Research Highlights

Research Highlights
Do Cover Crops Make Phosphorus Runoff Better or Worse?

Mini runoff weirs measured runoff from plots in different systems, including soybean stubble with and without interseeded cover crop. Photo: Ray Weil

By Laura Temple

Nitrogen (N) and phosphorus (P) are major nutrients that help plants grow. Just like N and P help crops thrive in the field, they also make algae thrive in the water — creating water quality challenges.

Adding too much N to brackish water systems creates imbalances that lead to algae blooms and hypoxia issues. Likewise, adding too much P to freshwater systems creates algae blooms in ponds, lakes and rivers that degrade water quality through eutrophication.

“The amounts of P lost from fields are tiny, agronomically speaking,” says Dr. Ray Weil, Professor of Soil Sciences at the University of Maryland. “But it also doesn’t take much additional P to impact freshwater systems. Just parts per billion can damage a lake.”

Farmers prefer keeping N, P and other nutrients in their fields, in forms available to their crops, both to support crop yield and protect water quality. No-till and cover crops are two common practices used in Maryland to help farmers retain nutrients and reduce runoff.

“Much less is known about the impacts of cover crops on P losses than on N losses,” explains Weil. “Cover crops could have contradictory effects on P. Some studies suggest that cover crops may increase soluble P at the soil surface, where it is susceptible to becoming runoff. However, cover crops also reduce overall runoff because of improved soil infiltration and structure.”

Weil is investigating the impact of cover crops on P availability and loss in a multi-year study funded in part by the Maryland Soybean Board. P tends to bind tightly to soil particles so that it is far less mobile than N and doesn’t move much through the soil. Because of this, most P is lost through soil erosion caused by heavy rains or snowmelt. 

“Our goal is to answer questions about the interaction of cover crops and P,” Weil explains. “Do cover crops reduce runoff volume? Increase the amount of rain required to start runoff? Do they reduce the concentration of P-carrying sediment in runoff? Or do cover crops increase the concentration of P in dissolved runoff water? Does cover crop plant uptake reduce P at the soil surface? Or does freezing injury to cover crop tissues release more P?”

To answer these questions, Weil’s research team is monitoring runoff from simulated and natural rain events in a variety of cover crop systems on silt and clay soils in Maryland. No-till, cover crop plots grew cereal rye, radish (which has been shown to leak P following winterkill) and a three-species rye–radish–clover mix. A fourth plot at each location was also no-till, but with no cover crop.

Stratification impacts P recommendations

Surface soil tests in the plots found that the type of cover crop had no significant effect on the soluble P present. These tests did show that no-till and cover crops cause P stratification, or uneven distribution of P in the soil. The top 1-inch of soil contains significantly more P than the next 5 inches.

“Cover crops, especially brassicas like radish, take up P from the soil profile,” Weil says. “As the cover crops decompose, that P remains near the soil surface because it doesn’t move through the soil. However, it’s easy to miss that P stratification in soil tests.”

Figure 1. Stratification in P was evident, with significantly more in the top in of the soil. Source: Ray Weil

When soil testing, he recommends analyzing the top 2 inches of soil separately for P in no-till, cover crop systems. He explains that the amount of P available near the soil surface may meet crop needs, but when diluted by a standard soil sample core of 6 to 8 inches, the results may show a need for applying P.

“Based on soil tests, farmers may be applying more P than the system actually needs, especially when cover crops make more organic and inorganic P available for crops,” Weil explains. “That adds costs and makes it more likely that extra P gets carried away in runoff. Basing P fertilization on soil surface tests could be part of a long-term strategy for farmers to maintain high productivity and be more sustainable both economically and environmentally,”

Weil believes P stratification is common in natural ecosystems, so it should not be considered a problem.

“Stratification appears to be the natural state of P in forests and prairies, where fallen leaves or last season’s growth provides P near the soil surface for currently growing plants,” he adds.

Monitoring runoff shows influence of cover crops

To see if P at the soil surface became runoff, small-scale instruments captured excess water leaving small sections of the trial plots. Rainfall simulators allowed his team to collect and measure runoff in specific, pre-determined conditions that mimic heavy, hard rains. Mini runoff weirs buried in the soil of fields with a slope of more than 5 percent allowed them to collect and measure runoff from nine runoff-generating events in the fall and winter of 2018-19, eight events in 2019-20 and 15 events in 2020-21. 

Overall, very little P was lost from these plots during fall and winter. Data trends show that rains in the fall, shortly after harvest, cause the most P loss. Although the results showed an increase in soluble P after the first rain following a long dry fall in 2019, they surprisingly did not show any increase in runoff P following cold damage or winterkill of the radish on its own or in the three-species mix. 

Figure 2. The most P was lost via runoff in the fall, shortly after harvest, as shown in the top chart, despite high ground cover. Cold damage to radish did not cause an increase in P in runoff. Source: Ray Weil

“The very low level of dissolved P loss in runoff from moderately high-P fertility soils under no-till management should be encouraging,” Weil says. “Cover crops provide many documented benefits to crops, and they can be managed to support P availability in soybeans and corn.”

To minimize P runoff in the fall after harvest, when it is highest, he recommends management strategies like seeding cover crops early and minimizing soil disturbance. His research shows that even drilling cover crops moves soil enough that an increase in erosion — and by extension, P loss — can be measured. 

“So far, our research does not indicate that cover crops add to P loads in runoff, but we need to look at additional scenarios,” he adds. 

Weil plans to take a similar protocol from research plots to fields in the fall and winter of 2021-22. Manure as a P source and a broader range of soil types will also be accounted for as this research continues.

“The Maryland Soybean Board demonstrates forward-thinking as they fund research that addresses questions like this,” Weil says. “We are working to solve long-term challenges, and that would not be possible without support from farmers.”

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.