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Potassium Fertilizer: What We Know and More Importantly What We Don’t Know

Evaluating inputs is an important process when planning crop management practices year-over-year. While a majority of that time is spent evaluating varieties of seeds and planting options, an important input that needs to also be evaluated is the soil. While this may seem like a constant in the equation of crop production, the nutrients in the soil are not. The composition of soils plays a large role in the nutrients available, it is important to understand just how soil composition plays a role in nutrient management, especially when talking about potassium. Potassium is one of the hardest nutrients to manage. Not only because there are numerous factors that go into the plant-available potassium in the soil, but it is also challenging to identify how much is actually there because of the inherent limitations related to soil tests.

Potassium Recommendations

For the past 20 years, potassium recommendations have been very similar, using the same soil test and recommendations provided by Universities in the Midwest. Research and general trends have started to show that although that may be one side of the story, that is not the whole story. Robert Miller is an Affiliate Professor of Soil Science at Colorado State University and Director of the Agricultural Laboratory Proficiency Program. Miller received his Ph.D. in Soil Fertility and Chemistry from Montana State University-Bozeman where his thesis was focused on potassium and soil fertility. He comments below:

“Over the past 70 years, Midwest nutrient management has focused on soil test method and crop response to an applied fertilizer, at specific locations, with little attention to crop nutrition. Moving forward in the 21st century improved management will fuse soil fertility and crop nutrition through observation data analysis. Utilizing a data base of multi-location grower field data, observation data analysis will be an effective tool relating a range of soil chemical, physical, and biological properties to crop nutrition and improve yields.”

The power of observations is something that Miller has focused on during his career in soil fertility and is something that he encourages all farmers and agronomists to start bringing into their operations. To complete the whole story of observations, there are multiple factors that can be measured and evaluated to help bring the story together – soil tests, plant analysis tests, tillage, and root depth. Below is one piece of the story that helps identify soil fertility. These equations recommended by Iowa State University and the University of Minnesota for determining recommendation levels of potassium per acre. As stated above, this is one piece of the puzzle of potassium fertility, but one that should be considered in the whole story.

University of Minnesota Equations

Dry Soil Test: lb. K2Orec. /acre = [1.166 – 0.0073 (soil test K, ppm)] x (expected yield)
Dry Soil Test: lb. K2Orec. . /acre = [2.2 – (0.0183) (soil test K, ppm)] x (expected yield)

Iowa State University Equations

Note: Iowa State Equation Recommendations are based on soils that test within the very low to low categories.
Moist Soil Test: lb. K2O rec./acre = (139.4 – 1.50 x ppm K) / (1 – 0.00859 x ppm K)
Dry Soil Test: lb. K2O rec./acre = (138.5 – 0.825 x ppm K) / (1 – 0.00532 x ppm K)

Moist Soil Test: lb. K2O rec./acre = (128.6 – 1.18 x ppm K) / (1 – 0.00678 x ppm K)
Dry Soil Test: lb. K2O rec./acre = (127.9 – 0.699 x ppm K) / (1 – 0.00476 x ppm K)

2-Year Rotation
Moist Soil Test: lb. K2O rec./acre = (240.2 – 2.23 x ppm K) / (1 – 0.00637 x ppm K)
Dry Soil Test: lb. K2O rec./acre = (238.4 – 1.13 x ppm K) / (1 – 0.00464 x ppm K)

An important factor to note when using these equations, and testing potassium and soil fertility in general, is to remain consistent with time of year, location, depth, and other factors. These factors all play a role in soil fertility and to help better understand and interpret data received from these tests, it is important to remain consistent in the collection of the data.

Factors Influencing Soil Fertility Tests

Soil composition has a significant impact on how potassium reacts to the different types of soil testing – dry and moist – which makes evaluating the tests challenging. Research has shown that soil type has a significant impact not only on soil potassium levels, but also in the soil test performances themselves.

A factor that plays a significant role in soil testing in general, but also specifically potassium presence, is the time of year the soil tests are taken. In the fall, the soil will be at the lowest nutrient potential that it will experience all year. After having completed a full growing season and having plant material harvested off the field, the nutrient availability will be limited. Those soil testing in the fall should understand that they should use this as a baseline low for their soil potential. Over the winter months, or the off-season, soils will break down leftover plant material and start to repopulate K into the soil. In the spring, the soil will reach its peak potential for potassium for the season, so when looking at soil tests, be sure that the amount of potassium in the soils in the spring will be enough for that growing season and that it will not reach critically low level during the growing season.

Another factor that plays a significant role in the potassium soil tests is the type of soil that is being tests. As we talked above, different soils types inherently have more or less plant available potassium, but the type of soil also affects how potassium shows up in a soil test. The largest factor when evaluating soil tests from clay-based soils, is to determine which type of clay the soil is. If it is a 1:1 clay soil, which are primarily found in southeastern Minnesota, this clay type does not put up a fuss when it comes to soil test evaluations. John Breker, Soil Scientist at AGVISE Laboratories has done extensive research in potassium availability, primarily focused in the Red River Valley and North Dakota regions. He comments,

“Much like nitrogen, potassium is more complex than a simple soil test. Our recent research in North Dakota has shown that soil type – specifically soil mineralogy – influences potassium availability. The mineralogical component has often been ignored, if not entirely forgotten, in soil fertility management. A long-neglected crop nutrient, potassium requires more work, both basic and applied research.”

When looking at 2:1 clay, such as Smectites and Illites, these are the clay types that cause issues when heated during soil tests and well as during the growing season. Smectites undergo a shrinking and swelling process when under pressure, either from drought or from saturation. This shrinking and swelling process can trap ions between the layers of clay making it unavailable to the plant, but also unavailable to detect during a soil test. The second type of clay is the Illites which do not undergo the shrinking and swelling process, but still house ions between the layers of clay which allows the potassium to be released as it needs to. Understanding the different soil types can help decode and understand the results of soil tests as well. Further information on this soil type locations in Minnesota can be found at mn_parentsoil.pdf

ROI of Potassium Fertilizer

Potassium has been a nutrient that is often left by the way side when executing a soil fertility plan for the lack of correlation between fertilizer application and increased yields. Although farmers and agronomist know the importance of potassium for plant functions, it is a nutrient that comes into question when evaluating ROI. AFREC funded research, in conjunction with the Minnesota Corn Growers, conducted by Jeffery Vetsch and Danial Kaiser, has shown the following results that potassium does provide economic return to farming operations.


Economic Returns on K2O Fertilizer Applications on Corn and Soybeans years 2012 to 2015.

The graph above shows the net economic return, in $/acre of potassium fertilizer on corn and soybean crop land on three sites across Minnesota across a four-year period of 2012 – 2015.
Source: information sourced from Effects of K Fertilization on Corn and Soybean Yield presentation at the U of M SROC Winter Crops Day 2019 presented by Jeff Vetsch.

The graph above shows the net economic return, in $/acre of potassium fertilizer on corn and soybean crop land on three sites across Minnesota across a four-year period of 2012 – 2015. The following information provides background to this graph:

Initial K ppm

  • Waseca – 100 ppm
  • Becker – 70 ppm
  • Rochester – 130 ppm


  • $4/bu corn
  • $10/bu soybeans
  • $0.35/lb. K2O, $420/t
  • $8 application/yr.

K2O Rates for Waseca & Becker

  • Control/Low – 0 lb./acre
  • Medium – 60 lb./acre
  • High – 80 lb./acre

K2O Rates for Rochester

  • Control/Low ¬– 0 lb./acre
  • Medium – 40 lb./acre
  • High – 80 lb./acre

As the graph above shows, there was significant economic ROI on the lowest testing soil, Becker, rendering around an $800/acre cumulative economic return when applying medium and high recommendation levels for potassium fertilizer. Similarly, in Waseca, we saw high economic return in both the medium and high range, with the high range pushing upwards of the $700/acre cumulative economic return. Although we did not see as high of return in the Rochester plots, the high recommendation application produced the higher economic return, around $300/acre cumulative, for that plot as well. This strengthens the argument that although potassium may be a hard nutrient to track, it does play a significant role in overall plant development and ultimately has a role in yield and can be, when applied responsibly and correctly, an economically viable plan for soil fertility.

Final Word

Potassium is an important nutrient involved in many different plant functions. These functions impact yield and overall plant growth and health. When evaluating potassium fertilizer application economically, it is important to note that although potassium fertilizer may not be at the forefront of yield influencers in terms of fertilizers, it is a necessary and important nutrient to help overall plant growth and health which in turn, impacts yield. Understanding what is within the soil and how those properties interact with the crops planted and harvested, gives you the opportunity to make management decisions that are best for your fields and farming business.

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