Research Highlights

Research Highlights
Cover Crops Support Nutrient Cycling

Planting date impacts the development of cover crop biomass in Louisiana. September planting generated the most biomass. Photo: Daniel Forestieri

By Laura Temple

One of the many agronomic benefits of cover crops is their ability to take up and store nutrients. Decomposing cover crop biomass then releases those nutrients in a plant-available form for future crops.

“Very little information is available to describe the nutrient contribution of cover crops and the resulting impact on the following cash crops,” says Brenda Tubaña, professor in the School of Plant, Environmental and Soil Sciences at Louisiana State University. “Understanding the relationships between cover crop biomass and nutrient recycling will help farmers take full advantage of this benefit of cover crops.”

The Louisiana Soybean and Grain Research and Promotion Board invested soy checkoff dollars in Tubaña’s research, allowing her team to establish a test plot to study cover crops nutrients in a soybean–corn rotation. Her study quantified nutrient uptake and biomass production of cover crops at different planting dates, as well as measuring changes in soil nutrient levels.

The trials used a cover crop mix of legumes, specifically crimson clover and hairy vetch, with tillage radish, a brassica. The legumes build up nitrogen levels, and both types of cover crop hold sulfur in their biomass in a readily available form for the next crop. This is important because sulfur emissions have gone down, resulting in less acid rain, which used to transfer enough sulfur from the atmosphere to the soil for crops to use.

Average nutrient recovery in pounds per acre per year by native weeds with no cover crop compared to a cover crop mix containing equal amounts of hairy vetch, crimson clover and tillage radish. Note: Boron, manganese and molybdenum were also recovered but at lesser amounts than zinc. Photos: Brenda Tubaña

“Our goal is to provide information on nutrient credits for each cover crop species on a given amount of biomass,” Tubaña says. “This information would help farmers plan for both the type of fertilizers and rates to apply for the next crop.”

Biomass clippings from cover crops and soil samples are captured before cover crop termination to measure nutrient cycling by cover crops. Photo: Daniel Forestieri, Louisiana State University

Her team varied planting timing and measured biomass and the nutrient content of each cover crop species just before burndown in February, to estimate nutrient uptake from cover crop biomass. The team also took soil samples in each plot before burndown and at key vegetative growth points for each cash crop. In soybeans, they sampled the soil before the R1 stage, and in corn, they sampled the soil at the V8 to V9 growth stage.

Available Soil Nutrients

“We saw improvement on soil nutrients in plots with cover crops compared to plots without cover crops,” Tubaña explains. “Soil showed noticeable differences in phosphorus, sulfur and potassium content.”

She says most of the nutrients in the cover crop biomass were released within six to eight months after burndown. Because nitrogen is more mobile and the turnover took place in just four to six weeks, her soil sample timing didn’t fully capture this flush of nitrogen to the soil. If cover crop termination is timed correctly, nitrogen should be available as the following cash crop is germinating and emerging.

The charts below (Fig. 1A-B) capture trends in the pounds per acre of phosphorous and sulfur during the first three years of the trials. The plots with cover crops consistently held more nutrients. The yellow arrows along the X axis indicate when the soybeans or corn were actively growing.

How does this nutrient availability impact crop yield?

Figure 1A-B. Soil Phosphorus (A) and Sulfur (B) in pounds per acre with and without cover crops over three years.

“During the first two years of these cover crop trials, they were yield neutral,” Tubaña says. “We started seeing increases in yield in year three, and we continue to monitor yield as these trials continue.”

Cover Crop Management

Tubaña notes that cultural management practices directly impact the amount of biomass cover crops produce, translating to the amount of nutrients they store. Her team planted cover crop trials in September, October and November.

“While earlier plantings in September generated the most biomass, we saw that they have the potential to compete with the cash crop,” she reports. “Our results suggest that in Louisiana, planting cover crops in October prevents competition for nutrients with the soybean or corn crop  the following spring, but still produces adequate biomass to absorb and store nutrients throughout the late fall and winter.”

The research also looked at the impact of providing the cover crops with starter fertilizer to improve stand establishment.

“We saw no effect from fertilizing cover crops,” Tubaña says. “If they are planted and established early enough, like in October, they don’t need starter fertilizer to produce a good stand.”

Published: Jul 4, 2022

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.