Ammonium Nitrate...How nervous should I be?

Last year I made a hay bale garden and part of the process is to break down the interior of the bales by adding ammonium nitrate to them for several days and watering it in well. When I was a kid (in the 60s), I remember my father bringing home a 50 pound of ammonium nitrate home every spring to spread on our lawn. I had no problem buying 50 pounds at my local feed store but I didn't use even 1/4 of the bag. It sat all winter and this year it has more or less solidified into a big greasy lump. I salvaged enough of the top layer to do this year's bales but I still have a big lump of it left. My dad taught me as a kid that it was explosive but what I don't know is, is it safe for me to use a hammer to break it up and spread the remainder in what will eventually be a fall garden plot or is it too unstable for that? I'm afraid to do a google search on "exploding ammonium nitrate". I don't want to be dragged out of the house in my jammies by men in black.