Nitrogen fertilizer is an effective and very widely used means of increasing crop yields in agriculture.  However, its widespread use, along with that of animal manure, has lead to greater contamination of surface and groundwater with nitrates.  Such contamination poses threats to both human and environmental health.

One of the main causes of the problem is overfertilization.  It is not always easy to determine what quantities of fertilizer will bring forth the maximum yield without excess, and it is by no means uncommon for farmers to overestimate this amount.  Fertilizer not consumed by crops can cause much contamination; it is easily dissolved and therefore easily spread by rain or irrigation water, thus broadening the scope of the damage.  Manure is also a strong nitrate source, which can do damage as well, as can nitrate from septic systems, cesspools, and nitrogen oxide particles from cars and other sources.

Nitrate contamination is a serious health risk. It is especially harmful to infants, in whom it can stop proper functioning of red blood cells, leading to insufficient oxygen and possible death.  In adults, nitrates may increase the risk of certain cancers.  Because of the heavy use of fertilizers (especially in Europe and the United States) nitrate contamination is found in many parts of the world.  It is one of the most common drinking water contaminants. 

Nitrates can also affect human health in less direct ways.  Nitrate runoff can pose problems that include the growth of toxic plants in bodies of water and resulting contamination of seafood supplies.  Fertilization runoff can degrade coastal waters, also causing a decline in seafood catch, an important source of food and income.

A WebQuest on fertilizer and agriculture