Lawn Watering - Using Soft Water for Irrigation

What do I need to add to my lawn when watering with soft water and how often?


We’ll start by answering a couple of commonly asked question: What is soft water and hard water? Is it OK to water plants with softened water?

Hard water is water that is high in dissolved minerals — specifically calcium (Ca2+) and magnesium (Mg2+) and sometimes iron (Fe) and manganese (Mn). Hard water reduces the effectiveness of soaps and detergents. It also leaves scale-like deposits on plumbing fixtures, stains and rings in tubs, sinks and toilets. The mineral buildup reduces the life of water heaters and home appliances.

Soft water vs. hard water for irrigation
Hard water leaves scale deposits on plumbing fixtures.

One solution when dealing with the problems caused by hard water is to soften it. Water softeners use salt (sodium chloride or potassium chloride) to replace the scale-building minerals with sodium or potassium ions. Note: Rainwater is also considered naturally “soft” because it does not contain dissolved minerals.

Softened water is therefore high in sodium and is not the best water to use for irrigating lawns and gardens because it can cause salt buildup on the soil surface.

Watering with soft water is risky in areas that get little rainfall. In arid climates, irrigation water will evaporate quickly leaving


salt deposits. Natural rain tends to wash salts from the soil so using soft water is less of an issue in areas that get a lot of rainfall.

Avoid using soft water for irrigation on sites that have sodic (“salty” clay) soils. Sodic soils are common in the Southwest.

To answer your question: The University of Arizona recommends applying gypsum at a rate of 25 lbs per 1000 ft2 twice a year (Turf Tips, February 2001) to lawns that are irrigated with soft water.

Soft water vs. hard water for irrigation
Watering with soft water can cause sodium buildup.

The gypsum will encourage sodium deposits to wash (or leach) out of the soil. I recommend core aerating along with the gypsum application to encourage water infiltration into the soil.

Another way to avoid salt buildup is to water heavily. Heavy watering will leach any deposited salts out of the soil while light watering will leave salt deposits.

Potassium chloride can be used in your water softener instead of sodium chloride. Potassium is a plant nutrient and it will not harm plants or soils. The downside to using potassium chloride is that it’s a lot more expensive.

Have you considered bypassing the water softener for irrigation? If this is a possible option, it is my opinion that it is best to avoid watering lawns and landscape plants with soft water.

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