Because of the high sodium content and lack of naturally occurring minerals in softened water, your plants may begin to suffer from prolonged exposure. Thankfully, there are some things you can do to minimize the negative effects of soft water and help your plants get the nutrients they need.
Check the post-softening composition of your water. As a general rule, if the soft water you are using is having a notably adverse effect on your plants, then it probably isn't too good for you either. Run the softened water through a water test kit to make sure all of the elements fall within the safe and acceptable levels. Water test kits are available from any home and garden store, and in many areas, your local water department will have them available for free.
Don't use water softeners for your outside faucets. There should be nothing you use your outside faucet for that requires softening , and generally adding the outside faucet just means that you burn through those expensive softening chemicals that much faster. When you go to water your plants, simply head outside with your watering can and fill it from the outside faucet. This will require a bit more lugging, but it assures that your plants are getting what they need.
Fertilize regularly with a nitrogen-rich fertilizer. Because the softened water uses salts to remove a lot of the natural minerals, your plants may require extra fertilizer in order to thrive. Look for a nitrogen-rich fertilizer at your local home and garden store and plan to fertilize about a third as often as is recommended by the instructions.
Elevate your plants. Never let plants sit in soft water, as the sodium will continue to be absorbed into the soil, which will in turn kill your plants. Placing your plants in elevated holders will allow the water to drain through and keep the soil from absorbing as much of the harsh minerals.
Re-pot your plants. If it appears that the plants are starting to suffer from extended exposure to soft water and the minerals contained therein, then it might become necessary to re-pot your plants. The fresh soil will be free of any of the harmful mineral build-up and provide a healthy boost for your suffering plants.