop Removal of Nutrients

Understanding the amount of nutrients removed with each bushel of crop produced is important to determine how much fertilizer should be replaced. Due to economics, it is not likely that many farmers will apply “buildup” for their soil tests this fall. With yields being highly variable within fields, VRT technology will be very beneficial this year for cost savings and applying the right amount of fertilizer in the right place.

Use the following guidelines for crop removal if your fertilizer application is every other year. For example, if applying dry fertilizer ahead of the corn crop in a corn-soybean rotation, and no dry fertilizer ahead of the soybean crop. If annual application is chosen, recommended application rates of potash will be 1.5 times higher.


CORN
0.43 lb. of P2O5 per bushel of corn grain x 200 Bu./A. = 86 lb./A. of P2O5 equivalent

0.28 lb. of K2O per bushel of corn grain x 200 Bu./A. = 56 lb./A. of K2O equivalent

0.10 lb. of Sulfur per bushel of corn grain x 200 Bu./A. = 20 lb./A. of Sulfur

0.0013 lb. of Zinc per bushel of corn grain x 200 Bu./A. = 0.26 lb./A. of Zinc

0.0029 lb. of Boron per bushel of corn grain x 200 Bu./A. = 0.58 lb./A. of Boron

Example: If using di-ammonium phosphate DAP (18-46-0) and muriate of potash (0-0-60), a 200 Bu./A. corn crop removes 187 lb./A. of DAP and 93 lb./A. of potash. In addition, 20 lb./A. of sulfur, 0.26 lb./A. of zinc, and 0.58 lb./A .of boron have been removed.


SOYBEANS
0.85 lb. of P2O5 per bushel of soybean grain x 70 Bu./A. = 60 lb./A. of P2O5 equivalent

1.30 lb. of K2O per bushel of soybean grain x 70 Bu./A. = 91 lb./A. of K2O equivalent

0.20 lb. of Sulfur per bushel of soybean grain x 70 Bu./A. = 14 lb./A. of Sulfur

0.08 lb. of Zinc per bushel of soybean grain x 70 Bu./A. = 5.6 lb./A. of Zinc

0.10 lb. of Boron per bushel of soybean grain x 70 Bu./A. = 7 lb./A. of Boron

Example: If using DAP (18-46-0) and potash (0-0-60), a 70 Bu./A. soybean crop removes 130 lb./A. of DAP and 152 lb./A. of Potash. In addition, 14 lb./A. of sulfur, 5.6 lb./A. of zinc, and 7 lb./A. of boron have been removed.


In summary, if a 70 Bu./A. soybean crop was produced in 2014, followed by a 200 Bu./A. corn crop in 2015, the total fertilizer needed to replace what has been removed in two years is 317 lb./A. of DAP and 245 lb./A. of potash. In addition, 34 lb./A. of sulfur, 5.9 lb./A. of zinc, and 7.6 lb./A. of boron has also been removed. 


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Chad Kalaher
Chad Kalaher>

Chad Kalaher

Beck’s Hybrids team sales agronomist for 22 counties in NE ¼ of IL and 7 counties in NW IN. Raised on grain and livestock farm in southern IL. B.S. Agronomy 1995 – University of Illinois, M.S. Weed Science 1997 – North Carolina State University. Previous positions in seed industry as researc

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Full biography

Beck’s Hybrids team sales agronomist for 22 counties in NE ¼ of IL and 7 counties in NW IN. Raised on grain and livestock farm in southern IL. B.S. Agronomy 1995 – University of Illinois, M.S. Weed Science 1997 – North Carolina State University. Previous positions in seed industry as research agronomist, district, and regional sales manager.

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