This corn field is showing nitrogen deficiency symptoms. The bottom leaves are yellow, a sign of the lack of N. Notice that N deficiency yellowing occurs along the midrib of the corn leaf. For almost all plants, the yellowing of the plant will take place when N is deficient. Maple leaves will show a general yellowing of the entire leaf. The main goal in a Nitrogen fertilization program is to keep the nitrogen available for plant use, while at the same time keeping it from becoming a pollutant or being lost to the atmosphere via denitrification.
Nitrates can pollute groundwater and create health hazards for humans and animals.
See Nitrates for more information about nitrates and human health or Nitrogen for more information on Nitrogen Contamination of Ground Water. The University of Minnesota is an equal opportunity educator and employer.