The skunks are digging in the soil looking for grubs and other insects that they eat. It may be that what you put down smells somewhat like those grubs and that may be why they are digging there. Not being a skunk, or any other anumal that eats grubs, and not finding out what the grubs smell like being a high priority item, I can't really say.

The main ingredient of Pro-Grow is urea -- it may be attractive to them, or smell like the insects they eat.

The National Gardening Assoc. says that they also like bone meal, fish meal, and peanut meal.

If the skunks aren't digging in untreated areas, you might change your fertilizer to a non-urea one and see if anything changes. If using another fertilizer solves the problem, you could spread the Pro-Gro on an area that you want rototilled (or skunk-tilled)!

Two tips that I've run across regarding skunks:

They don't like dogs and avoid them if possible. Perhaps you could invite a friend with a male, pee-happy dog to inoculate the shrubbery around your perimeter. OTOH, maybe human urine would do the same thing. Buy some beer and invite your male friends over, giving specific instructions on where to pee.

Also, skunks can be useful for their digging. They love ground-nesting yellowjackets. If you have some nests you want to eradicate, get some cheap syrup (dollar-store quality is fine) and pour some over the nest at dusk. The skunks should be attracted to the syrup and discover the nest, which they usually dig up and devour the occupants.

Sue