Cal-Mag is a class of supplement providing calcium and magnesium, typically calcium nitrate and magnesium nitrate in liquid form, sometimes with iron. It also adds nitrogen, which counts towards the feed total.
In practice
Growers use it to make up for calcium and magnesium missing from RO or very soft water, to buffer coco before planting, and with cultivars that genuinely demand more Ca and Mg. With hard tap water, a complete base often already supplies enough. Too much raises EC, pushes nitrogen up and can suppress potassium. Dose to a target in mg/L (ppm) of Ca and Mg, not by habit.
Don't confuse with
- Epsom salt: magnesium and sulfur only, no calcium.
- Calcium nitrate: calcium only.
- Coco buffer: a Cal-Mag use, not the product itself.
Myth check
"LEDs need more Cal-Mag" is a Myth (GenetixBay house position). LEDs do not create a Ca or Mg requirement. Demand is driven by cultivar, water source and coco cation exchange; leaf temperature differences under LED (Nelson and Bugbee) affect transpiration and environment settings, not nutrient need in themselves. See Cal-Mag and Mythbusters.
Calcium nitrate is the standard hydroponic calcium source, supplying about 19% calcium and 15.5% nitrogen, mostly as nitrate.Magnesium nitrate supplies about 9.5% magnesium and 11% nitrogen as nitrate, and unlike Epsom salt it can share a concentrated stock with calcium nitrate.RO water is water stripped of most dissolved salts by reverse osmosis; it has near-zero EC and little buffering, so pH moves easily and Ca and Mg must be added.Coco coir is a growing medium made from processed coconut husk, prized for holding water and air at once; it binds calcium and magnesium unless buffered.EC (electrical conductivity) measures how well a solution conducts electricity, a proxy for total dissolved salts; growers read it in mS/cm to gauge nutrient strength.An LED (light-emitting diode) grow light uses solid-state diodes to produce light; modern fixtures are the most efficient grow lights, at roughly 2.5–3 µmol/J.Cation exchange capacity (CEC) is a medium's ability to hold positively charged nutrients such as calcium, magnesium and potassium on its surfaces and release them later.Leaf temperature is the actual surface temperature of a leaf, which often differs from air temperature by 1–3 °C depending on light source and transpiration.Calcium (Ca) is an immobile secondary nutrient that builds cell walls and membranes; it travels only in the transpiration stream, so deficiency appears in new growth.Magnesium (Mg) is a mobile secondary nutrient at the centre of every chlorophyll molecule; deficiency causes interveinal yellowing on older leaves.Secondary nutrients are calcium, magnesium and sulfur: macronutrients needed in substantial amounts but not included in the N-P-K numbers on a label.
