Earth of every kind - savva-matti, the verse says, all earth - is refused because it is earth-bodied beings, because it breeds frogs and the like, and because it makes the eater ill. Salt is excepted.
Three reasons in the commentary, in that order: pṛthvīkāya, the one-sensed beings that earth is; the five-sensed beings such as frogs that damp earth generates; and the intestinal maladies of eating it. Chalk is named as covered. Salt is named as not, being necessary to life, which is the same exception that lets water through and stops snow.
The twenty-two, laid by their reasons - Earth highlighted