The National Soil Erosion Research Laboratory
Crop residues are the foliage, stubble and straw left on the soil by crops both before and after harvest. These residues absorb the destructive energy of raindrops and wind and decrease the amount of surface runoff, thus reducing erosion. While residues are most effective when left on the surface, they can also be worked into the soil. Residue cover as low as 30-40% can still significantly reduce soil erosion.
Corn Residue Cover
Examples of residue cover levels for corn and soybeans can be seen above and below this paragraph respectively. The photographs of corn and soybean residue levels here are taken from the Purdue University Agronomy Guide AY-269, "Estimating Corn and Soybean Residue Cover", by P.R. Hill, J.V. Mannering, and J.R. Wilcox, May, 1989.
Soybean Residue Cover