Softened water contains high levels of sodium.  If the levels of sodium in the soil get too high, it is toxic to plants.  The amount of sodium in your water is only one factor that could lead to sodium levels building up to toxic levels for plants.  Other factors include...

• the amount of softened water used for irrigation, 
• soil structure/composition, 
• the length of time soften water is used for irrigating, 
• the plant's tolerance of salts, and 
• the amount of leaching that occurs through rainfall. 

Unfortunately, it is difficult to say what level of sodium in your softened water will cause damage to landscape plants because it is more than just the level of sodium in the water that determines whether sodium levels build up in the soil to toxic levels.  

Measuring sodium levels in the water can be difficult to determine.  Even if you knew how much sodium was in your water, it would not directly correlate to potentially causing, or not causing damage to plants since those other factors in the bulleted list above play a big role as well.

Typically the use of softened water to irrigate only becomes an issue for potted plants that are watered exclusively with softened water over a long period of time, such as houseplants.  It would be rare for landscape plantings to see damage from softened irrigation water because typically in Iowa you would not be irrigating with the frequency, duration, or quantity of softened water needed to allow for salts to build up to toxic levels. That is not to say it couldn't happen.  Certainly in years where irrigation happens often over a long period of time with high amounts of water and when natural rainfall is sparse preventing good leaching, levels of sodium in the soil could rise to levels that are unhealthy for plants.  

With all that said, it is not recommended to water plants regularly with softened water.  Regardless of how much sodium is in the water, if it can be avoided, it is best to do so.  An occasional watering with softened water is very unlikely to cause issues, but making it a regular practice is not a good idea.  It is also not a good financial and resource decision as irrigation water that should not, and does not need to be softened is passing through the water softener anyway, wasting salt and money.  Most water softeners have a bypass valve that can be flipped during times of irrigation to allow unsoftened water to be used for watering and then flipped back when irrigation is done.  Sometimes outdoor faucets can be re-plumbed to be connected to the water source before the softener to provide unsoftened water outside and softened water indoors. 

Further questions can be directed to your local county Extension office. This local resource can be very helpful since they are familiar with trends and issues in your part of the state. Click on this link to get the contact information for the Polk county Extension office. https://www.extension.iastate.edu/polk/

Happy gardening! 

--Aaron