White residue in urine

I was playing with Mr. B today and he pee'd on one of his cardboard play boxes. After the urine dried up I noticed there was a white residue leftover.



After doing a bit of research, there was a past thread where a member said it's calcium being excreted, which is normalish but sometimes caused by a high protein diet. I kind of wonder if some of the...

White residue in urine