Research Highlights

Adjusting Nutrients to Increase Soybean Grain Yield

By Barb Baylor Anderson

Macronutrients like phosphorus (P) and potassium (K) and micronutrients such as iron (Fe) represent significant inputs for soybean farmers. So, making sure those nutrients are managed efficiently and effectively can lead to higher, more profitable soybean yield potential.

“Phosphorus and potassium are primary macronutrients, so a deficiency significantly affects yield,” confirms Dan Kaiser, University of Minnesota Extension soil fertility specialist and principal investigator of the Minnesota Soybean Research & Promotion Council funded research. “Because of the deficiency, I have gotten more questions about soybeans and whether more emphasis needs to be made regarding fertilizing soybeans directly. Farmers have to watch soil tests more closely now to ensure yield reductions are not occurring.”

Kaiser’s ongoing research goals are to evaluate the long-term impact of P and K rates and timing in a corn/wheat-soybean rotation. He is working on several long-term research projects to better understand when and where fertilizers need to be applied to increase soybean grain yield.

“When to apply fertilizer for soybeans used to be more clear-cut with fertilizer applied directly ahead of a rotational crop ahead of soybeans,” he says. “There is grower interest in applying fertilizer in front of every crop to achieve higher yield, so I want to help resolve that question.”

Kaiser’s findings, though, have shown corn yield is affected more consistently in a two-year corn–soybean rotation from fertilizer directly applied ahead of the crop. This is especially true for P where soybeans seem to do well if the appropriate rate is applied for a two-year rotation ahead of corn. For K, all of the fertilizer applied ahead of corn or as a split application before both crops were the best options. Applying high rates of K ahead of soybeans was more problematic.

“Past research has shown soybean yield decreases in single-year potassium fertility trials. However, long-term trials are more advantageous because the impacts of weather can be assessed at each location on the build-up of chloride (Cl) in the soil as well,” Kaiser says. “The chloride issue is interesting to me and needs to be investigated further.”

Kaiser explains that in the southern U.S., greater emphasis is placed on variety tolerance to chloride. Chloride-excluder varieties are selected to reduce the risk for yield loss where Cl additions are high. In more northern regions, almost all varieties are considered chloride-includers and can be susceptible to potential yield loss in the presence of high levels of Cl.  

“When it comes to grain quality, further work needs to study macronutrient phosphorus, potassium and sulfur application impact on the distribution of amino acids in the grain, too,” he adds. “Research has shown sulfur and potassium can impact cysteine and methionine content.”

Iron deficiency chlorosis (IDC) is a major concern for soybeans grown on high pH soils. Kaiser says research shows variety selection is still the primary management method to combat IDC. 

“Growers are increasingly interested in newer in-furrow iron products, but we have not found similar or greater yield from higher-yielding varieties more susceptible to IDC treated with in-furrow iron,” he says. “Selecting IDC tolerant varieties sets a good foundation to deal with it.”

Published: Oct 11, 2021