Research Highlights

Research Highlights
Fine-tuning the 4Rs for Phosphorus Could Improve Soybeans and Water Quality

Lysimeters are installed at a 12-inch depth in the fall after harvest and dug up in the spring prior to planting. Margenot and his team use these to evaluate nutrients that were not used by the crop and leached through the soil. Photo: Andrew Margenot

By Carol Brown

The 4Rs of nutrient management are important in any cropping system. The Right Source, Right Rate, Right Time, and Right Place can help with crop profitability as well as environmental protection.

Andrew Margenot, University of Illinois Crop Sciences Assistant Professor, is in the first year of a two-year project exploring the 4Rs of phosphorus (P) for soybeans for increased nutrient use efficiency as well as reducing the negative impact on water quality. His hope is to find the best outcomes for each of the 4Rs as well as finding ways to reduce nutrient loading in Illinois waterways.

“There has been a lot of talk about treating the soybean more kindly and investing in the soybean’s nutrition like many producers do for corn,” says Margenot. “This is one reason why we are looking at applying phosphorus in front of soybeans. We want to test the hypothesis of that P management for soybean may entail unnecessary losses of nitrogen (N), depending on the timing and source of phosphorus.”

Margenot’s research team, led by graduate students Yuhei Nakayama and Patricia Leon, is conducting intense studies at two locations: Champaign County near campus in central Illinois, and in Franklin County in southern Illinois. These locations will capture results of the two main soil types (Mollisols and Alfisols) that dominate the state and the Corn Belt. The first crop year has concluded and they recently finished soybean harvest this year.

“As a rule of thumb in a corn–soybean rotation, phosphorus and potassium application has historically been done ahead of corn, and the soybean must make-do with the nutrients left over,” he says. “We are looking at maximizing agronomic P use efficiency, so the farmer gets the most use per unit of phosphate they invested in, and by applying the right source of P, we’re looking to see if there is a decrease in nitrogen losses.”

The research team is testing four P sources: monoammonium phosphate (MAP), diammonium phosphate (DAP), triple super phosphate (TSP) and struvite, which is magnesium ammonium phosphate. They are applying these on their test field sites at two P rates, and applying in the spring and the fall, which addresses three of the 4Rs: Source, Rate, and Timing. For the timing in the spring, they will also apply via banding and broadcast to cover the fourth R: Placement.

Some of the 600 lysimeters are assembled to be installed at two research field sites in Illinois. They capture nutrients being leached through the soil profile. Photos: Andrew Margenot

To measure how much N and P are leached under the various P fertilizer treatments, Margenot and his team installed 300 lysimeters at each of the two field sites. The lysimeters capture the ammonium-N, nitrate-N, and phosphate-P leached through the soil profile out of the soybean rooting zone. The data from the previous crop year revealed some expected results but also some that were unexpected.

“We found strong location-specific outcomes of N and P leaching under P sources. At the central Illinois site, the amount of nitrate-N leached under MAP and DAP was on average approximately twice more when compared to the TSP and struvite,” he explains. “At the southern site, we found similar average nitrate-N leaching across all treatments, including the unfertilized control, but much more variable leaching under MAP and DAP compared to struvite.”

However, there was a clear difference in P leaching at the Urbana site, and it was on average three-fold greater with MAP and DAP compared to struvite. The unfertilized plots had the lowest amounts of phosphate leachate, but not necessarily the lowest amount of nitrate-N leached, pointing to contributions from soil organic matter to nitrate leaching via mineralization. Margenot is anxious to see results from this year’s crop to see if these results are repeated.

Nutrient Losses and Water Quality

Margenot’s team found high amounts of background nitrate-N leaching in their studies. The control sites had similar pounds per acre nitrate-N losses compared to the fertilized treatments. This is important to note, as there will always be some background leaching; even prairies will leach nutrients, Margenot says. The question he asks is how to mitigate the higher leaching of nitrate-N compared to this “baseline.” Changing their approach to fall-applied ammonium phosphates may be one means to achieve this.

Reducing nitrate-N and P entering Illinois waterways is a statewide goal, as it is for the other 11 states bordering the Mississippi River. The EPA’s Hypoxia Task Force called for these states to employ Nutrient Lost Reduction Strategies to reduce N and P entering the Mississippi River for improved water quality in the Gulf of Mexico. The Task Force goals are to decrease N and P loads by 15 and 25 percent, respectively, by 2025, and eventually by 45 percent for both nutrients.

Margenot’s results from this project have implications that could make strides in reducing nitrate-N loading into rivers and streams — in addition to updating the Illinois Agronomy Handbook for the 4Rs for P, and specifically for soybean.

“If we assume that half of the soybean acres are using fall-applied MAP or DAP, either in front of soybean or in front of corn, and half of N from the P sources is being leached and not used by the crop, it could mean millions of pounds of nitrogen statewide,” Margenot says. “Switching to nitrogen-free P sources such as TSP could contribute significantly to the N loss targets of the Nutrient Loss Reduction Strategy for non-point sources.”

The relationship and activity between nitrogen and phosphorus in crops, soils and water is complicated. Because soybeans can fix their own nitrogen, the plant doesn’t need more applied, the possibility of adding only a phosphorus-based fertilizer for boosting soybean health could be a solution for productive crops and improved water quality.

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.