Some farms use anhydrous ammonia. It's the cheapest form, but it's also a gas, so it makes it kind of dangerous to handle. (This is why meth labs are always exploding, right? I sure don't know.)
When the corn is about a foot high, we drive the tractor between the rows and knife in the nitrogen with a rented pull-behind applicator. We put on 28% liquefied nitrogen. If the corn is too short, the nitrogen might run out before the corn plant needs it, but you can't wait until it's too tall, because you wouldn't be able to drive through it.
Corn needs fertilizer to reach its full potential. Many farms put it on when they plant the corn. Since we spread manure on our fields during the winter, we don't. Because ... there's nitrogen in manure. Let's celebrate the wonder that is cow manure! Easily the cheapest fertilizer, and not at all explosive!