By Dave Franzen, NDSU Extension Soil Specialist
My colleagues in the soil fertility world have often assumed that shallow (inch or so) incorporation of urea was better than no incorporation at all.
A recent publication out of Quebec destroyed this assumption and found that even in a slightly acid soil, ammonia volatilization from urea was about 4 times more during the first 10 days after application when incorporated an inch deep, compared to not incorporating the urea at all (Rochette et al., Journal Environmental Quality, 2014).
This means that both surface and lightly tilled urea down to a depth of 2 inches should have a urease inhibitor (Agrotain and its NBPT containing brothers and sisters/Limus-NBPT+NPPT) to prevent ammonia volatility for about 10 days.
If urea is applied to the surface, it is better to leave it alone rather than incorporate, it in a no-till/strip-till system. Conventional till systems should set the cultivator 3-4 inches deep.