Magnesium Deficiency

What magnesium deficiency is

Magnesium (Mg) is a secondary macronutrient and the central atom of the chlorophyll molecule ^[1]. Without enough available magnesium, the plant cannot build chlorophyll, and photosynthesis drops. Because Mg is mobile inside the plant, when supply runs short, the plant pulls it from older leaves to feed new growth. That's why the classic symptom shows up on lower and middle leaves first, not the top Strong evidence.

The signature look is interveinal chlorosis: the veins stay green while the tissue between them turns pale yellow, sometimes progressing to rust-colored spots and crispy leaf edges. In cannabis, the yellowing often starts at the leaf tips and margins and moves inward ^[2] Strong evidence. Severe cases cause necrotic brown patches and leaf drop.

It's worth separating two different problems that look the same:

• True deficiency — there isn't enough magnesium in the feed or medium.
• Lockout — magnesium is present but unavailable because root zone pH is wrong, or because excess potassium, calcium, or ammonium is outcompeting it for root uptake ^[3] Strong evidence.

Why growers care about it

Magnesium directly drives photosynthesis. A plant losing chlorophyll in its fan leaves is losing the solar panels that fuel bud development. Catching Mg deficiency in veg costs you almost nothing — the plant grows out of it in a week. Letting it run untreated into mid- and late-flower means smaller, lighter, less resinous flowers because the plant is starving for energy during its highest-demand phase Weak / limited.

Magnesium also interacts with other nutrients. It's antagonistic with calcium and potassium at the root surface, meaning too much of one can block the others ^[3] Strong evidence. This is why Cal-Mag supplements exist as a combined product — the two are usually managed together, especially in coco coir and RO water setups where neither is supplied by the medium or water.

When to start treating

Start the moment you see consistent interveinal yellowing on multiple lower leaves that's spreading upward — not just one or two random leaves, which is normal senescence.

Before you treat, measure:

1. Runoff pH. Mg uptake in soil is best at pH 6.2–7.0; in coco and hydro, 5.8–6.2 ^[4] Strong evidence. Outside that range, Mg locks out regardless of how much you're feeding.
2. Runoff EC. A high EC (salt buildup) can also block uptake. If EC is well above your feed EC, flush first.
3. Your feed. Check whether your base nutrient line even contains magnesium, and at what ratio. Many bloom boosters are heavy on K, which can crowd out Mg.

If pH and EC are in range and Mg is in the feed at normal levels, you may have a real deficiency. If pH is off, you have a lockout — adding more Mg won't fix it and may make things worse.

How to fix it: step-by-step

Step 1: Diagnose, don't guess. Confirm interveinal chlorosis on older leaves with green veins. If yellowing is on new growth, it's probably iron or sulfur, not magnesium ^[2] Strong evidence.

Step 2: Check and correct root zone pH. Test runoff or slurry pH. If it's off, adjust your input water/feed pH and flush the medium with correctly-pH'd water until runoff matches. In many cases, this alone resolves the symptoms within a week.

Step 3: Check EC and flush if needed. If runoff EC is much higher than input EC, flush with 2–3x the pot volume of pH-corrected, low-EC water before adding more nutrients.

Step 4: Add magnesium if truly deficient.

• Epsom salt (magnesium sulfate): 1–2 g per liter (about 1 tsp per gallon) of water, applied as a feed. This is fast-acting and cheap ^[5] Strong evidence.
• Cal-Mag supplement: follow label rate, usually 2–5 ml/L. Preferred for coco coir and RO water grows where calcium is also low Weak / limited.
• Foliar spray: 1–2 g/L Epsom salt sprayed on leaves can deliver Mg within 24–48 hours for fast rescue Weak / limited. Don't spray in flower past week 2–3, and never under direct light.

Step 5: For soil grows, consider a long-term buffer. Dolomite lime mixed into soil at 1–2 tablespoons per gallon of medium supplies slow-release calcium and magnesium and helps stabilize pH ^[6] Weak / limited.

Step 6: Re-check in 5–7 days. Existing damaged leaves will not turn green again — that chlorophyll is gone. The signal that you've fixed it is that new growth comes in green and the chlorosis stops climbing up the plant.

Common mistakes

• Diagnosing by Google image without checking pH. Most "Mg deficiencies" posted in grower forums are pH lockouts. Adding more Mg to a locked-out plant makes the salt problem worse.
• Expecting damaged leaves to recover. They won't. Judge progress by new growth, not old leaves.
• Overdoing Epsom salt. Too much magnesium antagonizes calcium and potassium uptake, creating a new deficiency ^[3] Strong evidence. Stop dosing once symptoms stop spreading.
• Foliar feeding in late flower. Spraying sugary leaves and buds invites mold (botrytis). Stop foliar applications by week 2–3 of flower Weak / limited.
• Ignoring the water source. Reverse osmosis and very soft tap water contain almost no Mg or Ca. If you're on RO without a Cal-Mag supplement, expect deficiency Strong evidence.
• Confusing it with other problems. Iron deficiency = interveinal chlorosis on new growth. Nitrogen deficiency = uniform yellowing of old leaves (no vein contrast). Light burn = yellowing/bleaching at the top of the plant under intense light.

Related techniques and topics

• pH Management — the single biggest lever for any nutrient deficiency.
• Calcium Deficiency — usually appears alongside Mg deficiency, especially on RO water.
• Flushing — when and how to clear salt buildup from the root zone.
• Cal-Mag Supplements — what's actually in the bottle and when you need it.
• Reading Cannabis Leaves — a visual diagnostic guide for common deficiencies.

Sources

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