General hardness (GH), also called total hardness, is the concentration of divalent cations in water, mainly calcium (Ca²⁺) and magnesium (Mg²⁺). It is reported as mg/L (ppm) of calcium carbonate equivalent or in German degrees (dGH). Common classification for drinking water (US Geological Survey): soft below 60 mg/L, moderately hard 61–120, hard 121–180, very hard above 180 mg/L as CaCO3.
In practice
Hard water brings free calcium and magnesium. Water at 200 mg/L as CaCO3 from mostly calcium supplies roughly 60–80 mg/L of Cabeing reviewed. Growers on hard tap water may need little or no added Cal-Mag; growers on RO water need all of it from the nutrient. Hardness usually comes with high alkalinity, so hard water also needs acid.
Find GH in the utility's water report, test with an aquarium GH kit, or get a lab analysis that lists Ca and Mg separately.
Units and formula
- 1 dGH = 17.86 mg/L as CaCO3 (equivalent to 10 mg/L CaO)
- Hardness (mg/L as CaCO3) = 2.497 × Ca (mg/L) + 4.118 × Mg (mg/L)
Don't confuse with
- Carbonate hardness (KH): bicarbonate and carbonate, which is alkalinity.
- EC: total dissolved ions, including sodium, chloride and sulfate, not just Ca and Mg.
Myth check
Cal-Mag need is driven by water source, medium and cultivar, not by LED lighting. The LED claim is a Myth; see Cal-Mag.
