We traditionally use ammonium nitrate with an analysis of 33.5-0-0 or 34-0-0. In special situations, like corn crops, we occasion to use ammonium sulfate with an analysis of 21-0-0-24, where 24 represents the percentage sulfur. Ammonium nitrate is a slow release nitrogen that does not volatize like urea, a 46-0-0 product. Urea is estimated to lose about 20% of it nitrogen content if not incorporated into the soil or quickly rained in. All forms of nitrogen will leach with water deeper in the soil and with enough water below the root zone before the plant can use it.
Nitrogen is very water soluble and is not detected in soil tests. Small amounts of organic nitrogen are made available as plant tissues decay. Clovers and other legumes will release significant amounts of nitrogen as the plant is cut and the roots die. Nitrogen is responsible for green up and most of the above ground plant growth and is know as the farmer's favorite fertilizer. Of the three major nutrients, nitrogen is often the most limiting because it does not remain in the soil except in organic forms.
If you are interested in a more indepth explaination, you may like this link: