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Calcium and magnesium are the most abundant minerals in tap water and will influence the water hardness. Soft water, usually found in the Western/Northern parts of the UK, generally has an EC of below 0.4 and doesn’t contain much calcium or magnesium. Using a CalMag supplement in soft water regions will increase the levels of each element which are then absorbed, and used by the plants.

Hard water is at the opposite end with an EC largely above 0.8 and can mainly be found in the Eastern/Southern parts of the UK. Water in these regions contains higher levels of calcium and magnesium compared to soft water and as a general rule, if your grow room is in a hard water area, then there's no need to use a CalMag supplement, unless the plants show a deficiency.

Whether you're in a hard or soft water area, for ultimate control, we recommend running the tap water through a Reverse Osmosis (RO) filter, like the HydroLogic Stealth RO system. This will strip out all additional elements, including calcium and magnesium, to give an EC reading of 0. A CalMag supplement should then be added to the nutrient solution to raise the EC and give the plants the required levels of calcium and magnesium they need to flourish.