If you get large increases in corn yields from improving your soil fertility, then probably your corn grain from the unfertilized land is deficient in phosphorous. It may contain less phosphorus than your livestock and poultry need for adequate nutrition.
Pierre, W. H. and Englehorn, A. J.
"Rich Soils- Rich Corn,"
Farm Science Reporter: Vol. 1
, Article 4.
Available at: https://lib.dr.iastate.edu/farmsciencereporter/vol1/iss4/4