How long does it stay in the sun? Sunlight can turn the constituents to nitrates (e.g., potassium nitrate) and other chemicals while drying the stuff into powder. Bones and raw food may have some further effect as well. 'Tain't just one thing, it's complex.

As for the nitrates:

Saltpeter (potassium nitrate) was for many, many years extracted from, well, from shit. Cow shit, bat shit, dog shit, people shit, it didn't matter to the chemistry. Much guano was imported from Chile for fertilizer, but also to be made into saltpeter and then eventually into gunpowder. In fact, old urine ("chamber lye") was often added to boost the nitrate content. For a complete rundown on how to make your own saltpeter and gunpowder, see "Foxfire 5."

Actually, the older the shit the better.

So, when you see someone shooting a flintlock rifle or you read about the wars or hunters or fireworks or quarrying or whatever before, oh, say 1870, you can say with complete assurance, "Boy! ain't that some shit!"