Soybeans and cover crops can work together for better soil health, yields 

By Carol Brown, Soybean Research and Information Network 

There are many benefits when using cover crops in a crop rotation including reduced erosion from wind and water and improved soil organic matter. Cover crops are known to capture nutrients for the following season’s crops, but according to Shaun Casteel’s past experiences with cover crops and soybeans, he’s seen evidence of reduced yields. 

The Purdue University associate professor of agronomy and extension specialist is an expert in nutrients and nutrient cycling. He is leading a research project supported by the Indiana Soybean Alliance to take a closer look at how cereal rye and soybeans interact. 

“Our project objective is to see if cereal rye is immobilizing the sulfur and nitrogen that’s needed for soybeans,” Casteel explained. “I have seen a soybean yield drag following a cover crop. After one year of our trials, at one location we saw about a 6- to 7-bushel yield hit in soybeans with cereal rye compared to not using the cover crop.” 

Casteel knows that cover crops have soil health benefits, and he wants to determine how to also improve soybean yield to ensure cover crops are a win-win situation for farmers. In the fall prior to establishing the trial, he ran one vertical tillage pass across the corn stalks. He and his team then drilled the cereal rye treatment plots. The accompanying plots had no cover crops. The research team assigned one of four fertility treatments to each plot: 

• 20 pounds of sulfur applied as pelletized gypsum 
• 40 pounds of nitrogen from urea 
• A combination of 20 pounds of sulfur and 40 pounds of nitrogen 
• A check treatment with no applications 

There were four replicates of the different treatments at field scale. They terminated the cereal rye when it was between 12- and 16-inches tall before planting soybeans. Casteel said they were aiming for 1,000 to 1,200 pounds of rye biomass at termination. 

“After the first year of the project, we documented minimal nitrogen immobilization from the soil or the cereal rye, but the sulfur was being immobilized,” he said. “The plots with applied sulfur with and without cereal rye yielded the same at 71 bushels of soybeans. This showed us that sulfur was immobilized with cereal rye, but we were able to overcome the 6-to-7-bushel yield drag by the addition of the sulfur fertilizer.” 

Although it takes some time to build soil fertility when farmers use cover crops, there are other immediate benefits. In this project, Casteel also observed how cereal rye biomass helped curtail weeds and reduce soil temperature, which protects it from evaporative water loss. 

Because of the observed sulfur response, Casteel cautiously thinks soybeans after cereal rye may benefit from added sulfur. But the best test for any field is to use crop reference strips first. 

“If farmers are using a cereal rye cover crop, they should create some reference strips with 20 pounds of sulfur applied with a soluble source like pelletized gypsum, ammonium sulfate or ammonium thiosulfate,” he explained. “Every field may not need additional sulfur, but with reference strips, farmers will have a better idea of how their fields are responding to cover crops and applied nutrients.” 

The project will be repeated this coming crop year to gather two seasons of data.