Ever seen those circular rings of stunted growth around trees (just under the drip line)?
from :"Natural History: The work of nature - ecological systems":
"Hardwoods require more nitrogen for growth than do conifers"
Hardwoods (oaks, hickories and most of the deciduous trees which grow in the midwest) suck the extra nitrogen the farmer puts on his crops and compete with the crops for their value. Explains why grasses do so well under trees, since they require little N to grow, mostly P and K - while corn and soybeans get real paltry results.