Cannabis Magnesium Deficiency: Causes, Effects & Tips

Magnesium (Mg) sits at the center of every chlorophyll molecule and activates many of the enzymes that drive energy production. Without it, plants can't photosynthesize efficiently, no matter how much light they receive. Demand increases in mid-to-late flower, and deficiency is especially common with RO water, coco coir and high-intensity LED lighting. 
magnesium deficiency dosatron

How to recognize magnesium deficiency

Magnesium is mobile, so symptoms start on older, lower and middle leaves. 

  • Interveinal chlorosis: yellowing between the veins while the veins stay green.
  • Yellowing that starts at leaf margins and moves inward.
  • Rust-brown or purple spots on affected leaves as the problem progresses.
  • Leaves that curl upward, turn brittle and die off.
  • Reduced vigor and smaller buds when the problem hits in flower.

Common causes

Target roughly 6.0–7.0 in soil and 5.5–6.5 in hydroponics and coco. Low pH in particular reduces magnesium availability. 

How to fix magnesium deficiency in cannabis

  1. Check and correct pH at input and runoff (or in the reservoir).
  2. Add a Cal-Mag supplement, especially with RO water or coco.
  3. For quick results, use magnesium sulfate (Epsom salt) in the feed or as a light foliar spray in veg — follow label rates and avoid spraying buds.
  4. Rebalance potassium if you are using heavy bloom boosters.
  5. Watch new growth: damaged leaves won't recover, but spreading should stop within about a week.

Deficiency at a glance

 

Magnesium deficiency 

Mobility 

Mobile — older and middle leaves first 

Key signs 

Yellowing between green veins, rust spots, upward curl 

Peak demand 

Mid-to-late flower 

Ideal pH 

Soil 6.0–7.0, hydro/coco 5.5–6.5 

Common mix-up 

Nitrogen deficiency (veins also yellow), potassium deficiency 

How Dosatron helps

Magnesium and Cal-Mag supplements only work if they are dosed consistently. Dosatron proportional pumps inject concentrated solutions at a fixed ratio into the irrigation line, so every plant receives the same amount, whatever the flow. Running separate units for base nutrients and Cal-Mag also helps avoid precipitation in concentrated stock solutions. 

Need help on how to dose Cal-Mag and nutrients precisely

FAQ

Older and middle leaves, because magnesium is mobile. 

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