Iron Deficiency

Iron deficiency is the plant being unable to take up iron, which it needs to manufacture chlorophyll. Iron is immobile inside the plant, so every symptom lands on the newest growth and nowhere else.

What you are seeing#

Look at the very top of the plant, at the two or three newest sets of leaves.

New leaves emerge pale, almost lime or lemon yellow, with the veins standing out as thin green lines against the yellow. The contrast is stronger and the yellow is brighter than in magnesium deficiency, and it appears on leaves that have never been green in the first place.

In a severe case the newest leaves come in nearly white with only the largest veins showing green. Leaf size and shape stay normal, which separates this from zinc.

The lower two thirds of the plant look completely fine. That contrast, a healthy green plant with a bright yellow cap, is the signature.

Why it happens#

There is almost always plenty of iron in your medium. Above roughly pH 6.8 in soil, or 6.4 in coco and hydro, iron oxidizes into a form the roots cannot absorb. The iron is in the pot and the plant is starving next to it.

Beginner causes:

• pH creeping high. Tap water is often pH 7.5 to 8.2 and drifts the root zone up over a few weeks.
• Overwatering. Waterlogged, low oxygen root zones cannot take up iron even at correct pH.
• Heavy phosphorus from a bloom booster, which binds iron directly.
• Cold root temperatures below 60 F.
• Very rarely, an actual shortage in a soilless mix with no micronutrient package.

How to fix it#

1. Test runoff pH today. This is not optional for this page, it is the diagnosis. Target 6.0 to 6.5 in soil and 5.5 to 6.0 in coco and hydro. Deliberately run the low end of the range, because iron is most available there.
2. If runoff reads high, flush with 2 to 3 pot volumes of water adjusted to pH 6.0 in soil or 5.7 in coco.
3. Let the medium dry more between waterings. Lift the pot and water when it feels light. Root zone oxygen matters as much as the number on the pen.
4. Pause any bloom booster or high phosphorus additive for a week.
5. Over the next week, if the yellowing continues at confirmed correct pH, add a chelated iron supplement, ideally DTPA or EDDHA form, at the label rate. Plain iron sulfate is largely wasted above pH 6.5.

How to tell it apart#

Versus magnesium deficiency. Same yellow-between-green-veins pattern, opposite ends of the plant. Magnesium is mobile and starts on the oldest bottom leaves. Iron is immobile and starts on the newest top leaves. Magnesium also produces rusty brown spots, which iron deficiency does not. Look at the plant as a whole: yellow bottom means magnesium, yellow top means iron.

Versus manganese deficiency. Both hit new growth with interveinal yellowing. Manganese adds distinct necrotic flecks and dead speckles inside the yellowed tissue fairly early. Iron deficiency yellows cleanly with no spotting until it is very severe. If the new leaves are yellow but unmarked, lean iron.

Versus zinc deficiency. Zinc also hits new growth, but zinc distorts it: blades come in narrow, twisted and bunched together with very short spacing between nodes. Iron deficiency leaves normal leaf shape and normal node spacing. Check the stem, not just the color.

Versus pH lockout. These are usually the same event. Iron deficiency is the most pH-driven symptom on the plant.

How long until it looks better#

The yellow leaves will green up partially, unlike most deficiencies, because the tissue is alive and simply never got its chlorophyll built. Expect partial, not full recovery on existing leaves.

After a pH correction, expect visible improvement in new growth within four to seven days. The newest leaf at the top should emerge a normal green rather than lemon yellow. If the plant is still capping yellow after ten days at a verified pH of 6.2 in soil, look at drainage and root health next, because saturated roots block iron just as effectively as bad pH.