Because they are harvesting their entire corn product for bio fuel they clean up all the remains of the corn that is typically left and will saturate back into the soil. When they aren't harvesting their corn for bio fuels what they harvest the remains are left on the ground to reabsorb into the soil which helps to provide the soil with nutrients and keep it a renewable source along with other means of conservation.

[Linked Image from w3.the-kgb.com]