Magnesium Deficiency

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What magnesium deficiency is

Magnesium (Mg) is a secondary macronutrient and the central atom of the chlorophyll molecule ^[1] Strong evidence. Without enough available magnesium, the plant cannot build chlorophyll efficiently, 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 critical to separate two different problems that look identical:

• 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.

This distinction changes how you fix the problem. Adding more Mg to a locked-out plant makes the salt problem worse and delays recovery.

Why growers care about magnesium

Magnesium directly drives photosynthesis. A plant losing chlorophyll in its fan leaves is losing the solar panels that fuel bud development and resin production. Because Mg is mobile, deficiency symptoms appear quickly but also respond quickly to treatment.

Magnesium also interacts with other nutrients. It is antagonistic with calcium and potassium at the root surface, meaning too much of one can block uptake of 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.

Catching Mg deficiency in vegetative growth is low-cost to fix. In late flower, when the plant's energy demand is highest and leaves cannot be regrown, untreated deficiency can reduce bud density and potency because photosynthesis is compromised during the most critical growth phase.

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 and rule out lockout:

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 contains magnesium, and at what ratio. Many bloom boosters are heavy on potassium, which can crowd out Mg uptake.

If pH and EC are in range and Mg is listed on your feed label, you likely have a true deficiency. If pH is off, you have a lockout — adding more Mg won't fix it and will worsen salt accumulation.

How to fix magnesium deficiency: step-by-step

Step 1: Confirm the diagnosis. Confirm interveinal chlorosis on older leaves with green veins. If yellowing is on new growth only, it's probably iron or sulfur, not magnesium ^[2] Strong evidence. New-growth yellowing that skips veins is iron deficiency; uniform old-leaf yellowing without vein contrast is nitrogen deficiency.

Step 2: Measure and correct root zone pH. Test runoff or slurry pH using a reliable meter. If it's outside the target range (6.2–7.0 for soil, 5.8–6.2 for coco/hydro), adjust your input water/feed pH and flush the medium with correctly pH'd water until runoff matches ^[4] Strong evidence. In many cases, pH correction alone resolves symptoms within a week without any Mg supplementation.

Step 3: Check EC and flush if needed. If runoff EC is much higher than input EC, flush with 2–3 times the pot volume of pH-corrected, low-EC water before adding more nutrients. This clears salt buildup that blocks root uptake.

Step 4: Add magnesium if truly deficient.

• Epsom salt (magnesium sulfate): Dissolve 1–2 g per liter (about 1 tsp per gallon) of water, applied as a feed. This is fast-acting, cheap, and shows results within 5–7 days 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 depleted Weak / limited.
• Foliar spray: 1–2 g/L Epsom salt sprayed on leaf undersides can deliver Mg for fast rescue. Do not spray in flower past week 2–3, and never under direct light, to avoid mold and light burn Weak / limited.

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 over time Weak / limited.

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

Common mistakes

• Diagnosing by image search without checking pH. Most "Mg deficiencies" posted in grower forums are actually pH lockouts. Adding more Mg to a locked-out plant makes the salt problem worse and delays recovery.
• Expecting damaged leaves to recover. They won't. Once chlorophyll is lost, that leaf is a sunk cost. Judge success by new growth.
• Overdosing Epsom salt. Too much magnesium antagonizes calcium and potassium uptake, creating a cascading deficiency ^[3] Strong evidence. Use the recommended rate and stop dosing once new growth comes in green.
• Foliar feeding in late flower. Spraying sugary leaves and buds in weeks 3–8 of flower invites mold (botrytis) and bud rot. 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, Mg deficiency is predictable, not a sign of poor nutrient choice Strong evidence.
• Confusing it with other nutrient problems. Iron deficiency = interveinal chlorosis on new growth only. Nitrogen deficiency = uniform yellowing of old leaves without vein contrast. Manganese deficiency = interveinal spots with a netted appearance on new growth. Copper deficiency = twisted, pale new leaves. ^[2] Strong evidence.

Related techniques

• pH Management in Cannabis Cultivation — the single biggest lever for any nutrient deficiency.
• Calcium Deficiency Signs During Mid Flower — usually appears alongside Mg deficiency, especially on RO water.
• Calcium-Magnesium Supplementation in Coco vs. Soil — when and how to use Cal-Mag supplements.
• Iron Deficiency in Cannabis — how to tell it apart from Mg deficiency.
• Nitrogen Deficiency in Cannabis — how to distinguish it from Mg deficiency by leaf pattern.

Sources

How this page was made

Related

• Calcium-Magnesium Supplementation in Coco vs. Soil — Why coco demands cal-mag from seedling stage, and soil usually doesn't.
• Calcium Deficiency Signs During Mid Flower — Recognize immobile calcium deficiency in flower and distinguish it from lockout or other n...
• Copper Deficiency in Cannabis — A rare micronutrient problem that is usually misdiagnosed as something else.
• Iron Deficiency in Cannabis — Diagnose interveinal chlorosis, correct root-zone pH, apply chelated iron.
• Nitrogen Deficiency in Cannabis — Bottom-up yellowing signals nitrogen shortage; diagnosis and correction differ by growth s...
• pH Management in Cannabis Cultivation — Control nutrient availability by maintaining pH in your growing medium.