Ask the Agronomist

Potassium Fertilization for 2020 Success

Dr. Robert Mullen (Progressive Farmer image provided by eKonomics)


Q: Why do native potassium (K) levels vary in soils, and how does weather impact those amounts?

Dr. Robert Mullen: There are three main reasons for the variable levels.

1. Some soils have more native potassium than others, depending on the types of minerals from which the soils were derived. In the central plains and west, soils tend to be fairly rich in potassium, combined with lower rainfall that reduces the leaching of K. Further east, there are more soils with less native potassium fertility and higher rainfall that increases leaching -- which historically has required more K fertilizer application.

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2. Components of the soil play a role. The 2:1 clay soils, which predominate the bulk of Midwest and southeastern areas, hold more potassium than coarse-textured sandier soils that leach higher K amounts. Yet, dry weather periods can lead to potassium fixation, as K gets tied up between clay layers when you see cracks in the soil.

3. Cropping system history and modern genetics also play a role in reducing the finite amount of available K in soils. As farmers have switched away from small grains to more corn and soybeans, the crops extract higher amounts of K during the growing season -- especially the higher-yielding genetics.

Q: How much potassium is taken up and removed during grain harvest, and how does the removal of the stover affect potassium replacement requirements?

Mullen: Using corn as an example, the amount of K removal has dramatically increased over the past 10 to 15 years. If 150 bushel per acre corn was removing 40 pounds of K2O per acre in the early 2000s, then we jump to 250 bushel yields today, that removes 70 pounds per acre of K2O. That 30 pounds per acre increase over 10 years means 300 more pounds of K2O is being removed. Regarding soybeans, a 60 bushel per acre crop removes 78 pounds of K per acre. Producers need to make sure they cover what is being removed to maintain soil test levels that account for this higher level of productivity. If not, growers take from a finite supply of potassium in the soil, which at some point could become exhausted. And if farmers remove the corn stover after harvest, for bedding or ethanol or other uses, that same 250-bushel corn crop now has an estimated 300 pounds per acre of K2O that must be replaced. Remember that only 25 to 30% of potassium gets shuttled into the grain. Be sure to check out our website,, to use our Nutrient Removal Calculator and read the latest news on this topic.

Q: When does potassium get released by the plant back into the soil, and how does that impact soil testing?

Mullen: Potassium can be released from plants fairly quickly back into the soil. As soon as the cell walls and plant tissue begin to break down, potassium is released. For example, if harvest occurs in early October, and soil is sampled two weeks later, it's possible that potassium from the unharvested material would appear on the test. However, in a really dry environment, no added K would register in that test until spring when residue has released it into the soil. In that same dry environment, if you have 2:1 clay soil, the potassium can get trapped between clay layers. So, you can imagine how soil test levels for K change given different environments.

Q: Given these tough economic times, will cutting back on potassium cause yield loss in 2020?

Mullen: For potassium management, the whole goal is to ensure that the soil environment has an adequate supply of K to support your yield goals. While nitrogen is vital, yield can still be limited by inadequate potassium availability. New research released by Purdue University this fall showed a lower-testing potassium field all of a sudden couldn't produce a crop. Translation: Growers can't focus solely on nitrogen. I've become a firm believer in more frequent soil tests, even annually, to track fertility trends that can help optimize a farmer's application strategy.

For More Information:

> Visit for news and fertility management information. It also contains valuable tools like a rainfall tracker and Growing Degree Day calculator to help farmers assess possible fertility loss and plant development needs.


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