Calcium and magnesium dissolved in water cause “hard” water. Hard water leaves mineral deposits in pipes and bathtubs, and keeps soap from cleaning effectively. It does no harm to plants, though, and can be used to water them without causing any problems.
Water softeners, which are used to soften hard water, replace the calcium and magnesium with sodium. Water with sodium in it is “soft.” It has none of the negative attributes of hard water, but it is bad for plants. Some plants, such as citrus, avocado, and orchids, are sensitive to it in small amounts. Their leaves burn at the edges if they take up too much sodium.
Sodium also destroys the structure of soil if allowed to accumulate. Regular use of softened water on houseplants — especially without sufficient leaching — can cause the soil to become impermeable to water. The sodium also makes the soil very alkaline. If it becomes alkaline enough, it begins to dissolve some of the organic matter in the soil, and the drain water becomes dark brown.
There are three solutions to softened water. The simplest is to draw water for houseplants from a faucet that does not have softened water. Outside hose bibbs are usually left off the water softener line; draw houseplant water from them or from a neighbor with unsoftened water.
Another solution for softened water is to use potassium chloride to recharge the water softener. Although it’s more expensive than sodium chloride, it replaces the calcium and magnesium with potassium instead of sodium. Potassium is less damaging than sodium to plants, and won’t hurt them if the soil is kept leached.
A third solution is to add calcium to replace the sodium in the softened water. Dissolve 1/2 teaspoon of gypsum (calcium sulfate) in a gallon of softened water. Gypsum dissolves slowly, so give it some time. Horticultural gypsum is available from nurseries. The calcium in the water will keep the sodium from accumulating in the soil and harming the plants.