Research Highlights

Research Highlights
Does Fertilizing Cover Crops Boost Nutrient Capture?

Farmer concern about differences in cover crop growth prompted this study. Radish cover crops seeded at the same time into standing corn (left) and standing soybeans (right) appeared noticeably different after cash crop harvest. The radishes in corn appear to have taken up very little N compared to the radishes growing in soybean residue. Photos: James Lewis

By Laura Temple

Cover crops scavenge nitrates, keeping nutrients in fields where future crops can use them. It’s a primary reason farmers seed cover crops in the sandy soils of the Chesapeake Bay watershed.

“Our research shows that our soils have lots of nitrogen (N) deep in their profiles, even if not much is available near the soil surface,” says Ray Weil, professor of soil sciences at the University of Maryland. He has led practical cover crop research for years. “Those nutrients move down through the soil during the winter. To keep them in place, cover crops need to take them up during the fall.”

A farmer told Weil that cover crop radishes grew well in his fields following soybeans, but they remained very small following corn harvest in the fall, before planting soybeans. 

“He shared his concern that the stunted cover crops weren’t adequately providing the benefits he planted them for,” Weil explains. “This farmer and others looked to us for solutions.”

Weil and his team, which included graduate research assistant Melissa Stefun, hypothesized that applying a small amount of N to early planted cover crops in low-nitrate soils could stimulate cover crop growth and deeper roots that would capture more N from deep in the soil. However, Maryland nutrient management regulations don’t allow late fall applications. The Maryland Soybean Board invested in Weil’s research to test his theory and see if data could be generated to show benefits from this practice. 

In early December 2020, this radish-clover-ryegrass cover crop mix responded to both 15 and 30 pounds of nitrogen per acre applied immediately after corn harvest in October. However, the response was too small to justify the fertilizer application. Photos: University of Maryland

During the three years of the study, the team gathered data from a total of 45 site-years, located both in university research plots and on farms. They targeted plots only in sandy Coastal Plain soils, where the primary function of cover crops is to take up N deep in the soil profile before it leaches into groundwater. To give the cover crops as much time to grow as possible, they and the collaborating farmers interseeded cover crops into standing corn

The first year of the study, they used large-scale plots and equipment to apply low rates of N, 15 or 30 pounds per acre, as urea ammonium nitrate (UAN) to the cover crops following corn harvest. Cover crop response to the added N was inconsistent and not great enough to justify applying the fertilizer. They reasoned that the small response may have been due to the very few days of growing season left for cover crops between post-harvest fertilization and the onset of cold weather dormancy.

In 2021 and 2022, the team applied nitrogen solution in early September to micro-plots of cover crops inter-sown into standing corn. Photos: University of Maryland

So, for the next two years of the study, they switched to applying the N earlier, while the corn was still standing. Stefun hand-fertilized small plots of young cover crop plants with a solution of ammonium nitrate before corn harvest to maximize the growing degrees available for the cover crop to take up the N during the fall. 

“We took soil samples in each plot just before applying fertilizer,” she says. “We also measured ground cover, aboveground biomass and N concentration in the cover crop tissue. If radishes grew sufficiently, we measured the mass and nutrient content of their roots as well.”

Stefun took soil samples at 6 and 12 inches deep. She assessed cover crops through the fall, harvesting biomass in December once the cover crop went dormant for the winter and the plants had captured all the N possible prior to winter leaching season.

The cover crop response to applications of 30 pounds of nitrogen per acre was variable and not nearly great enough to get back all the N applied. Source: University of Maryland

“Based on what I saw in the plots, I could understand farmer concerns,” she adds. “Most of the cover crops were very small when we took December measurements, though the amount of biomass varied widely.”

The team seeded rye and a three-way mix of forage radish, rye and crimson clover in university research plots, in addition to checks with no cover crops. In on-farm plots, they studied whatever cover crop the cooperating farmer preferred, which included barley, rye, turnip and radish.

“Our hope was that applying a small amount of N to non-legume cover crops shortly after emergence would increase N uptake from a mere 5 to 15 pounds per acre to as high as 50 to 100 pounds per acre,” Weil says. “However, just a couple of the plot pairs showed much positive response. In most plots, we measured no statistically significant difference in the amount of N taken out of the soil by fertilized and unfertilized cover crops.”

Stefun adds that the soil and cover crop analyses showed only a weak relationship between fertilization and N uptake. 

“Most of the cover crop tissue analyzed contained about 2% N, regardless of fertilization rate,” she reports. 

Weil and Stefun find the results puzzling. They increased the fertilizer N rates to 40 pounds per acre for the third year of the study, but the responses still weren’t enough to make up for the N applied to the soil. They acknowledge that other factors, like limited light filtering through standing corn to cover crops or allelopathic compounds released by the corn that might interfere with other plants, could slow cover crop growth. 

Since the applied nitrogen had no effect on the cover crop tissue N concentration and variable effects on dry matter, only 13 of 96 pairs of plots, the green symbols, showed enough response to equal the amount of N applied. Source: University of Maryland

Based on this data, farmers with similar growing and field conditions probably should not invest in fertilizing their cover crops in fall. However, Weil and his team continue to ponder solutions to improve cover crop growth and the ability to capture N prior to planting soybeans. 

“This challenge in capturing nutrients in these fields doesn’t take away from the other benefits cover crops provide,” he says. “Soybeans still benefit from weed suppression and soil moisture conservation provided by the mulch of cover crop residue. That organic matter also contributes to overall soil health.”

Published: Dec 4, 2023

The materials on SRIN were funded with checkoff dollars from United Soybean Board and the North Central Soybean Research Program. To find checkoff funded research related to this research highlight or to see other checkoff research projects, please visit the National Soybean Checkoff Research Database.