They are two different problems that usually go together. Salinity is an excess of soluble salts: the plant has water around it but cannot drink it, because the salt retains it. Sodicity is excess sodium in the exchange complex: the soil disperses, loses structure, becomes impermeable and becomes waterlogged.
The practical effect is the same: the list of what can be planted is narrowed. In mild degrees it is managed with tolerant species; in severe degrees the land is left for grazing, and in the extreme it is a salt flat without agricultural use.
In the depressed pampas, saline impact is one of the factors that weigh the most on the price per hectare, and it is not seen in a photo or walking through the property on a dry day.