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Research Highlights

Research Highlights
Examining Soybean Nutrient Uptake and Yields with Early Planting

Photo: United Soybean Board

By Laura Temple

Farmers are asking lots of questions about planting soybeans early, according to Jarrod Miller, assistant professor and Extension specialist in agronomy at the University of Delaware.

“Soybeans tend to yield better if they flower before the summer solstice,” he explains. “The start of indeterminate soybean reproductive stages depends on the plants’ detection of the length of the night. Later-planted soybeans have less time to develop biomass before shifting to reproductive growth.”

Questions from farmers and growing national interest in earlier soybean planting dates prompted Miller to add a few questions of his own for Delaware and the Mid-Atlantic region.

“I wondered how disease pressures associated with cooler, wetter soils impact early planted soybeans, though springs are getting warmer,” he says. “Interest in nutrients made me consider if planting date influences nutrient availability from the soil or nutrient uptake by soybeans. Does our knowledge about nutrients match what happens in the field when soybeans reach reproductive stages early?”

To learn more, the Delaware Soybean Board funded a research study to compare yields, disease pressure, and tissue and soil contents from soybeans of the same maturity group planted at three different timings. Miller began the study in 2020, which will continue at least through 2022.

Weather conditions in 2020 caused the first planting to be in early May, with the subsequent plantings in mid-May and early June. In 2021, the study planting dates were April 12, April 28 and May 10.

“Both years, there was no statistical difference in yield between the planting dates,” Miller reports. “However, our absolute yields were 20 bushels per acre less in 2021. We have to account for a variety of environmental factors.”

Planting date timing study trials at the University of Delaware Carvel Research Center are answering farmer questions and raising new ones about soybean growth patterns and nutrient uptake. Photo: Jarrod Miller

The earliest 2020 planting at the beginning of May missed some of the cooler spring weather that may cause disease or poor growth. In 2021, he saw poor emergence at the earliest planting date. His trials did not use seed treatment for consistent comparisons.

“It is possible that with better emergence, the earliest 2021 planting would have yielded higher,” he says.

Growth Patterns

Though yield has been similar, Miller has observed differences in growth patterns. To get additional information about how planting dates influence soybean growth, he captured drone images bimonthly.

He notes that the first 2020 planting had the most biomass throughout the season based on vegetative index data from drone imagery. The June planting was always behind in growth, though it remained green longer at senesce in October.

The first planting in 2021 did not have the same ground coverage, especially early in the season, likely due to poor emergence. By late June, the first planting had the lowest vegetative index rating. The final planting in this trial, planted on May 10th, had the most biomass through most of the season.

“Soybeans are elastic, and they grow to fill in space,” Miller says. “However, the drone data supports what we know about soybean growth, even though very little difference in growth could be seen when walking the fields. It also supports the idea that the earliest-planted soybeans in 2021 may have had higher yields if more plants had survived.”

New Nutrient Use Questions

To identify differences in nutrient uptake, Miller’s team took soil samples and soybean tissue samples of each planting at the R1 or R2 stage.

“There were differences in nutrient uptake, with interesting patterns that we don’t currently understand,” he explains. “While all nutrient values fell within sufficiency ranges, the results raise new questions to explore. We may learn that field conditions give rise to the need for separate nutrient sufficiency levels based on planting dates.”

Miller thinks one of the most interesting observations in both seasons was a significant drop in aluminum uptake, a toxic non-essential element, as the season continued, especially since soil aluminum levels were highest in the plots with the latest planting. Concentrations weren’t high enough in early plantings to reduce yield, but currently he has no explanation for this pattern.

He found several other interesting and unexpected patterns in the 2021 data.

“At the first planting date, yield has a negative correlation to iron and aluminum concentrations in soybean tissue and potassium and boron concentrations in the soil,” he says. “Does that mean the plants couldn’t take up enough potassium and boron at that stage?”

He notes that at the second planting date, soil sulfur had a positive influence on yield. At the third planting date, higher tissue calcium concentrations were correlated with higher yields.

“The reasons for these observations are unclear,” Miller continues. “But they could lead to future research to help us understand the potential for plant stress related to nutrient availability at different planting dates for the same maturity group soybeans.”

In 2021, he encouraged researchers at the University of Maryland to do similar planting date research to look at different soil types within the same region. Results from those studies will likely also inform future fertility research.

“While this research is raising lots of new questions, it also has confirmed that Delaware soybeans can yield well at a variety of planting dates,” he adds. “That knowledge should help farmers make decisions to spread out spring workloads.”

This project was funded by the soybean checkoff. To find research related to this research highlight or to see other checkoff research projects, please visit the National Soybean Checkoff Research Database.