Calcium and magnesium both play important roles in the growth of plants. Calcium helps plant roots develop and increases the resistance and strength of plant tissues and stems. Magnesium is an indispensable mineral that helps in the production of chlorophyll, the light-absorbing green pigment that serves as an energy source for plants. It also increases vitamin concentrations and aids in uptake of phosphorus within the plant body.

As per Pool and Spa Water Testing, the presence of calcium in the system is desired to ensure filming on those places where the temperature is relatively high, like in boilers and pipes transporting warm water. Scaling must be avoided because it reduces heat transfer and pump capacity, and causes cloudiness in the water. It is recommended to maintain the calcium hardness value within the range from 200 to 400 ppm as calcium carbonate (CaCO₃) and it should be monitored weekly to ensure the pool or spa water is well balanced to avoid corrosion and scale formation.

Calcium presence in water supplies results from passage over deposits of limestone, dolomite, gypsum and gypsiferous shale. The concentration may extend from 0 to several hundred milligrams per liter, depending on its source and treatment. Calcium is necessary in plant and animal nutrition since it is an essential constituent of bones, shells and plant structures. Calcium in water as carbonate is one of the primary components of water hardness which can cause pipe or tube scaling.

Water hardness has traditionally been defined as the capacity of water to precipitate soap. The ionic species in the water causing the precipitation was later found to be primarily calcium and magnesium. Thus, water hardness is actually a quantitative measure of these ions in the water. The measure and subsequent control of water hardness is essential to prevent scaling and clogging in water pipes. In other words, water, with exception to distilled water, contains dissolved salts (magnesium and calcium carbonates). The concentration of these salts determines the water hardness, which can be expressed in calcium carbonate or magnesium carbonate. The sum of these two represents the total hardness level. In addition, water hardness is also related to the phenomenon of pipe rusting in water heating and cooling systems, reverse osmosis, and demineralization plants.