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Research Highlights
Foliar Fertilizers Rarely Increase Soybean Yield

By Laura Temple

Do foliar fertilizer applications during early reproductive growth boost soybean yields?

Soybean Extension specialists across the U.S. were hearing this question from farmers and agronomists more often. High soybean prices, interest in pushing yields to the next level or plans for in-season fungicide and insecticide applications often prompted this question.

“As we talked to contacts in other states, we realized very little information was available about current foliar fertilizer products,” says Rachel Vann, assistant professor and Extension soybean specialist for North Carolina State University. “We decided to work together to find answers. The soy checkoff charged us with cooperating across states to help farmers through the Science for Success initiative, and this topic emerged as one that would benefit farmers throughout the country.”

Funded by the soy checkoff, Science for Success brings U.S. soybean Extension specialists together to collaborate on research and outreach for soybean production. In 2019 and 2020, they worked together to test applications of foliar fertilizers. Trials in individual states were funded by a combination of state soy checkoffs and leveraging those checkoff investments to secure industry and university support. The process of combining data and developing recommendations was supported by the United Soybean Board.

“We used the same research protocol at trial locations in 16 states over two years,” Vann says. “We compared six macronutrient and micronutrient fertilizer products applied at the R3 growth stage, and aggregated data from 46 total trials.”

She says this approach allows researchers to effectively capture data across a wide range of environmental variability and soybean production systems in a relatively quick timeframe.

“Based on this work, we can confidently say that applying foliar fertilizer to soybeans at early pod development when no visible symptoms of nutrient deficiencies are present provides no yield or economic benefit,” she reports. “Given the uniformity of the response across all the trials, we will not recommend blanket foliar fertilizer applications. Some inputs make a difference for soybean yields, but this is not one of them.”

Researchers came to this conclusion based on a variety of trial data. They collected leaf tissue samples following fertilizer application to learn if current high-yielding soybean varieties need supplemental fertilizer. They also collected yield and soybean composition data and provided pricing for an economic analysis. Emma Matcham, a graduate student at the University of Wisconsin-Madison, took the lead in aggregating the data.

The results showed slight increases in nutrient concentrations in leaf tissues following fertilizer applications, but that didn’t translate to yield. Differences in soybean protein and oil content were observed between sites, but not among treatments. And, soil properties didn’t impact foliar fertilizer responses.

A key to Science for Success is sharing research information with farmers. The team worked together to produce a full report for farmers, Foliar Fertilizers Rarely Increase Yield in U.S. Soybean.

“The collaborative approach of Science for Success allows us to make more robust recommendations,” Vann adds. “We can identify consistencies across soybean production, as well as areas where the environment interacts with a practice to make a difference.”