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Research Highlights
Do High-Yielding Soybeans Get the Nutrients They Need?

Fertility trials in North Carolina determined that existing recommendations for phosphorus and potassium meet even the needs of soybeans in fields that routinely yield 60 to 70 bushels per acre. Photo: Luke Gatiboni

By Laura Temple

North Carolina soybean production has changed dramatically in the past 40 years, as it has throughout the U.S. Advances in management practices, genetics and other factors have more than doubled average soybean yields in fertile soils.

“While high-yielding soybeans can produce 60 or even 70 bushels per acre, fertility recommendations in North Carolina haven’t changed since the early 1980s,” explains Dr. Luke Gatiboni, Extension soil fertility specialist for North Carolina State Extension. “High-yielding soybean fields, defined as those routinely producing more than 60 bushels per acre, likely use more nutrients than average-yielding fields.”

He says that raised key questions.

“Do those recommendations for phosphorus and potassium support the needs of current high-yielding soybeans?” he asked. “Do they need to be recalibrated to better fit with current agronomic practices and varieties?”

To answer these questions, Gatiboni led a research project supported by the North Carolina Soybean Producers Association. Fertility trials focused on soybean uptake of phosphorus and potassium in fields expected to yield well above the state average.

Fertility Trials for High-Yielding Soybeans

Trials were placed in the two primary soybean-producing regions of the state. In the Piedmont of central North Carolina, Union County, acidic clay soils and sloping fields support no-till production systems. The Coastal Plain of eastern North Carolina, including Beaufort County, has sandier soils with very different organic matter content that works well with conventional tillage systems.

In both locations, Gatiboni and his team applied five different rates of phosphorus and potassium. Dry fertilizer was broadcast at planting, and the results allowed them to determine the critical level of each nutrient to ensure it didn’t become a yield-limiting factor.

2020 fertility trials in two North Carolina locations showed that current phosphorus and potassium recommendations are sufficient even for high-yielding soybean fields. Source: North Carolina State Extension

“In 2020 and 2021, the trial results showed that high-yielding soybeans did not respond to fertilization above the current levels,” Gatiboni reports. “We will repeat the trial in two more locations — one in each region — in 2022, but we expect to see consistent results. North Carolina soybean producers can keep using the current fertilizer recommendations, even in high-yielding fields, and they will have adequate nutrients.”

Value of Context for Fertility Recommendations

When Gatiboni investigated the work behind the current soybean fertility recommendation, he understood why they still work for today’s high-yielding soybeans. Critical levels for each nutrient describe what needs to be in the soil to meet crop yield goals.

“Soil fertility experts in North Carolina took a unique approach when they established soybean nutrient recommendations roughly 40 years ago,” he explains. “They determined the critical level for phosphorus to raise 25 to 30 bushels of soybeans was 25 to 35 parts per million (ppm). However, at that time, farmers only tested their soil every four or five years. To ensure that crops would have the nutrients needed between soil tests, they set the phosphorus recommendations at 60 ppm, more than what is needed.”

Gatiboni says his research found that phosphorus needs in high-yielding soybeans actually have increased about 5 ppm over time. However, because soil tests are often done more frequently, those recommendations still meet soybean needs.

Harvest of these North Carolina fertility trials determined that existing recommendations for phosphorus and potassium meet the needs of high-yielding soybeans. Photo: Luke Gatiboni

He adds that North Carolina soils tend to be high in phosphorus, in part because of the availability of organic fertilizer in the form of manure. The state is a top producer of both pigs and poultry within the U.S. Manure is often applied to meet nitrogen needs, resulting in an abundance of phosphorus available for crops.

However, the context for potassium recommendations in North Carolina is completely different. The critical level set for soybeans in the 1980s accounted for just what the crop needed. In addition, potassium tends to leach from the coarse-textured soils of the Coastal Plain region.

“The existing potassium recommendations are adequate, but they don’t include any excess,” Gatiboni says. “Understanding how the recommendations were developed can help farmers make more informed decisions about the fertilizer applied in high-yielding soybean fields.”

Because soybean production has advanced everywhere, farmers benefit from understanding how fertility recommendations for their state and region have been developed. With fertilizer costs rising, knowing when recommendations were calibrated and how they were set can help farmers apply soybeans need, while also managing profitability.

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.