Nitrogen is the most commonly used mineral nutrient. It is important for protein production. It plays a pivotal role in many critical functions (such as photosynthesis) in the plant and is a major component of amino acids, the critical element constituent component of proteins. These amino acids are then used in forming protoplasm, the site of cell division and plant growth. Nitrogen is necessary for enzymatic reactions in plants since all plant enzymes are proteins. It is a necessary component of several vitamins, e.g., biotin, thiamine, niacin and riboflavin. N is part of the nucleic acids (DNA and RNA).
Nitrogen is taken up by plant roots as either nitrate or ammonium. Some crops have a definite preference for nitrate.
Nitrogen deficiency causes chlorosis (yellowing) of the leaves due to decreased levels of chlorophyll. Since nitrogen is highly mobile within the plant yellowing shows first on older leaves, with progressively younger leaves yellowing as the deficiency becomes more severe. Nitrogen deficient plants tend to be stunted, grow slowly, and produce fewer tillers than normal. Certain N-deficient crops may reach maturity earlier than plants with adequate N.
Stunted chlorotic plants with reduced yield.
Plants will have dark green overly vigorous foliage which may have increased susceptibility to disease and insect attacks. There may be increased sensitivity to low temperature. Extremely high levels of nitrogen may actually result in decreased yield. Excess nitrogen may negatively affect fruit/seed quality and may delay fruit maturity.
LINKS
http://www.uwex1.edu/ces/pubs/pdf/A2519.PDF
http://www.back-to-basics.net/efu/pdfs/nitrogen.pdf
http://www.gov.on.ca/OMAFRA/english/crops/facts/90-201.htm#c