Don't look at the colour — look at where (reading a deficiency)

v1 — 2026-07-14.

You see a yellow leaf, you search "yellow leaf", and you get twenty different answers, each with a photo and each perfectly confident. The problem is that you are staring at the least informative piece of evidence.

The colour tells you little. Where it appears tells you a great deal.

The plant cannibalises itself

Nutrients split into two groups depending on whether the plant can move them around inside once they are in place. A nutrient is mobile if the plant is able to move it from one part of the plant to another as it is needed.

When a mobile nutrient runs short, the plant doesn't sit still: it scavenges it from the older growth and sends it where demand is greatest, which is the new growth. It is doing triage — sacrificing the old to save the new. That is why the symptom shows up low.

Immobile nutrients can't do that: they get locked in place, and that is where they stay. If one runs short, the plant cannot steal it from anywhere, so the new shoot pays. The symptom shows up high.

That single fact turns an ugly leaf into a diagnosis.

Where to look first

• Symptom on old / lower leaves → a MOBILE nutrient. The mobile ones are nitrogen, phosphorus, potassium and magnesium (also chlorine, molybdenum and nickel).
• Symptom on new growth / upper leaves → an IMMOBILE nutrient. The immobile ones are calcium, sulfur, boron, copper, iron, manganese and zinc.

That alone halves the problem before you judge a single shade of yellow. Some textbook examples:

• Nitrogen (mobile): general yellowing that starts on the old leaves.
• Iron (immobile): interveinal chlorosis — yellow with the veins still green — on the young leaves.
• Calcium (immobile): distorted or curled young leaves, often with tip burn.

Look at the iron/nitrogen pair: both are "yellow". The difference isn't the colour, it's which end of the plant.

Before you feed: check the pH

And here is the trap that catches the most people. A deficiency symptom does not prove the nutrient is missing.

When pH is too acidic, elements such as potassium, calcium and magnesium may exist in a chemical form that roots cannot absorb, even if these elements are present in sufficient amounts. And the other way round: at alkaline pH, iron, manganese, zinc and boron sit in forms that are unavailable.

Read that again: present in sufficient amounts, and the plant still starves. That is not a deficiency, it's a lockout — and the cure is not more fertilizer, it's fixing the pH. Feeding a plant that cannot pick the food up only adds salts and pushes pH further off.

So the right order is: pH first, fertilizer second. The long version is in pH does not feed your plant, and the pH dosing calculator helps you correct it without overshooting.

The honest limits of this method

The mobility rule is a compass, not a microscope. It's worth saying where it fails:

• It is quite easy to mistake a nutrient deficiency symptom for other abiotic problems, such as pesticide drift, or for a plant disease.
• Problems rarely arrive alone: several shortages at once overlap and muddy the picture.
• A photo of a leaf is not a diagnosis. Mobility tells you which half of the table to search, not which exact element it is.

What you can always do: look at where it appears, check pH and watering before touching the fertilizer, and change one thing at a time so you know what actually worked.

The summary

Colour is ambiguous; position is not. If the symptom starts low, suspect a mobile nutrient (N, P, K, Mg) — the plant is robbing its own old leaves. If it starts high, suspect an immobile one (Ca, S, B, Cu, Fe, Mn, Zn) — it had nobody to rob. And before you feed, check pH: a nutrient that is present but locked out disguises itself as a shortage, and feeding it more only makes things worse.

Sources

• Shober, A.L. & Denny, G.C. (2025), "Identifying Nutrient Deficiencies in Ornamental Plants", University of Delaware Cooperative Extension (retrieved 2026-07-14)
• Goldy, R., "Knowing nutrient mobility is helpful in diagnosing plant nutrient deficiencies", Michigan State University Extension (retrieved 2026-07-14)