some of the shallow caves around here have large quantities of very old crap so old it's turn to powder clumped up melted together to make one dirt clod sometimes several feet thick. I don't know if it's bat or rat/mouse crap. I'm gonna collect some and try it on a plant. I think the crap at the bottom is hundereds of years old. it's very arid here so stuff doesn't rot as much as it just drys up.