Most water is termed “hard,” which means that it contains a significant amount of minerals, mostly calcium and magnesium, both of which are essential to the human diet. Other vital minerals and trace elements are also found in hard water. In other words, the substances that make water hard are not contaminants. Instead, they are the naturally healthful components of water. Overall, water is commonly rated in five categories (very hard, hard, moderately hard, slightly hard, and soft) by how many grains of minerals per gallon it contains.
An example of naturally soft water (with little or no mineral content) is rainwater, but when we speak of water softening, we’re referring to removing the minerals by artificial means. This process typically makes the water salty to varying degrees and may make it unsuitable for routine drinking or plant watering. Why? The softening process may replace the calcium and magnesium ions with sodium ions.