There's a good way to find out.... 

Go out and pull a series of soil samples.  I would probably go 24" deep if you can.  Use a series of buckets (or containers).  Label them so you know which one is which. I would break up each core - say one core section 0-4" deep - goes in the first bucket; next core 4-8" deep - goes in the next bucket; next corn 8-12" deep - goes in third bucket; and so on.   Pull multiple cores like this, and split each one accordingly.   Then put each subsample in a separate soil test bag, and send them off.  Be sure to analyze for nitrate nitrogen.   When you get results back, you should get a picture of where you nitrogen is - at least your nitrate- nitrogen.