The reason plants die when grown in salt water is because of the change of osmotic potential of the soil. Typically, there are more solvents in the water inside the plant (higher concentration) than the solvents outside the plant. Therefore, plant roots pull in water through osmosis from low concentration to high concentration. When the water is salty, it makes the water outside have higher concentration that the water inside the plant cell sap. This makes the water and nutrients to move reversely.