Soft water is not ideal for watering plants. Not only does it lack the nutritious minerals of harder waters, but it actually leaches nutrients out of the soil. However, if soft water is your only option, it is possible to safely water your plants with it. By paying careful attention to the soil's nutrient content, you can avoid many of the common problems that arise in plants watered with soft water.
Know which plants to water with soft water. Soft water is slightly acidic, and should only be used to water plants that like acidic soil like blueberries, azaleas, rhododendrons, camellias, tomatoes, roses and bearded irises.
Keep the soil nourished by testing its mineral content with a TDS meter at least once a month. Mix a sample of your soil with water (the amount will be dictated by the manufacturer's instructions) and insert the TDS meter which will give you a reading which will reveal the mineral content of your soil. Soft water can leach important minerals like calcium and magnesium carbonates out of the soil. Replace these or any other missing nutrients with a foliar spray or water-soluble fertilizer.
Dilute soft water with an equal amount of rainwater or sparkling mineral water if your plants' soil is continually nutrient deficient.
Do not use softened water that has been chemically softened by a water softener to water your plants. Water softeners work by adding sodium and potassium chloride to the water which can slow your plants' growth and kill your lawn.