Soil School: How clay content impacts K availability

Tom Bruulsema, Plant Nutrition Canada

In 2020, 44 per cent of soils tested in North American showed potassium levels below the critical level. What does that mean in terms of impact on crop production?

On this episode of the RealAgriculture Soil School, Bernard Tobin and Plant Nutrition Canada chief scientist Tom Bruulsema dig into how K is available in the soil; the impact a soil’s clay content can have on the nutrient’s availability; and how to best sample and test potassium levels in your soil.

Overall, Bruulsema says the percentage of fields that tested low for potassium is not alarming. He notes that the results indicate that farmers would lose yield if deficiencies are not addressed, but he presumes most farmers are reading those soil tests and adding the required levels of potassium.

“If you have a test that’s below critical you can still operate, you can still often get pretty good yields. The one thing you can’t do is skip an application without consequences,” says Bruulsema. “If you want to do that, you probably would be looking at trying to build your soil test potassium or maintain it at a level at least a little bit above the critical.”

When it comes to crop removal, the amount of nutrients taken up varies by crop. Bruulsema notes that soybeans extract surprisingly high levels of potassium and are often the biggest removers. “Potassium in a plant is usually mostly in the vegetative part rather than in the grain. But even a 200 bushel corn crop will remove 40 pounds of K2O in the grain. If you’re harvesting silage, you could be removing five times that, like 200 pounds per acre.”

New research is also revealing how soil structure, specifically the makeup of clay in the soil, can impact levels of available K. (Story continues after the video.)

“For example, in North Dakota, the research found that the different types of clays in the soil there behave differently with respect to potassium,” says Bruulsema. When looking at soil, he notes that there are dozens of types of clays but they are broadly classified into two groups — smectites and illites.

“In Ontario, we have a lot of the illites, but there may be some that have higher levels of smectites. What they found in North Dakota is that soils that were higher in smectite had a lower critical level than soils that were dominated by illites,” Bruulsema notes.

Those findings have led North Dakota to produce soil maps that allow farmers to determine their critical K level based on their location in the state. “We don’t have that yet in Ontario. But clay can affect the availability of potassium in your soil, particularly of course in the high clay soils,” says Bruulsema.

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