Calcium and magnesium are nearly always sold together, but they are two elements with two different stories. Knowing which of the two is missing buys more precisely than reaching for a combined bottle by default.
The two deficiencies do not look alike
Magnesium is mobile inside the plant: when it runs short the plant pulls it out of old leaves to feed the new ones, so you see it start low down, as yellowing between the veins while the veins themselves stay green. Calcium does not move: it stays where it was built in, so a shortage shows up in the newest growth instead, as twisted or malformed young leaves and damaged growing tips. If the problem is at the top, adding magnesium will not fix it.
Combined, or separate
Most brands supply the two as one bottle: Hy-pro Cal-Mag, Atami Ata Calmag, Plagron Calmag Pro and Bio G Power Calcium MG. That is the practical choice when you are topping up preventively. CANNA deliberately does it the other way and sells CaO Calcium and MgO Magnesium as two straight fertilisers, so you correct only what is actually short. Druid Nutrients Uruz is a powder for dry dosing.
Calcium tolerates little company in the bottle
This is the practical reason CANNA keeps the two apart. Undiluted, calcium reacts with phosphate, sulphate and magnesium, which is exactly why an A+B feed comes in two bottles. So never mix a straight calcium product undiluted with anything else: dissolve it in the water, stir it through, and only then add the next component. The CANNA magnesium is a sulphate on top of that, which puts it on the same side of the warning.
Measure before you top up
A yellowing leaf is not automatically a deficiency. If the pH sits too high the calcium is in your water but out of the plant's reach, and then feeding more makes it worse rather than better. So measure pH and EC with something from Measuring & Control and correct with pH Control before you open a bottle.
Frequently asked questions
Do I need CalMag on tap water?
In potting soil usually not, because both the soil and the tap water bring calcium and magnesium along. On reverse osmosis you do, because the water starts at zero. See also Soil.
And on coco?
That depends on your line, not on the medium alone. A proper coco line already has the extra calcium and magnesium built in; run a soil or hydro line on coir and you do have to add them separately. More on that under Coco.
Can I give too much?
Yes. Excess calcium blocks uptake of potassium and magnesium, so a generous dose creates a second deficiency. Watch the EC rather than defaulting to the maximum on the label.
