I added too much salt

You can--it is correct English--but it sounds formal, academic, and "literary." The normal way to say it is "I put too much salt in the soup" or "I over-salted the soup." If you really want to say it that way, I think a better phrasing would be "I have added a disproportionate amount of salt to the soup." (By the way, when that happens, you can fix it by adding potatoes).

I have added salt in proportion to the amount of soup.

You could just say, "I added (way) too much salt to the soup". If you want to be more specific, you could say "I added too much salt for the amount of soup there is/I added too much salt for how much soup there is"

I've added the salt to the soup.