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Larry Sagers Horticultural Specialist Utah State University Extension Service Thanksgiving Point Office © All Rights Reserved
Listeners question. Will it hurt to water my plants with softened water?
The answer to this question is yes with a slight qualifier. There is a big difference between naturally soft water and softened water from a water softener.
Water softeners are specific ion exchangers that remove positively charged ions. They remove primarily calcium (Ca2+) and magnesium (Mg2+) ions. Calcium and magnesium are the ions that cause hard water.
Ion exchange water softeners replace calcium and magnesium ions in the water with sodium or potassium ions. The exchanger ions are added to the ion exchanger reservoir as sodium and potassium salts (NaCl and KCl).
In this process, water passes through a media bed of sulfonated polystyrene beads. The beads are supersaturated with sodium. The ion exchange process takes place as hard water passes through the softening material. The hardness minerals attach themselves to the resin beads while sodium on the resin beads is released into the water.
When the resin becomes saturated with calcium and magnesium, it must be recharged. The recharging is done by passing a salt (brine) solution through the resin. The sodium then replaces the calcium and magnesium ions which then drain into the sewer or septic tank.
Hard water treated with an ion exchange water softener using sodium chloride salt always has sodium added. According to the Water Quality Association (WQA), the ion exchange softening process adds sodium at the rate of about 8 mg/liter for each grain of hardness removed per gallon of water.
For example, if the water has a hardness of 10 grains per gallon, it will contain about 80 mg/liter of sodium after being softened in an ion exchange water softener if all hardness minerals are removed.
Water-quality standards approved by the U.S. Environmental Protection Agency (USEPA) for sodium concentrations for drinking water are 20 mg/L and water starts to taste salty at levels that are slightly higher than that.
Plants vary in their tolerance of sodium salts but greenhouse irrigation water is considered unsuitable if it has a sodium content of more than 30mg/L and the recommendation is that it should be much lower than that if possible.
That being the case, it is evident that most water softeners are putting out water that is likely much higher in sodium than any recommended levels and so soft water should not be used to water plants either inside or outside the home.
Most fruit trees, many flowers and vegetables and interior plants are sensitive to even small amounts of sodium. If they take up too much sodium, their leaves burn on the edges or tips.
Sodium also destroys the soil structure if allowed to accumulate. Regular use of softened water on houseplants without sufficient leaching causes the soil to become impermeable to water.
There are simple solutions to softened water. The simplest is to draw water for houseplants from a faucet that does not have softened water. Outside faucets should not be connected to the water softener.
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.
