High levels of Sodium (Na) is not good for plants. That's why adding table salt (NaCl) to freshwater tends to cause poor health in plants. That's one of the reasons why few plants located near marine littoral zones do well. The Mangrove and Coconut trees are outstanding exceptions. Excessive accumulation of Na+ interferes with uptake of Potassium (K+), Calcium (Ca++) and Manganese (Mn++). This endangers plant metabolism and affects growth due to ionic imbalances within the cell. It's effect on the macronutrient K+ is particularly sinister because K+ is vital for osmoregulation and protein synthesis, for maintaining cell turgor (the pressure of the cell's contents against the cell walls) and for stimulating photosynthesis.