Magnesium Deficiency

Magnesium deficiency is a shortage of the atom sitting at the center of every chlorophyll molecule. Without it the leaf tissue cannot stay green, but the veins hold their color longest, which produces the most recognizable pattern in the whole diagnostic set.

What you are seeing#

Start at the bottom third of the plant. Magnesium is mobile, so the plant strips its oldest leaves first.

The tissue between the veins turns pale green, then bright yellow, while the veins themselves and a thin band of tissue on either side stay clearly green. The result looks like a green fishbone or skeleton laid over a yellow leaf. The pattern is sharp and unmistakable once you have seen it.

Next come rust or tan colored spots inside the yellowed areas. These spread and merge into dead brown patches. Leaf edges and tips crisp up and curl slightly upward. Badly affected leaves drop.

The yellowing works its way up the plant over one to two weeks. New growth at the top stays green until it gets severe.

Why it happens#

The magnesium is usually available but outcompeted or absent. Cannabis under strong light uses far more magnesium than most bagged nutrient lines supply on their own.

Beginner causes:

• Coco coir or reverse osmosis water with no Cal-Mag supplement. This is the leading cause by a wide margin.
• Heavy potassium or calcium feeding, which competes with magnesium at the root and blocks uptake.
• Soil pH below 6.0, where magnesium availability drops off sharply.
• Strong LED lighting increasing demand without a matching feed increase.

How to fix it#

1. Add a Cal-Mag supplement at 5 ml per gallon at the next watering. That is the whole fix in most cases.
2. Check runoff pH and correct to 6.2 to 6.8 in soil, 5.8 to 6.2 in coco and hydro. Below 6.0 in soil this keeps recurring no matter how much you add.
3. If you have no Cal-Mag on hand, Epsom salt works. Use one quarter teaspoon of magnesium sulfate per gallon, dissolved fully in warm water first. Do not exceed one half teaspoon per gallon.
4. Back off any bloom booster or potassium heavy additive for one week. Excess potassium is a common hidden cause.
5. Over the next week, keep the supplement in every feed rather than dosing once and stopping. Magnesium demand does not go away.

How to tell it apart#

Versus nitrogen deficiency. Both start on the lowest leaves, which is why they get mixed up. Nitrogen yellows the entire leaf uniformly, including the veins, and the leaf then goes limp and drops fairly clean. Magnesium leaves the veins distinctly green against yellow tissue and adds rusty brown spots that nitrogen deficiency never produces. If you can see a green vein pattern, it is magnesium.

Versus iron deficiency. The pattern is nearly identical: yellow between green veins. The discriminator is position, and only position. Magnesium is mobile so it starts at the bottom of the plant. Iron is immobile so it starts at the very top, on brand new leaves, while the bottom stays green. Look at which end of the plant went yellow first and you have your answer.

Versus pH lockout. In soil, most apparent magnesium deficiency at correct feeding rates is a low pH problem. Measure runoff before adding anything. Correcting from 5.6 to 6.5 in soil often resolves this with no supplement at all.

How long until it looks better#

The yellowed and spotted leaves will not turn green again. That chlorophyll is gone and the plant has already moved the magnesium out of those cells.

Magnesium corrects faster than almost anything else. Within three to five days of a Cal-Mag dose at correct pH, the yellowing should stop moving up the plant. New leaves emerging after that point should be an even green with no vein pattern. If the pattern shows up on fresh top growth a week later, recheck your pH, because you are locked out rather than short.