As the water evaporates from the soil, the sodium concentrates in the soil. Over time, the concentration of sodium causes the problem to get worse as sodium levels increase. There are potassium-based water softening systems which may cause less problems. In these systems the hard water minerals are replaced by potassium, a plant nutrient that is beneficial to plants. However, as the potassium accumulates in the potting soil, it too can reach levels which are damaging.
Potassium can accumulate to higher levels before the problems develop.
There are other methods of removing hard water minerals from soil. As long as plant toxic minerals are not added to the water, they should cause no problems. In many cases the easiest solution is to just use water from a faucet that receives unsoftened water. In New Mexico ground water usually has high mineral content, but only in some areas is the content high enough to cause plant injury. However, leaching the salts from the soil on a regular schedule is important.