how to ruin a bucket of salt

[ScubaSteve]

Rock solid, yes. I've had that happen a few times. Phosphates, no. Phosphates don't travel in humidity, so the moisture that made your salt hard didn't bring phosphates in. The phosphates would have had to have been there in the first place. The only other possibility is the moisture allowed bacteria to grow, which then died and broke down to produce Phosphates. But this is unlikely as the bacteria likely wouldn't grow in those conditions and they'd need a phosphate source to grow and survive anyways... So again the phosphate would have to be there in the first place.

I'm gonna say bad batch.