Anhydrous ammonia is a gas with no (zero) charge. It is a very strong base and as such, wants to pick up a hydrogen ion. When it does, it becomes an ammonium ion and has a plus one (+1) charge. Ions cannot exist (under normal conditions) without a counter-ion. Ionic chemical substances must have the same number of positive and negative ions. This ammonium ion will find a negatively charged ion to give a net neutral charge. In your example, this ion is a nitrate ion which has a negative one (-1) charge. One ammonium ion (+1) and one nitrate ion (-1) provides a neutral molecule; ammonium nitrate.
If you use anhydrous ammonia (neutral; zero charge) instead of ammonium nitrate, the ammonia gas will react with water to form ammonium hydroxide. This molecule has an ammonium ion (+1) and a hydroxide ion (-1). Ammonia is basic enough to ionize water. This is how ammonium hydroxide (like you can buy at the grocery or hardware stores) is made. Since ammonium hydroxide is a strong base, it is corrosive to your skin, eyes, lungs, and even some metals (like aluminum).
Ammonium nitrate is neutral and not a strong base and is less corrosive. If you want to fertilize your land, ammonia gas is cheap but hazardous and corrosive. Ammonium nitrate is a solid and much easier to handle.
Check the Safety Data Sheets for ammonium nitrate for the correct safe handling procedures. These are available from your supplier or on the internet.
