What does magnesium deficiency look like, and why does it start at the bottom?
Magnesium is a mobile nutrient inside the plant. When supply runs short, the plant redistributes it from older leaves (the lower ones) toward actively growing tissue. That's why interveinal chlorosis — veins staying green while the blade turns yellow — shows up first on the bottom fan leaves and works its way up as the deficiency deepens.
In advanced cases the affected leaves go from yellow to bronze or brown, curl and crinkle, and eventually drop. The green-vein-on-yellow-background pattern is the telltale sign — it's distinct from salt tip burn or viral mosaic.
How do you tell magnesium deficiency apart from iron deficiency?
This mix-up is extremely common because both show up as interveinal chlorosis. The key is where it appears:
- Magnesium: interveinal chlorosis on OLD leaves (lower canopy). Mg is mobile and gets pulled from mature leaves first.
- Iron: interveinal chlorosis on NEW leaves (top growth). Fe is immobile, so its deficiency hits young tissue hardest — where the plant needs it most.
- Nitrogen: uniform yellowing (not interveinal) that also starts on older leaves, but without the distinctive green-vein pattern.
If the interveinal chlorosis is starting from the bottom → magnesium. If it's starting from the top → iron (and check your chelate source, see /guides/iron-eddha-vs-edta).
Why is it so common in coco and with soft water?
Coco has a high cation exchange capacity (CEC): the fibers hold onto positive cations like Ca²⁺ and Mg²⁺, temporarily pulling them out of the solution available to the roots. The effect is most pronounced with fresh, unwashed coco. This is why cal-mag demand is structurally higher in coco than in soil or pure hydroponics. For a deeper look at coco CEC and the Ca/Mg relationship, see /guides/coco-cec-calcium-magnesium.
Soft water (low in dissolved calcium and magnesium — typical of rainwater, reverse osmosis, or many well sources) makes things worse: the plant gets less baseline Mg with every watering. Tap water from harder-water regions usually has more buffering, but it can still fall short of the optimal range if your feed program doesn't compensate.
How much magnesium does cannabis actually need, and what does the science say?
Studies on Cannabis sativa in hydroponic systems and inert substrates place the optimal Mg range in nutrient solution between 35 and 70 ppm depending on the growth stage and cultivar:
- Morad & Bernstein (2023) found in a controlled trial that Mg must be maintained in a balanced ratio with Ca to avoid cationic antagonism; Mg deficits impaired the uptake of other nutrients and reduced foliar yield.
- Cockson et al. (2019) analyzed nutrient profiles in indoor cannabis and reported Mg sufficiency concentrations between 35 and 65 ppm in veg and up to 70 ppm in bloom.
- Veazie et al. (2025) confirmed that cation availability — including Mg — is strongly pH-dependent: if substrate pH is out of range, Mg can be present in solution yet still unavailable to the plant.
Cationic antagonism is the underlying mechanism: excess Ca²⁺, K⁺, and NH₄⁺ compete with Mg²⁺ for the same root uptake sites. That's why adding magnesium alone without reviewing overall nutrient balance may not be enough.
What role does pH play in magnesium availability?
Mg is well absorbed in the pH range of 5.5–6.5 (hydro/coco) and 6.0–7.0 (soil). Outside those ranges, magnesium can be present in solution and still not enter the plant:
- pH below 5.5: Mg competes heavily with Mn²⁺ and Zn²⁺, which become much more soluble at low pH.
- pH above 6.5 in hydro/coco: Mg starts to precipitate with phosphates or carbonates, reducing mobility to the root zone.
- Before adding cal-mag, always check the pH of your substrate or runoff. If it's out of range, correct the pH first — adding more Mg without doing that may produce no visible change at all.
How do you correct a magnesium deficiency?
A three-step protocol:
- 1. Check pH: measure at the drip point or from runoff. If it's out of range (below 5.5 or above 6.5 in coco/hydro), dial that in first.
- 2. Add cal-mag to your feed: a well-formulated liquid calcium-magnesium supplement delivers Mg in ionic form that's directly available to the roots. Dose based on your source water hardness and target EC; follow the manufacturer's guidelines and monitor total EC.
- 3. Track new growth: already-damaged leaves won't recover — dead tissue doesn't reverse — but new growth should look healthy within 5–10 days if the issue is resolved.
