The Encirca website talks about Nitrogen Management.

With it and other websites you can predict the amount of Nitrogen loss.

Encirca says most of the nitrogen loss occurs 30-60 days after planting.

Before planting the soil is usually too cold for large amounts of N loss.

60 days after planting the corn plant has taken up large amounts of the N.

Wet and warm soil from the middle of May till the middle of June is the biggest problem.

If you have tile it can leach through the tile.

If you do not have tile it is subject to denitrification and evaporation.