What No One Tells You About Softened Water

Gardeners are often told that rainwater is the best water for plants, and that soft water is better than hard. Both statements are true for rain. Neither is true for water from a household softener, which is soft for a different reason and brings something rain never does.

What makes water hard

Rain falls with very little dissolved in it. As it soaks through soil and rock it picks up minerals, mainly calcium and magnesium, and by the time it reaches a well or a reservoir it may be hard. The U.S. Geological Survey classes water with up to 60 milligrams per liter of hardness, measured as calcium carbonate, as soft, 61 to 120 as moderately hard, 121 to 180 as hard, and more than 180 as very hard. Hard water leaves scale in kettles and spots on glass, but for most garden plants it is harmless. Calcium and magnesium are plant nutrients, and calcium in particular helps hold clay soil in a crumbly, open structure.

What a softener does

Most home softeners work by ion exchange. Water passes through a bed of resin beads loaded with sodium, and the beads swap that sodium for the calcium and magnesium in the water. When the resin is spent, the unit flushes it with brine made from salt to reload it, and sends the flushed hardness and excess salt down the drain.

The result is water with almost no calcium or magnesium and extra sodium in their place. Each grain per gallon of hardness removed adds roughly 8 milligrams of sodium per liter, so a home with very hard water at 20 grains per gallon ends up with about 160 milligrams of sodium per liter in every tap the softener serves.

Why plants and soil object

Sodium harms plants in two ways. Sensitive plants take it up and show scorched leaf edges and stunted growth, and in pots the salts collect as a white crust on the surface and the rim. The slower damage happens in the soil. Soil scientists judge irrigation water by its sodium hazard, the proportion of sodium to calcium and magnesium, expressed as the sodium adsorption ratio. Colorado State University Extension explains that soils with a high clay content show reduced infiltration and crusting when watered with high-sodium water, and that the effect is stronger when the water is otherwise low in salts. Softened water is an extreme case, with plenty of sodium and almost no calcium or magnesium to balance it.

Sodium pushes clay particles apart. The soil loses its crumb structure, seals at the surface, and drains slowly, so water runs off or puddles, roots get less air, and more water is needed to wet the same depth. Rain helps leach sodium down in wet climates, but in dry regions it accumulates year after year.

The problem downstream

Salt sent down the drain does not disappear. Treated wastewater is discharged to rivers or reused for irrigation, and conventional treatment does not remove dissolved salt. In the Santa Clarita Valley of California, chloride from softener brine raised salt levels in the Santa Clara River, which supplies farms downstream. Installation of new automatic softeners was prohibited there in 2003, and in 2008 voters approved a measure requiring existing salt-discharging softeners to be removed, an approach judged far cheaper than adding salt removal at the treatment plants.

Keeping softened water off the garden

Softened water feels pleasant in the shower, uses less soap and protects water heaters from scale, so many households will keep their softener. A few steps protect the garden.

Bypass the outdoor taps

Plumb the softener so that outdoor faucets and any irrigation line draw from the supply before it. Many installers do this as standard, but older homes sometimes have every tap softened. Check by finding where the softener sits on the main line.

Soften only what needs it

Some households soften only the hot water, which carries most of the scale problem, and leave the cold supply hard.

Consider potassium

Softeners can be regenerated with potassium chloride instead of sodium chloride. The water then carries potassium, a plant nutrient, instead of sodium. It still sends chloride to the drain.

Water houseplants from another source

Use rainwater, or water drawn from a tap ahead of the softener. If softened water is the only option, water until it runs freely from the bottom of the pot so that salts are flushed out rather than left to build.

If the soil already shows white crusting, slow drainage and scorched leaves, a soil test that includes sodium will show how far it has gone, and it is the starting point for any correction.