Calcium Deficiency

Calcium deficiency is a shortage of the element the plant uses to build cell walls. Because calcium does not move once it is placed, the damage always appears in the newest growth, never the old.

What you are seeing#

Look at the top of the plant, at the youngest leaves and the growing tips.

New leaves come in small, misshapen and often crinkled, with edges that hook or twist rather than lying flat. Small necrotic spots appear, tan to rust brown, scattered irregularly across the blade with no relationship to the veins. The spots start pinhead sized and spread into dead brown patches with a yellow halo around them.

Growing tips can turn brown and die back entirely in a severe case. Overall growth slows and new node spacing gets short and stunted.

Older lower leaves stay green and normal. That is the diagnostic key. Calcium is immobile, so the plant cannot rob its old leaves to fix the new ones.

Why it happens#

Coco coir holds onto calcium and magnesium and releases sodium and potassium instead, which is why coco growers see this constantly. Reverse osmosis and distilled water contain none at all.

Beginner causes:

• Growing in coco without a Cal-Mag supplement.
• Using reverse osmosis or heavily filtered water with a nutrient line that assumes tap water calcium.
• pH below 6.0 in soil, where calcium uptake falls off.
• Strong LED light, which increases growth rate and therefore calcium demand faster than the feed keeps up.
• Very low humidity or no air movement, which reduces transpiration. Calcium only moves in the transpiration stream, so a plant that is not transpiring cannot deliver it.

How to fix it#

1. Add a Cal-Mag supplement today at 5 ml per gallon, which is the standard dose for most brands. In coco with reverse osmosis water, treat this as permanent, not a fix.
2. Check runoff pH and bring soil to 6.3 to 6.8. Calcium uptake is best at the upper end of the soil range.
3. Confirm you have real airflow. A clip fan rustling the canopy raises transpiration and moves calcium to the new growth. A still tent will keep showing this symptom no matter what you feed.
4. Keep humidity in the 45 to 60 percent range in veg. Below about 35 percent the plant closes its stomata and calcium delivery stalls.
5. Over the next week, in soil only, a tablespoon of dolomite lime per gallon of medium topdressed and watered in gives a slow, pH-stabilizing calcium source.

How to tell it apart#

Versus magnesium deficiency. Position is everything. Magnesium is mobile and shows in the old lower leaves as yellowing between green veins. Calcium is immobile and shows in the new top leaves as spots and distortion with normal green color between them. If the bottom of the plant is the problem, it is magnesium. If the top is, it is calcium.

Versus pH lockout. Below pH 6.0, calcium is present but unavailable, and the plant shows this page's symptoms while the pot is full of calcium. Adding more Cal-Mag then does nothing except raise your salt level. Measure runoff before you dose.

Versus light burn. Both damage upper leaves. Light burn spares anything shaded and produces bleached, pale or yellow tissue at the tips of the leaves nearest the lamp, with no distortion of new growth. Calcium deficiency produces distinctly shaped rust spots and twisted, crinkled new leaves even on tops that sit well below the lamp.

How long until it looks better#

The spotted leaves do not recover. Those spots are dead tissue and they will stay exactly as they are, or spread slightly, for the life of the leaf.

You are judging this purely on the next set of leaves. Within five to seven days of adding Cal-Mag at correct pH, new growth should emerge flat, evenly green and spot free. Compare a fresh leaf to a photo you took today rather than trying to remember. If new growth is still spotting after ten days, your pH is wrong and the calcium is not reaching the plant.