Short answer. Cal-Mag is a calcium-plus-magnesium supplement, usually calcium nitrate and magnesium nitrate, sometimes with a little iron. You need it when your water plus base nutrient supply too little Ca and Mg: RO or very soft water, new or unbuffered coco, or a base formulated for hard water. You do not need it on hard tap water with a complete base feed. Overuse adds nitrogen and pushes K and Mg out of balance. "LEDs need more Cal-Mag" is a myth. Demand is set mainly by the cultivar, then by your water, your medium, root-zone pH and transpiration, not by the light source.
What's happening
Many base nutrients are designed assuming some Ca and Mg come from the tap. Hard water commonly supplies tens of mg/L of each; Purdue classes water above about 60 mg/L Ca or 25 mg/L Mg as hard [3]. RO water supplies essentially none. A feed that is balanced on tap water can therefore be Ca- and Mg-short on RO.
Coco makes it worse when new: its exchange sites hold K and Na and soak up Ca and Mg from the feed until they are saturated [4] (see buffering coco).
Cal-Mag is not Epsom salt. Epsom salt is magnesium sulfate only. Most Cal-Mag products are nitrate-based, so they also add roughly 1 mg/L N for every 1.2 mg/L Ca (from the calcium nitrate) plus N from magnesium nitrate. Check your label.
What actually drives Ca and Mg demand
- Cultivar. Genetic demand varies. Two cultivars in the same room, on the same feed, can show very different Ca and Mg status. In thirteen hemp cultivars grown side by side, leaf Ca had the widest spread of any nutrient [7], and two medical cannabis genotypes differed in how they took up and moved nutrients on the same K feed [5]. If one plant in a mixed room shows Ca/Mg signs and the rest don't, suspect the genetics before the bottle.
- Water source. RO and soft water bring almost no Ca or Mg; hard tap water can bring most of what the plant needs [3].
- Medium. Fresh coco's exchange sites are loaded with K and Na and pull Ca and Mg out of the feed until buffered [4].
- Root-zone pH. Low pH cuts Ca and Mg availability; see pH and nutrient availability.
- Transpiration and VPD. Ca moves only in the transpiration stream [6]. Cool leaves in a humid room transpire little, so new growth gets little Ca whatever the feed holds.
Greenleaf aside: the LED myth. "LEDs need more Cal-Mag" is a Myth. A photon doesn't consume calcium. The kernel of truth is transpiration: LEDs put out less radiant heat, so leaves often run cooler than under HPS at the same air temperature [8], leaf VPD drops, and Ca delivery to new growth slows [6]. That is an environment setting, not a light-source requirement. Many growers also moved to LEDs at the same time as they moved to coco and RO water, which gets the blame mixed up. If you switch to LEDs, set leaf VPD (raise air temperature a degree or two, or lower RH) and check your water and medium before you reach for a supplement.
Numbers that matter
| Parameter | Target | Notes |
|---|---|---|
| Total Ca in solution (water + base + supplement) | 150–200 mg/L | Cannabis research solutions ran 200 mg/L [1] |
| Total Mg | 40–70 mg/L | Veg optimum 35–70 mg/L [2] |
| Ca:Mg (mass) | ~3:1 to 4:1 | Grower practice |
| RO base before nutrients (if your base assumes tap) | ~Ca 40–60, Mg 15–25 mg/L | Grower practice; roughly EC 0.3–0.4 mS/cm from the supplement.being reviewed |
| Hard-water threshold | >60 mg/L Ca or >25 mg/L Mg | [3] |
Decision steps
- Read your water report (or measure source EC: RO is usually <0.05 mS/cm, many tap waters 0.2–0.8).
- Read the base nutrient label: what Ca and Mg does it supply at your dose? Many list it as % CaO and % MgO; convert (Ca = CaO × 0.715; Mg = MgO × 0.603).
- Add only the gap between water + base and the targets.
- Coco: buffer first; then a smaller ongoing top-up is often enough.
- Re-check K. If K is very high (PK boosters), you may be fighting K antagonism rather than a Ca/Mg shortage [5].
Common mistakes
- Adding full-rate Cal-Mag to hard tap water "just in case".
- Using Cal-Mag to treat broad-mite damage or pH problems.
- Forgetting the extra nitrogen late in flower.
- Adding Cal-Mag after the base nutrient in the same concentrate bottle (precipitation with sulfate and phosphate). Add it to the water first, at working strength.
Evidence
- Ca and Mg shortage on RO water and in fresh coco: Supported (substrate chemistry [4], water chemistry [3]; no cannabis trial specific to Cal-Mag).
- Target Ca and Mg levels: Supported [1][2].
- "LEDs need more Cal-Mag": Myth (GenetixBay house position). No study shows the light source itself raises Ca or Mg demand; the transpiration effect of cooler leaves [6][8] is a VPD setting, fixed with the environment.
- Cultivar differences in Ca and Mg uptake: Supported [5][7].
