From Corn and Soybean Digest:

Even though planting season has not yet started in full force, many fields across Iowa are already turning green. These fields were planted to cover crops last fall with the goal of reducing soil erosion and taking up excess nitrogen.

Reducing erosion is an important step in improving Iowa’s water quality. Planting cover crops is a proven and practical way to help reduce the loss of nitrogen and phosphorus into the state’s waterways.