Personally I wouldn't toss it if there is anyway you can spread it out to dry - like on some plastic tarp in the sun. You can even spread it out over the garden and then mix it in after it dries some. It isn't going to hurt anything. Just don't dump it where it will wash into a pond or body of water.

It will lose much of the N but will still be somewhat beneficial. Otherwise you can dump it into a compost pile as a sort of kick-starter or even spread it out over a lawn area.

Dave

It's still fertilizer, though you may have lost some nitrogen. Depending on how wet it got, the chemistry may be largely unchanged. If you smell ammonia, though, the nitrogen is probably compromized. It is, however, probably badly clumped, and won't be spreadable with a drop spreader. But you could just dissolve it in water and apply it to plants that way. Be careful not to overdose! Or just toss a scoop on your compost pile every month or two, or spread it lightly near mature plants. If it's still wet, be careful. Use gloves when handling. It's pretty corrosive.

Thanks! I'm glad I can use it for something.

I should add, on further review, that ammonium nitrate, when combined with water, generally doesn't release nitrogen. It takes heat to make it do that, and then what you get is nitrous oxide (get a laugh out of that!) This is, of course, why ammonium nitrate works fine as a fertilizer, when dumped on moist soil. That is, your fertilizer gets plenty wet when you put it on the ground, and it still works. So to the extent that your fertilizer is ammonium nitrate (urea is very different) the chemistry is pretty much water-proof.