Cover Crops at Work: Keeping Nutrients Out of Waterways
The Sustainable Agriculture Research & Education (SARE) program provides an excellent overview of cover crop impacts on nitrogen and phosphorus losses from agricultural systems in its Ecosystem Services from Cover Crops fact sheet series. Nitrogen can be lost from agricultural fields in runoff water and groundwater. This displaced nitrogen may then travel into waterways and cause imbalances in the nutrient levels of these sensitive ecosystems. Farmers want nitrogen to remain on the land to fertilize their crops and support productive growing systems, and society as a whole doesn’t want excess levels of nitrogen in the water.
So how can cover crops help? They scavenge soil nitrogen and prevent it from being leached, and they can provide natural sources of nitrogen to cash crops and thus reduce the amount of fertilizer needed for production. Cover crops reduced the amount of nitrogen leaving a field by 1% to 89%, with a median value of 48% (across 10 studies and 16 observed reductions).
What about phosphorous? Compared to the impact of cover crops on erosion or losses of nitrogen, the impact of cover crops on phosphorus in the field is less studied and the research inconclusive. Phosphorus can be transported to waterways by above- or below-ground water flows. Some studies report finding no significant effect of cover crops on total phosphorus losses, sometimes because the cover crops may have reduced total phosphorus losses but increased soluble phosphorus losses (often in below-ground, leachate water).
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