Do you consider variable rate, field potential, yield target approach? If so, GeoPard could be a good starting point:
Field potential maps -> soil points (grid based could be very misleading) -> yield target / Equation based calculation of fert needs. 

For John Deere users there is a free trial, 250 acres, register with the JD account geopard.tech/johndeere

Let me know if it could be helpful.