Is it cal-mag?

The short answer

• “Cal-mag” is two different nutrients with two different presentations, and the reason the bottle bundles them is convenience, not physiology.
• Calcium, magnesium and potassium are the three that get mistaken for each other — spots, interveinal yellowing and scorched edges overlap on a single leaf. The reference text says visual symptoms may not pinpoint the element and recommends a tissue test to confirm.
• Before you buy anything, read the root-zone pH. All three lock out together when the medium drifts acidic, and a lockout looks identical to a shortage while responding to the opposite fix.

What each one actually looks like

From the controlled single-element deficiency study in the field’s reference handbook, restated:

• Calcium. Small brown spots and dead patches between the veins, toward the tips and edges, on new growth — calcium cannot be moved from old leaves, so it shows up top. It is the subtle one: no bright yellowing, just spotting and distortion on leaves that are otherwise green.
• Magnesium. Yellow between the veins with the veins staying green, progressing to dead tissue. The handbook records it on younger leaves in cannabis, against the classic rule that a mobile nutrient shows low first — which is exactly why the position rule cannot settle it, and why magnesium is so easily confused with iron.
• Potassium. Interveinal yellowing that becomes scorched, browning leaf margins, with weak stems. The bright, burnt-edge one.
• Phosphorus, for completeness, because it lives in the same confusion: leaves stay dark green with dead tips and edges and water-mark-like spots. Not the purple stems that every guide shows — the handbook’s controlled study did not produce that.

Why the photo cannot settle it

Two pieces of evidence, one from the literature and one from our own bench.

The handbook, after laying out those presentations, says plainly that visual symptoms may not be able to pinpoint the exact cause, and recommends tissue nutrient analysis to confirm. That is the reference text for the field telling you the leaf is not enough.

Our own measurement agrees, and we give it as ours. We had a careful reader label 148 photographs of deficient leaves blind — without being told what the source called them — and compared the reads to the source labels. Nitrogen, magnesium, iron and sulphur came back clean: the reader and the source agreed almost every time. Calcium came back agreed 5 times and disagreed 40, read as magnesium 18 times and potassium 22. Photographs an expert source had labelled calcium looked, to a careful eye that did not know the answer, like magnesium or potassium more often than not. That is not a bad reader; it is the same overlap the handbook describes, measured.

So when a thread asks “this should be cal-mag, right?” and three people say yes, understand what has happened: the picture is consistent with calcium and with two other things, and everyone picked the one the bottle is named after.

The thing to check before the bottle

Calcium, magnesium and potassium share more than a look. They lock out together when the root zone drifts acidic — below roughly pH 6.2 in soil-type media, the group becomes chemically unavailable even though it is present in the pot. A locked-out plant shows the same spots and edges as a starved one, and if you answer it with more feed you push the pH further the wrong way and make the lockout worse. This is the single most common way a “cal-mag deficiency” gets treated for weeks without improving.

So the first measurement is not a leaf, it is runoff pH. Water the plant, catch what comes out the bottom, and read it with a pen you have calibrated. Soil wants roughly 6.0–7.0; coco and hydro roughly 5.5–6.5. If the number is under the range, correct the pH and wait for new growth before deciding anything about elements — the damaged leaves will not recover, but the new ones will tell you whether the problem was ever a shortage.

If the pH is fine, then narrow it

• Where is it? New growth at the top: calcium (or iron, if it is yellowing rather than spotting). Lower and middle leaves with scorched edges: potassium. Interveinal yellow with green veins: magnesium, and check the top as well as the bottom, because in cannabis it goes either way.
• Spots or edges? Small dead spots between veins on green leaves: calcium. Whole margins browning inward: potassium. Yellow fields between veins, then death: magnesium.
• What is the medium? Coco holds calcium and magnesium against the plant until it is buffered, and it is the medium where a genuine shortage of both is most common. Living soil rarely runs short of either; a “cal-mag” look in living soil is more often a pH or watering problem.
• What is the water? Very soft or reverse-osmosis water carries almost no calcium or magnesium, and plants on it need them supplied. Hard tap water often carries enough calcium that adding more is pointless.

If after all that it is still ambiguous, that is the expected outcome, not a failure of the method. The handbook’s answer at that point is a tissue test. The practical answer for a home grower is: correct the pH, feed a balanced supplement that carries both if the water or medium argues for it, and judge by the new growth in ten days, not by the old leaves.

How to check this yourself

• Runoff pH first. Calibrated pen, catch the runoff, compare to the range for your medium.
• Then position and pattern. Top or bottom; spots, edges or interveinal fields.
• Then the water and the medium. RO or very soft water, or unbuffered coco, makes a real shortage plausible. Hard water or living soil makes lockout or overwatering more likely.
• Mark the newest leaf and wait ten days after any correction. Old damage stays; the verdict is in what grows next.

What would show that we have this wrong

If the runoff pH is squarely in range, the water is soft, the medium is coco, and the new growth is spotting and twisting at the top, that is about as clean a calcium case as a photo ever gives — feed it and stop reading this page. And if the spots are ringed with yellow on older leaves rather than between veins on new ones, look at manganese toxicity, which the handbook notes is more common than manganese deficiency and looks nothing like a shortage.

Where our device fits, stated narrowly

Croplock does not name the element for you. The classifier on the box reports the family a leaf belongs to — spots-and-margins, interveinal, whole-leaf — and names a single element only when the top two candidates are far apart and in different families, precisely because of the overlap this page is about. For the calcium–magnesium–potassium group that means it will usually tell you the family and the cue, not the bottle. We wrote up why.

Sources

• Cannabis in Controlled Environment Production (CRC Press), Ch. 5, Rootzone Management, §5.3.1–5.3.2. DOI 10.1201/9781003150442 — the single-element deficiency presentations and the statement that visual symptoms may not pinpoint the cause. Restated in our own words.
• The 148-photograph blind agreement measurement is our own, from June 2026, and is given as such.
• The pH ranges by medium are conventional working ranges; the group lockout below about 6.2 is from our own diagnosis reference, derived from the handbook.

More of these at Answers. The device is at croplock.com.

Related: pH or EC — which number is actually telling me something? · Is it a deficiency or a disease? · Is my pale new growth a deficiency?

Also read: Is this the fade, or a deficiency?