Water hardness is caused by high calcium and magnesium levels. However a water test is needed to find out how much hardness is accounted for by bicarbonates, called temporary hardness and how much is composed of sulphates and chlorides (permanent hardness). For a discussion of alkalinity and hardness please see Water test interpretations.
Lime softening uses calcium hydroxide to raise the pH of the water to reduce temporary hardness. At around pH 10 calcium is precipitated and at around pH 11.5 magnesium is precipitated.
The theoretical dose of lime can be calculated from water test results. In practice the dose of calcium hydroxide and final pH adjustment needs to be tested on actual samples in laboratory trials. Too little lime will only reduce calcium. Too much will create an imbalance of calcium and magnesium in the final sample and will make pH adjustment more difficult.
In the Apps Laboratories lab we were successful in significantly reducing the hardness in a bore water sample from a rural property. After several trials we arrived at a lime dose that achieved a good final balance of salts and hardness.