cultivating it in is a whole different ball game than spreading it over the top and letting it lie there. Basically if you get a rain right after spreading top dressing can work ok. If you don't get a rain right after spreading you start losing it to volatilization. Agrotain may help a bit but a matter of degree.
Why not side dress it below the surface and covered? Or use 28% liquid side dressed?
Jim at Dawn | 
Top Dress Corn with Urea
