Water containing calcium and magnesium carbonates is normally suitable for growing plants. Both calcium and magnesium are macronutrients required by plants. Normal hard waters will not exceed the needed levels of these elements in a nutrient solution. Most nutrient solutions use 200 ppm of calcium and 50 to 60 ppm of magnesium. The amount of these elements in the raw water must be deducted from those called for in the nutrient formulation before calculating the weights of fertilizers used to provide the correct levels in the nutrient solution. In fact, the presence of calcium and magnesium in raw water can reduce the amount of fertilizers required in making up the nutrient solution.

The presence of low levels of carbonate in raw water stabilizes the pH of the nutrient solution by resisting sudden drops in pH. This is termed 'buffering capacity.' It is advisable to have 30-50ppm of carbonate/bicarbonate level in your nutrient solution to reduce rapid shifts in pH. If youÂre raw water is very pure, without any carbonate or bicarbonate to assist in stabilizing the pH of the nutrient solution.

The presence of sodium chloride in raw water is a different story. If levels of sodium chloride exceed 50ppm, there may be potential problems in the growing of specific crops, depending upon their tolerance. Some herbs such as watercress and mint will tolerate higher levels of sodium chloride than do hydroponic tomatoes or cucumbers. Also, if a closed hydroponic system is used, the level of sodium chloride will build up with the circulation of the solution past the plants. The plants will absorb water faster than the elements, thus reducing the volume of water, necessitating the addition of water as it circulates back to the nutrient tank. At the same time, as the plants do not assimilate the sodium chloride, the level will increase in the solution with the addition of water, eventually arriving at a concentration toxic to the plants. In this case, a water treatment facility using a process such as reverse osmosis water filtration system must be installed to remove the sodium chloride.

Here is a link that might be useful: Hydroponics