How Sulfur Applications Can Improve Productivity of Early Planted Soybeans

Planting soybeans early helps maximize yield by increasing the plant’s productive life. When April planting dates come with cool, wet soils, sulfur management is key to ensuring yield potential. 

Sulfur is an essential nutrient for nitrogen fixation, nodule formation, and seed protein content. Soybean plants draw sulfur from organic matter within the soil, but the amount readily available in early spring can be limited. 

“When the soil is cooler, mineralization of microbial activity is limited, so there is an opportunity to bolster it with a soluble source of sulfur,” said Shaun Casteel, an agronomy professor at Purdue University. 

Granular products, such as ammonium sulfate (AMS), can be applied within a few days of planting. Casteel targets a rate between 15 and 25 pounds per acre, with 20 pounds per acre as the sweet spot. 

While sulfur deficiencies most commonly occur in sandy, loamy soils, soybeans planted early in soils with up to 4% organic matter have shown a positive yield response to sulfur applications. 

Over five years of research, Casteel has seen an average yield response of 8–10 bushels per acre (bpa), and highs around 20 bpa. 

“I want to be clear that there are some conditions where we won’t see a response,” Casteel noted. 

“I’m a big fan of doing those earlier plantings, and using strip trials to see if you have a site that’s responsive.” 

Be aware that sulfur isn’t the only yield-reducing factor to note in early spring. Low pH values in the soil can also negatively impact nitrogen fixation, and need to be corrected to achieve high yields. 

A Farmer’s Perspective

Meagan Kaiser of Carrollton, Missouri, has been transitioning to early-planted soybeans over the past four years. “We’ve continually seen that the early-planted beans flat-out yielded better across varieties and brands,” she said.

Kaiser plants the majority of her soybeans between April 1 and April 20. She applies two forms of sulfur: AMS near planting, and an elemental sulfur in the fall. 

“The interesting thing about elemental sulfur, which is a 90% sulfur product, is that it takes microbial activity to break it down and make it available for plant uptake,” Kaiser said. “Putting it on in the fall allows it time over winter and early spring to become available for the plant.” 

To avoid extra tracks, Kaiser utilized sprayer tracks, resulting in an every-other-row application. All other management has remained the same. While the numbers are different every year, Kaiser has seen yield increases of over 10 bpa. 

“The areas where we applied AMS out-yielded areas where we didn’t apply. But we also noticed that the field as a whole achieved a higher yield than the normal average,” Kaiser said.