Hold on everyone. Don't get the two things confused.
!. Fall application of NH3 is practiced to spread out the workload for the next season. It is applied to the ground that will be growing corn the following year. It is applied after the soil temperature is below 50 degrees F. This is so that the microbes in the soil will not convert the NH3 to a form that can be leached from the soil. Also in MOST cases the cost of the NH3 is less in the fall and availability of the product and applicators are not as critical as they are in the spring when a lot is put on. And as NH3 is a liquid that converts to gas at air temperature it is injected into the soil to contain it. As a gas it is attracted to moisture therefore it attached to soil moisture and stays where it is applied. Then as the ground warms in the spring it is converted to a form that the corn plant will use.
2. Surface applied N whether liquid or urea are applied during the growing season. In corn fields if applied they need to be incorporated via mechanical or moisture or they will volatilize into the air and not be of use to the growing plants. The same is true in pastures and since mechanical incorporation is not practical in pastures, moisture is used. Unless the temperatures approach 85-90 degrees the the volitalizeing is slow so rain and dew will attach the N to the soil. Therefore most people will apply the surface N as early in the year as the soil begins to warm with the idea that they will hopefully get spring rains to incorporate the N for plant use later in the season.
I know this is not a complete scenario of N application but maybe it will get everyone to thinking and asking more questions. JLP