The hardness of water is formed by dissolved salts. In natural waters, hardness is formed mainly from magnesium and calcium hydrogen carbonate and sulfate dissolved from the soil and / or aquifers. 

The more salts dissolved in the water, the harder the water becomes. Since the composition of the geological subsoil varies greatly from region to region, a wide variety of water hardnesses and compositions are encountered in practice.

Other input pathways include dissolution by acidic components of precipitation or by nitric acid from nitrification resulting from agricultural fertilization.