Research Highlights

Making a Difference with Nitrogen Management in Early Planted Soybeans

By Carol Brown

When planting soybeans earlier in the spring, more factors come into play beyond cold weather and the threat of frost. Michigan growers are increasingly seeing the benefits of getting soybeans in the ground earlier than historically recommended, and researchers are looking at management strategies so farmers can reap the maximum yields.

Michigan State University Extension soil specialist Kurt Steinke has been focusing on nutrient management strategies to help soybean farmers with their production. Through a research project supported by the Michigan Soybean Committee, he is comparing nitrogen application and biological nitrogen fixation in early planted soybeans. 

“When planting earlier, oftentimes we have less than stellar growing conditions. Soils are cooler and, depending on the year, they can be wetter or drier. I wondered if nutrient management practices behave differently early in the season versus later,” Steinke says. “Also, about 45% of Michigan growers apply some form of nitrogen to their soybean crop. Are these N applications offsetting the plant’s nitrogen fixation capabilities?” 

Steinke’s research project compares soybeans planted in April to those planted in mid-May with six different fertilizer strategies under both irrigated and non-irrigated conditions. He and his research team evaluated the impacts of liquid vs. granular 2×2 starter fertilizer, and nitrogen applications at pre-planting, vegetative stage 4 (V4) and reproductive stage 2 (R2). They measured soybean growth and development, mineral nutrition and nitrogen fixation at research plots in 2021 and 2022. 

“Soybeans rely on a combination of nitrogen in the soil and biological nitrogen fixation to satisfy their requirements,” Steinke says. “The percentage from each source varies with soil temperatures, soil moisture, soil physical properties and nutrient concentrations as well as soybean genetics. Earlier planted soybeans display different responses to nutrient applications than those planted under warmer conditions.”

As with many growing seasons, both years were different, which impacts results as well as farming plans.

“In 2021, we had extremely dry conditions and we had to irrigate earlier than we would have liked to ensure seed emergence,” says Steinke. “And 2022 was a more normal year. We saw responses in different conditions, which helps to illustrate what a farmer would see.”

Part of Steinke’s research is to measure whether biological nitrogen fixation, or BNF, can fully satisfy the soybean plant’s needs. Many growers apply nitrogen as a supplement to reach optimum levels for the plant. But is biological nitrogen fixation sufficient?

“Across each year, our data showed that nitrogen application rates under 25 pounds per acre band-applied as starter fertilizer didn’t reduce biological nitrogen fixation measured at R2 or R6, but applied nitrogen above 100 pounds per acre negatively impacted BNF,” Steinke explains. “We’ll need to take a closer look at N rates between the 25- and 100-pound rate thresholds. We discovered that nitrogen applied at R2 did not negatively impact BNF, but this timing also didn’t provide any yield benefits across years.”

Planting date had less of an effect on BNF than the fertilizer applications did. In 2022, Steinke saw earlier planting increased the share of nitrogen derived from the atmosphere in the soybean plant but only under non-irrigated conditions.

Earlier planted soybeans reach canopy closure sooner and can produce higher yields, but spring weather variability and colder soils can restrict root growth and slow microbial activity. But these are risks that Michigan farmers are willing to take. With early planting, adding mineral fertilizers may help offset any delays in BNF by the soybean plants. 

The question that remains is whether helping early soybeans along through extra nitrogen fertilization will lead to higher yields and a better return on farmers’ investments. Thus far, under the conditions tested, Steinke’s treatments have accelerated early and mid-season soybean growth but yield benefits haven’t followed.

Published: Oct 16, 2023