Making Cover Crop Mixtures Flourish to Help Your Farm Thrive

Authors: Sheryl Hosler, Madeline Luthard, Brosi Bradley, Jason Kaye

Managing Diverse Cover Crop Mixtures for Nitrogen Retention

What is the issue?

Cover crops are a common agricultural practice used to manage off-season nitrogen losses. As they grow, cover crops take up nitrogen to satisfy their growth requirements, and thus prevent it from leaching through the soil and into streams or groundwater. Different cover crop species vary in their ability to mitigate off-season nitrogen losses depending on factors like how much cover crops grow (biomass) and the relative amount of nitrogen they store in their tissues (carbon-to-nitrogen ratio; Figure 1). This research aimed to characterize how well different cover crops, and also different mixtures of cover crop species, are able to reduce potential nitrogen leaching. While previous research has shown that winter-hardy grasses are highly effective at retaining nitrogen in agricultural soil, we sought to identify multi-species mixtures that might be as effective as these grasses, while providing additional benefits.

Figure 1. Expected leaching patterns based on cover crop stand characteristics.

What did we find and why does it matter? 

All cover crops remove nitrogen from soil that would otherwise be vulnerable to leaching. When there is no cover crop present (bare fallow control), we see high levels of nitrogen throughout the soil profile (Figure 2 - top). Because there are no plants growing, nitrogen is vulnerable to leaching beyond the plant rooting zone. When there is a cover crop in place, we see significant reductions in potential nitrogen leaching compared to the fallow plots (Figure 2 - bottom). This is especially true for cover crops in the grass and brassica (mustard) families, as well as diverse mixtures of cover crop species that contain species of two or more plant families.

Legumes and diverse cover crop mixtures add nitrogen to surface soil, but still reduce leaching below the plant rooting zone. Cover crop species in the legume family have high nitrogen levels in their tissues, yet they still significantly reduce potential nitrogen leaching compared to the fallow control (Figure 2 - top). Mixtures of cover crops with species from different plant families are also highly effective at reducing potential nitrogen leaching compared to the fallow control. When mixtures include species from the legume family, the legumes in the mix supply some nitrogen to the surface soil that can be used by the subsequent cash crop, while the non-legumes in the mix help to reduce nitrogen deeper in the soil profile and minimize potential nitrogen leaching (Figure 2 - bottom). As expected, high biomass, a high ratio of carbon relative to nitrogen, and more species in a mixture are qualities of cover crop stands that are associated with the best nitrogen retention.

Figure 2. Depth profile of total inorganic nitrogen for the most effective monocultures (winter-hardy [WH] grass and brassica) and polyculture (4-species mix), compared to legumes (winter-hardy [WH] and winter-killed [WK]) and fallow (top). Nitrate (NO₃^-) trapped on resin bags, showing cumulative leaching potential from August to May (bottom). Graphs present model-predicted values, where a statistical model was used to estimate average inorganic nitrogen (top) and resin nitrate (bottom) based on data from 10 years of data collection.

What did we do?

For 10 years, different cover crop species and mixtures were planted at Penn State’s Russel E. Larson Agricultural Research Center in Centre County, Pennsylvania. Potential soil nitrogen leaching was measured under the growing cover crops and under bare fallow plots, which serve as a no cover crop control treatment. We captured potential nitrogen leaching using:

1. Deep soil core samples to 30 inches (Fig. 2 - top, collected at the end of the cover crop season in May and analyzed for total inorganic nitrogen: nitrate (NO₃^-) plus ammonium (NH₄⁺).
2. Resin bead bags (Fig. 2 - bottom) buried 10 inches below the surface in August when cover crops were planted, then removed in May before the cover crops were plowed. Resin bead bags trap nitrate (NO₃^-), a form of N that is very mobile and likely to leach through the soil profile.

Publications completed for this work

Arrington, K. E., Ordóñez, R. A., Rivera-Ocasio, Z., Luthard, M., Tierney, S., Spargo, J., Finney, D., Kaye, J. P., & White, C. M. (2024). Improving a nitrogen mineralization model for predicting unfertilized corn yield. Soil Science Society of America Journal, 88(3), 905–920. https://doi.org/10.1002/saj2.20665

Hosler, S. C., Murrell, E. G., Arrington, K. E., Baraibar, B., Barbercheck, M. E., Bradley, B. A., Burgess, M., Finney, D. M., Hunter, M. C., LaChance, J. C., Mortensen, D. A., White, C. M., Lowry, C. J., & Kaye, J. P. (2025). Managing cover crop mixtures over a decade via species replacement and seeding rate adjustment. Agricultural & Environmental Letters, 10(2). https://doi.org/10.1002/ael2.70029