Identify and fix magnesium deficiency in cannabis

Magnesium deficiency is an undersupply of plant-available Mg²⁺ in the tissue. Magnesium is the central atom of the chlorophyll molecule — without Mg there is no functional photosynthetic apparatus.

Cannabis shows Mg deficiency especially during the rapid stretch phase and early flowering stage, when demand rises and competing cations (K⁺, Ca²⁺, NH4⁺) antagonize uptake.

Mg²⁺ binds in chlorophyll a/b and activates over 300 enzymes, including RuBisCO activase and ATP-dependent kinases. Without Mg, CO₂ fixation collapses and reactive oxygen species damage the mesophyll.

Because Mg is phloem-mobile, the plant breaks it down from older leaves and relocates it to the growing tips — which is why chlorosis always starts at the bottom.

Mobile nutrients (N, P, K, Mg) move from old to young tissue when deficient. This is the most important distinguishing feature: Mg and N deficiencies start at the bottom, Ca, Fe, and S deficiencies start at the top.

With Mg, the leaf veins remain noticeably green while the tissue between them lightens — a 'fishbone' pattern that later turns into rust-brown necrotic spots.

Stage 1 (mild): Dull light green between the veins on the lowest 2–3 leaf tiers, veins remain deep green.

Stage 2 (moderate): Distinct yellow patches between the veins, leaf margins start curling upward; the chlorosis moves up one tier.

Stage 3 (severe): Rust-brown to purple necrotic spots, leaves become brittle and fall off; during flowering, bud weight measurably decreases.

1. pH blockage (most common cause): substrate/solution pH below 5.8 (coco/hydro) or 6.0 (soil) drastically reduces Mg uptake, even though enough Mg is present.

2. Cation antagonism: excessive K or Ca dosing displaces Mg at the root uptake sites.

3. True deficiency: RO/soft water without Mg supplementation, one-sided nutrients without an Mg fraction.

4. Leaching: overwatering in coco/hydro flushes the readily soluble Mg out of the root zone.

Step 1: Is the chlorosis on old or young leaves? Old → Mg or N likely.

Step 2: Do the veins stay green (interveinal)? Yes → Mg. No, evenly pale → more likely N.

Step 3: Measure the root-zone pH (runoff/substrate). If it's below the Mg window → primarily pH blockage, don't increase dosing.

Step 4: Check the EC. Very high EC with a lot of K/Ca → antagonism; flush and readjust the ratio.

1. Bring pH into the target range first: coco/hydro 5.8–6.2, soil 6.2–6.8. This often resolves the 'deficiency' without extra feeding.

2. Supplement Mg through the root: Epsom salt (MgSO4·7H₂O) 1–2 g/L in the nutrient solution, check EC afterward.

3. Emergency foliar spray: finely mist 20 g/L MgSO4, not under full lighting — it acts quickly on existing leaves but does not replace root feeding.

4. Defuse antagonism: lower the K and Ca fraction until the Ca:Mg ratio is around 3:1 to 4:1.

Use Cal-Mag additives consistently with RO/soft water; aim for 50–70 mg/L Mg in the finished solution.

Keep pH management stable: measure runoff pH daily instead of just checking the feed.

Don't scale K aggressively upward during flowering without proportionally carrying Mg along.

Low root temperatures (< 18 °C) additionally slow Mg uptake — a supposed deficiency can be a temperature problem.

High transpiration (low VPD management, strong PPFD) increases Mg throughput; with tight supply, the deficiency then shows up first.

Ca, K, and Mg compete for the same transporters — the ratio matters more than the individual dose.

Increasing the Mg dose without checking pH — with a pH blockage this only worsens salt stress.

Expecting yellow leaves to turn green again — they don't; judge success by new leaves.

Foliar spray as a permanent solution instead of root correction.

In recirculating hydro systems, antagonists accumulate; a periodic reset of the reservoir solution keeps the Ca:Mg ratio stable.

A leaf analysis (dry matter) provides certainty: Mg levels below ~0.2% of dry matter are considered deficient.