From Plants: In North America you can get salt from the root of the hickory tree. You can boil the roots until it evaporates and salt crystals are left. Foods that naturally have salt: Carrots, beets, poultry and most animals.
Salt Licks: A salt lick is a salt deposit that animals lick to get there intake of salt. Follow game trails to see if there are any deposits near you. You can also find them in areas were cattle are often herded and graze.
Ranchers set them out for their animals. Just as a side note, even if you are near the sea, do not drink strait salt water as it can damage you kidneys and do other nasty things. Dilute it it with fresh water and then drink or evaporate the water until the salt is left then use that for seasoning food and the like.
It will all depend on where you are salt was as valuable as gold and used as money in some places and not all that long ago. Salt is still a major resource in many places today. If you are going to try to extract salt from plants or trees you need to know the complete chemical makeup remember that cyanide can also be extracted from plants, in fact it is a form of salt.
Areas that have natural salt deposits are great if you happen to be in one. As someone said follow the animals to see where they are getting salt. Clay is just wet dirt, silt, it won't necessarily have a high enough salt content to be of any use.
If the surrounding area doesn't have salt the clay won't either. If the clay does have salt, and you are sure it doesn't have things you don't want, you can consume it without chewing on it. Even if you are sure it has salt be careful.
Try a very small amount to see if you have any ill effects.
Here in the South, there are places called salt licks. These are natural deposits of salt that attracted animals that the Indians and longhunters hunted in the 18th century. Longhunters found that they could heat a cast iron pot and pore in the water from a salt lick.
The water would boil away, leaving the salt.
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