Nitrogen

Nitrogen is recognized as very necessary to produce high yielding grain crops. Legumes such as soybeans and clover fix nitrogen from the atmosphere. Nitrogen is the most volatile and difficult to manage nutrient. It has all kinds of ways to get into the soil. Lightening fixes small amounts. Plant residues decaying to humus contain lots of nitrogen. Some is released each year. Manure is a wonderful source of nitrogen, but if you put on enough for the crop, you will tend to over apply phosphorous or potassium depending on the source. Nitrogen sources containing ammonia forms will acidify the soil. Nitrogen is needed for proteins to form in plants. IN order to minimize nitrogen needed in fertilizer, apply close to when the crop needs it. Try the NRate Calculator to determine economical rates.
http://extension.agron.iastate.edu/soilfertility/nrate.aspx