Plants need sunlight, water and a few minerals in order to thrive. One mineral they don't need is sodium. Sodium chloride (NaCl) is also known as table salt--an essential (and essentially overused) nutrient for humans. Plants, however, are adversely affected by the presence of salt. The degree to which they suffer from salt poisoning depends on the concentration to which they are exposed.
How Salt Enters Soil
In nature, salts are formed in soils as a result of acid and base interactions. If the soil in the pots of houseplants came from the backyard, some salt will likely be present, but seldom enough to be a problem. Houseplants more commonly come into contact with harmful concentrations of salt through softened water. In water softeners, salt traps magnesium and calcium in "hard" water, replacing it with sodium. Plants actually use both magnesium and calcium for cell functions and stem structure, so the hard water would be better for them.
Sodium and Plants
Salt dissolves very easily in water. The positively charged sodium atoms separate from the rest of the salt molecule, and are chemically close to the positively charged potassium atoms that plants use in several critical cell functions. As a result, plant roots readily absorb the sodium, crowding out potassium and causing the plant to lose nutrients.
How Plants React to Salt
Salt also has a desiccating effect. By osmosis, water tends to be drawn toward salt, and therefore away from where it was. Thus, plant roots lose water, even though there may be plenty of moisture in the pot. Houseplants react to these "drought conditions" by closing the stomata in their leaves, so less water is lost through transpiration. Unfortunately, it also means that the leaves cannot take in the carbon dioxide they need in order to make food for the plant. Plants also produce proline, which acts to stabilize root membranes against water loss through osmosis.
Salt Poisoning and Plant Appearance
Houseplants mildly poisoned by salt exposure take on a blue-green hue as the sodium replaces potassium in their leaves. Root and overall plant growth will become slow and stunted. The plants will wilt from water loss. Over prolonged exposure, the leaves will yellow as the green chlorophyll in them is destroyed to use the energy elsewhere in the plant. Eventually, the plant will die.
How to Mitigate Salt Poisoning in Houseplants
Removing salt from soil is very difficult. It would be easier to replace the soil in the pot of the houseplant. Otherwise, flush the soil with a large amount of clean, salt-free water. Water houseplants with hard water, not softened water.