February 27, 2024

Potassium more than just a crop nutrient

JOHNSTON, Iowa — Potassium has been referred to as the mother of all nutrients for the numerous benefits it provides plants.

“Potassium can influence the efficiency of many other nutrients in our crop production system such as water use efficiency, drought tolerance, stalk quality, standability and disease tolerance,” said Troy Deutmeyer, Pioneer field agronomist.

Deutmeyer noted several field trials across the Corn Belt that show the impacts of both potassium and phosphorous on crops and optimum soil test range recommendations from those trials.

Iowa State University trials found year-to-year yield variability can be reduced by building up soil test potassium levels.

“Pioneer recommends for your soil test for potassium to generally be in that 200 to 250-plus parts per million range, that way we eliminate a lot of yield variability,” Deutmeyer said.

“In the soil test initiative conducted by Pioneer, we took thousands of soil tests in our customers’ fields and found 60% of those fields had potassium levels below 160 ppm. That’s a long way from the 200 to 250 that we recommend to try to maintain the yields stability from year-to-year.”

Nitrogen Efficiency

Customers often ask Deutmeyer what they can do to improve their nitrogen use efficiency and better manage their nitrogen.

“A lot of times their questions refer to the form of nitrogen they’re using or the rate or timing or application method. One of the first things that I ask them is what their soil test potassium levels are,” Deutmeyer said.

He referred to a study conducted at Ohio State University comparing nitrogen rates, soil test potassium levels and corn yields.

Plots with low potassium levels at 80 ppm and 280 pounds per acre of nitrogen yielded 167 bushels per acre. Plots with potassium levels of 139 ppm and 180 pounds per acre of nitrogen applied yielded 211 bushels per acre.

“That 44-bushel advantage tells us they had a nitrogen use efficiency of 0.85 which is a very high level of nitrogen use efficiency. That tells us that we can become very efficient with our nitrogen if we have adequate soil test levels,” Deutmeyer noted.

Like nitrogen, potassium levels have a similar maximum return on investment.

Pioneer conducted trials in 20 eastern Iowa fields and found the yield curve flattens out at potassium soil test levels of 300 ppm.

“We really don’t see a need to be above this. At about 250 ppm the curve in the trials really starts to flatten out. That’s why we are recommending around that 200 to 250 ppm range,” Deutmeyer said.

Yield Trends

Data from five years of Pioneer’s Intensively Managed Product Advancement Characterization and Training testing of corn across 133 locations found a 47-bushel difference between the low-yielding and high-yielding plots.

“There was a distinct trend as we move from high test to low soil testing in both phosphorous and potassium in our corn plots,” Deutmeyer explained.

“It also replicated in soybean plots where we had a 20-bushel difference between our low-yielding soybeans plots and our high-yielding soybean plots. There’s a very strong correlation in both phosphorous and soil test potassium levels as we move between those different yield levels.”

Optimum Phosphorous

For phosphorous, trials found optimum yields were achieved when soil test levels ranged from 30 to 50 ppm.

“In our soil sample initiative, we found that 51% of the fields that our customers had that we were doing research in tested very low to low phosphorous levels,” Deutmeyer continued.

“Phosphorous is important for a whole bunch of different plant processes as well — photosynthesis, cell division, respiration, energy transport, storage and many other items.”


For livestock producers utilizing manure as a fertilizer, Deutmeyer referred to data that found when phosphorous soil test level reached 100 ppm or higher, yields decreased.

“There may be too much of a good thing. Take a look at your soil test. If you have fields approaching 100 ppm or above of phosphorous, it would probably be a really good idea to move those manure applications to another field,” he said.

“Not only can high phosphorous levels start to potentially limit and decrease yield, phosphorous is also a water quality concern, as well. The algae blooms that we see in our lakes, rivers and ponds are primarily due to excessive phosphorous.

“In summary, shoot for soil test potassium levels of 200 ppm to 250 ppm range. On phosphorous soil test let’s shoot for 30 to 50 parts per million range. That way we’ll be able to maintain those yields near 100% potential and that really almost weather proofs us a little bit in regards to protecting us from Mother Nature.”

Tom Doran

Tom C. Doran

Field Editor