Soybean nutrient concentration research aims to improve fertilizer use efficiency

By Laura Temple, North Central Soybean Research Program

Modern soybean varieties produce much higher yields than decades ago. And researchers have identified other differences. New soybean cultivars have lower concentrations of phosphorus (P) and potassium (K), according to a meta-analysis of soybean composition over time.

“What do lower nutrient concentrations mean to soybean plants?” asks Dr. Alvaro Sanz-Saez, an assistant professor in Crop, Soil and Environmental Sciences at Auburn University. “Maybe new cultivars have higher, unrealized yield potential. Or maybe they need fewer nutrients. Or maybe breeding for higher yields has limited their ability to take up nutrients.”

The answers to these questions could impact fertilizer applications and costs or lead to knowledge to further increase soybean yields. Sanz-Saez and a team of researchers investigated differences in nutrient concentrations between older and newer cultivars in a project funded by a soy checkoff investment from Alabama Soybean Producers.

“This research could help us detect characteristics that make future soybeans absorb K and P more efficiently, reducing fertilizer application and farming costs,” Sanz-Saez said.

Field trials compared nutrient P and K content in three cultivars developed in the 1940s and 1950s and three cultivars developed around 2000. These trials confirmed differences in nutrient concentration, which Sanz-Saez attributes to the higher yields of newer cultivars diluting the absorbed nutrients.

“The recent research meta-analysis demonstrated a decrease of 18 and 13 percent in seed P and K concentrations compared to older soybean cultivars,” he said. “While the values from our trials vary due in part to drought conditions in 2019, we still consistently found that the older cultivars contained statistically higher concentrations of both P and K, while the newer cultivars produce higher yields.”

The trials also investigated if P and K applications beyond the standard local recommendations would increase yield and nutrient concentration in plant leaves and soybeans. Each cultivar received treatments of an additional 60 pounds per acre of P, K, and a combination of P and K.

“We demonstrated that additional P and K does not increase nutrient concentration or yield,” Sanz-Saez said. “We believe that newer soybean cultivars do not need more fertilizer. However, we do want to understand any limitations in root structure or nutrient transporter level that prevent newer cultivars from being able to use additional nutrients to further increase yield.”

To learn if transpiration capacity – which supports nutrient uptake – impacts nutrient concentration, cultivars used in this trial with the most difference in P and K concentrations were grown under elevated carbon dioxide levels. These conditions reduce plant transpiration.

“We found that although elevated carbon dioxide levels reduced transpiration, it did not reduce P and K concentration percentages in the seed,” he said.  “We believe other factors must influence how nutrients are absorbed. More research will help us learn how to improve nutrient absorption and use.”

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