The fall fertilizer application window is wide open right now. Aside from visiting your Inputs Monitor Regional Pricing Index, consider the following...
While the air has cooled significantly in the last few days, it may take some time for the soil to follow. Bearing in mind that NH3 applications (click here for more) are best made when soil temps are 50 degrees F or below, visit ISU Extension's real-time soil temperature data map to pinpoint the best time to apply for maximum effect.
ISU's real-time soil temperature map can be found here -- extension.agron.iastate.edu/NPKnowledge/
Nitrogen carryover in a drought year can be tricky to figure, (click here for more) but ISU Extension experts have that covered as well with their corn nitrogen rate calculator. This handy tool is updated frequently and the accompanying data contains 214 trials for corn-on-soybeans and 111 trials for corn-on-corn.
ISU's Corn Nitrogen Rate Calculator can be found here -- extension.agron.iastate.edu/soilfertility/nRate.aspx?mode=advanced
Your Inputs Monitor always reccomends doing your homework and these are great tools to help you maximize your efficiency.