Salt, while not technically a spice, serves the same purpose as a spice: flavoring and preserving food. So I'd say salt is covered in the game.
Salt was used way before plantations...
The thing about salt though, is that it had way more importance than spices and was even used as currency. It should be in the game, along with opium, tea, coffee, and tobacco.
First of all, I will use the same argument that defeated saltpeter: salt is very common and evenly distributed around the world relative to other resources, so it would defeat the whole point of having resources, which is to replicate how certain civilizations benefited from having an abundance (or monopoly) of those resources.
Also, we have plenty of mineral resources, so we would have to take one out, which gives us an issue: Iron, Copper, Aluminum, and Uranium can't really be taken out until Civ V (still unlikely even then) due to game mechanics, so you would have to a) make salt a luxury resource (even though it fits much better as a general or food resource) and b) get rid of gold, gems, or silver, which had more importance historically and better reflect how certain regions got lucky with resources (Malinese gold, S. African gems, Spanish silver), which is the whole (aforementioned) point of having resources in the first place.
i agree with all of you that salt is very important in the real world but like i posted before, it would mess up the game
there should be no more resources added. if you add salt, cotten,chickens etc it would mess up the game. since there are so many resources it would be rare to get the important ones (horses, iron,copper,oil,uranium) the chances of getting those resources would be so small if you add all of these new resouces. (its already hard enough to get aluminum and uranium)
Also, we have plenty of mineral resources, so we would have to take one out, which gives us an issue: Iron, Copper, Aluminum, and Uranium.