Remove salt from dirt
Assuming you mean NaCl - the common "salt" (chemists call lots of things salt!).

If you extract the salt by physical means, it's a physics question. 

Assuming that the "dirt" is not (or poorly) soluble in water, I would simply dissolve the salt in water, filter the liquid, then recrystallize (by evaporation of the liquid) and weigh the resulting crystals. Not sure if you call that chemistry or physics... it's physical chemistry. It is well described in text books and online - see for example this Scientific American page

Depending on how accurate you want to get, there are a slew of precautions to take. That would be the realm of the chemistry.stackexchange sister site.