Almost every gardener is familiar with the concept of salting the earth, a practice designed to prevent crops from growing. So, no gardener would willingly do it to their own garden, right? Sadly, they might if they were unaware that the primary means of softening water is to add salt. In areas where the water is particularly hard, many homeowners add whole-house water softeners designed to prevent mineral buildup on the pipes, calcium deposits in the shower and ease of cleaning themselves and their home. But many home water softening systems create soft water by adding sodium chloride--salt--to the water. If you would never consider pouring rock salt on your garden, then don't use softened water to irrigate it either.
Soft Water Retards Growth
Because of the high sodium content in soft water, plants are unable to absorb enough of the moisture they need when being irrigated with soft water. Just as salt can dehydrate a person, it can dehydrate many of your ornamental plants, harm your grass or simply slow the growth of your garden.
Soft Water Removes Soil Nutrients
Because of the nature of soft water, it was designed to prevent calcium and other mineral deposits in your shower and other plumbing fixtures. Those qualities don't stop at the spigot, and soft water can literally wash away calcium and other beneficial minerals in your garden. Without proper nutrition, your garden may wither and die or simply fail to thrive.
Soft Water Salts the Earth
In especially arid climates, like the Southwest, irrigating with soft water deposits salt into the soil, and there is insufficient rainfall to rinse the soil clean. Instead, the salts in the water continue to accumulate in the soil, eventually turning your garden into a barren wasteland.