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Water Quality Answers
Should softened water be used for watering house plants or for sprinkling the garden or lawn?
Where the amount of hardness minerals in the water is only moderate (less than 200 milligrams per liter), it is doubtful whether the sodium concentration would be sufficient to be a serious hazard to plants. Most house plants require specific soil conditions for healthy growth. Many thrive best in slightly acid soils. If there is a high hardness concentration in the water being softened, the chances are good that necessarily high sodium concentration of the soft water would be harmful to plants.
Distilled, reverse osmosis, or deionized water or rain water are better choices for watering plants to prevent mineral accumulation in the soil. Or, if using softened water, water heavily to wash previously deposited minerals through and beyond the plants' root zones. Heavy sodium or potassium salt concentrations in the absence of calcium or magnesium may affect swelling of soils and retard the growth of plants.
For outside sprinkling purposes the use of softened water is, first and foremost, wasteful. Again, where the concentration of hardness minerals is heavy, the sodium salts replacing them might retard growth and might be sufficient to kill the grass.
Note: Where the rainfall is rare, sodium accumulation is apt to be greatest. Heavy rain "rinses" the earth.