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Water that is very high in salts such as sodium, c alcium and magnesium can injure houseplants if used over a prolonged period. Water high in salts can cause leaf necrosis (speckling of the leaf), leaf burn (drying of leaf tips and margins), and in severe cases, leaf drop.
If water high in salts is used, in time the salts will accumulate and show up as a white or tan crust at the edges of plant pots. The same crust may form on the soil surface and around the plant stems. Misting houseplants with water high in salts will cause a white residue to form on the leaves, which can lead to permanent leaf spotting.
Solving Salt Problems
One approach to these problems is to reduce the salt buildup in various ways. Begin by removing the salt crust from the soil and pot. Next, flush the pot with rain water or melted snow, which are very low in all salts. Some of the salts in the soil will dissolve in the water and drain away. Flushing with tap water will leach some salts away, but because the tap  water itself will be high in salts, it is not as effective as rain water or melted snow. Watering houseplants with a strong tea solution will help to offset the damaging effects of high levels of salts in the soil.
A more direct approach is to reduce the salt content of the water before using it. Some homeowners boil water to remove the salts. This procedure does remove some salts, which build up as a scale inside the kettle, but many salts remain in the water. There are a number of different systems for salt-removal available to the homeowner. Before I discuss them, here are a few more facts about water and salts.
Hard and Saline Water
Water with a high concentration of calcium and magnesium is referred to as "hard" water. How high is high? Hard water has greater than 150 parts per million (ppm) of calcium and magnesium. Saline water has a high concentration of many different salts, including calcium and magnesium. The salinity of water is measured by its electrical conductivity; the dissolved salts increase the conductivity of the water, so the higher the salinity, the higher the conductivity.
Soaps used for laundry and washing are not very effective in hard and saline water, and for this reason many households with high-salt water supplies "soften" their water before using it.
Salt-Reducing Systems
The ion exchange or salt water softener - This is the system most commonly used by homeowners to soften water for laundry and washing. But it may not be suitable if you want water that is safe for your houseplants. Somewhat paradoxically, this system uses common salt (sodium chloride) to soften the water. Small beads of resin capture the calcium and magnesium salts in the water; as part of this process, sodium is released into the water. The amount of sodium released is equal to the amount of calcium and magnesium in the untreated water. If the water is very hard and contains high amounts of calcium and magnesium, the treated water will contain an equally high amount of sodium. Though the treated water will be suitable for laundry and washing, it will be most unsuitable for plants because sodium in high concentrations is even more harmful to plants than calcium and magnesium!
Reverse Osmosis System - This system is commonly used to produce purified water for drinking. The treated water is very low in salts and safe for houseplants. In contrast to the ion exchange system, the reverse osmosis system has a relatively small capacity and is not practical for treating large volumes of water. It uses a "semi-permeable membrane" that allows water to flow through and captures most salts and minerals. The membrane must be cleaned and eventually replaced.
Deionizing and distilling systems are also very effective in removing salts from water. Like the reverse osmosis s ystem, these systems have a small capacity. The costs of purchasing and operating them are quite high, though the treated water is very pure and safe for houseplants. Deionized water can be purchased at some stores.
Urban systems - Some urban centres choose to soften their water supply. Some cities add calcium oxide to the water. When the water is aerated, calcium carbonate precipitates out and is removed.
Many urban centres also add chlorine and fluoride to water supplies for health reasons. Some plants may be damaged by fluoride and chlorine, even at the very low levels present in our water. Susceptible plants include Hawaiian ti, spider plant, wandering jew, prayer plant, some palms and calathea. Let the water from the tap sit for a day at room temperature; this will allow the chlorine and fluoride to evaporate.
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