By Jonathan Eisenthal

Potassium (K) is a macronutrient that governs the corn plant’s ability to take up nutrients and moisture, and transport both within the plant tissues. The proper functioning of the stomata—the openings that allow the plant to breathe, taking in carbon gases and releasing oxygen—also depends on potassium nutrition.

In the last decade, scientists have made rapid gains in understanding how the mineral make-up of soil affects the ability of plants to get the potassium they need, according to Dan Kaiser, an associate professor at the University of Minnesota’s Department of Soil, Water and Climate.

He is in year two of a three-year research project to map the soils of Minnesota according to mineral content, which has a direct impact on how potassium chemistry behaves when soils are wetting and drying.

“We’re looking at the ratio of (the clay species) illite to smectite,” Kaiser reported.

Research in North Dakota has identified that the illite-to-smectite ratio can impact the “critical soil test level” of potassium—the soil test value where maximum yield is achieved.

Smectite clays, which are prevalent in the Red River Valley, have more of a ‘shrink-swell’ capacity than other soil types, and may need a higher rate of potassium because of their tendency to fix the nutrient, and make it unavailable to the plants at key points in the growing season. Fixation occurs as potassium is trapped within the small spaces between clay particles, out of the reach of the corn plant roots.

Illite clay does not shrink and swell and has a lower tendency to hold onto essential nutrients. Illite contains potassium that ‘weathers out’ and becomes available to plants. So, soils high in illite may not require as much added potassium to reach maximum corn yield.

Kaiser plans to create a general map of Minnesota that shows that illite-smectite ratio across the state.

“My goal is to better develop our potassium fertilizer guidelines to ensure yield is maximized as economically as possible, and to keep the cost of soil analysis lower while allowing higher throughput of samples to get data out to farmers,” Kaiser said.

Additionally, Kaiser’s work is looking at Cation Exchange Capacity (CEC), which measures the soil’s ability to hold onto essential nutrients, to see if it also could help predict critical soil test potassium levels. Surprisingly, research has shown that lower rates in sandy soils at times result in yield increases. Kaiser’s project seeks to map this phenomenon as well.

“I am thankful the corn growers found it worthwhile to fund this work,” Kaiser said. “Much of the fertilizer research focuses on nitrogen, and then phosphorus also attracts a good share of the attention. Not long ago, sulfur came along and kind of kicked potassium out of the third spot in terms of attracting research interest. But, there are circumstances when potassium is an issue, and changing the rates can result in a yield boost.”