Research Highlights

Examining Factors in Potassium Availability

By Laura Temple

Most soybean nutrient recommendations are based on target yield. Farmers usually vary target yields for each field based on a variety of factors. 

“Perhaps one of those factors should be the nutrient availability in a field,” says Jarrod Miller, assistant professor and extension specialist in agronomy at the University of Delaware. “That ties to soil type, focusing on how they hold and release nutrients.”

He explains that each nutrient moves through soils in a certain way. Soils also have different capacities to hold specific nutrients. 

“Should our nutrient recommendations account for those variations? Do we need to apply higher rates if nutrients are less available?” he asks. 

Miller hypothesized that irrigation and soil cation exchange capacity, or CEC, could affect the availability of potassium, or K, and other macronutrients like nitrogen and phosphorous, as well as micronutrients like boron, calcium, copper, iron, magnesium, manganese and molybdenum. Soybean yields could be influenced more by the availability of those nutrients to the plants than just fertilizer application rates.

“For example, lower concentrations of K do not move readily with soil water,” he explains. “It moves slowly, so it may be less available than expected based on fertilization rates.”

To learn more, the Delaware Soybean Board funded an observational study led by Miller to compare nutrient content in soybeans and soils under irrigated and rainfed conditions. During the early reproductive stages of two growing seasons, Miller’s team pulled soil and tissue samples from soybeans under irrigation and from soybeans in the dry corners of the same fields.

“With this information, we were able to compare soybeans under the same management, with the exception of soil moisture,” Miller explains. 

The team observed that soils with lower CEC also contained less K than soils with higher CEC, often showing deficiencies compared to K recommendations. However, regardless of CEC, the soils in dry corners of fields had higher K concentrations than expected. At the same time, they saw no statistical differences in K concentration in any soybean tissue. 

“K may have built up in dry corners over the years because less was taken up by crops, while fertilizer rates remained consistent for the entire field,” he says. “We were surprised that tissue samples saw no difference in K, but that may be because higher concentrations allowed for easier crop uptake.”

Soybean tissue samples from plants under irrigation did show higher concentrations of nitrogen and phosphorus, though both irrigated and dryland soils contained the same amount of phosphorous. Most other micronutrients showed no difference in soil or tissue concentration between irrigated and rainfed soils.

“Something is happening based on nutrient holding capacity and soil moisture, but it isn’t easy to define,” Miller says. “The good news for farmers is that means the current nutrient recommendations appear to be reliable for both irrigated and dryland soils.”

He notes the soil CEC between and within fields can vary widely. He believes farmers should consider varying both irrigation and K application rates based on that factor, which represents the soil’s ability to hold those compounds and make them available to crops.

Published: May 1, 2023