Yellow leaves on houseplants: a nutrient deficiency chart

Start with one question: are the old leaves yellowing, or the new ones? Nutrients that move inside the plant, including nitrogen, magnesium and potassium, cause symptoms on old leaves first. Nutrients that cannot move, including iron, zinc and calcium, show on new growth first. That single split narrows the diagnosis by more than half.

Yellow leaves are the most common plant complaint and the most commonly misdiagnosed. The usual response is to add a general fertiliser, which helps in maybe a third of cases and does nothing in the rest. The pattern of the yellowing tells you far more than the fact of it.

First, rule out the causes that are not nutritional

Before reading the chart, check three things. Overwatering is the single most common cause of yellow leaves on indoor plants, and it looks similar to nitrogen shortage. If the mix is soggy and the yellow leaves feel soft rather than dry, water is the problem, not fertiliser.

Low light causes even, gradual yellowing on the leaves furthest from the light source. Adding nutrients to a plant sitting in a dark corner makes no difference.

Natural ageing accounts for one or two lower leaves yellowing on an otherwise healthy plant. That is normal and needs no action.

The deficiency chart

Work down this table using the location of the symptom first, then the pattern.

The three you will actually meet most often

1. Nitrogen

Even, pale yellowing that starts at the bottom of the plant and moves up. The plant is moving nitrogen out of old leaves to feed new growth, which is why the pattern is so consistent. Any balanced feed corrects it. A water-soluble NPK 19:19:19 at 1 to 2 g per litre works, and you should see new green within a week.

Nitrogen shortage is normal in a plant that has been in the same potting mix for more than a year. Potting mix is not soil; it holds very little nutrient reserve.

2. Iron

Bright yellow leaf with a clear network of green veins, appearing on the newest leaves while older leaves stay green. This is the most distinctive pattern on the chart and the easiest to recognise once you have seen it.

The cause is usually not a lack of iron but alkaline water or mix locking it up. Hard tap water above pH 7.5 causes this reliably. The fix is chelated iron EDTA as a foliar spray at about 1 g per litre, which bypasses the soil chemistry entirely. Response is visible in five to seven days on new growth. Old damaged leaves will not turn green again.

3. Magnesium

When more than one nutrient is short

Mixed symptoms across old and new leaves usually mean the mix is exhausted rather than short of one specific element. In that case a chelated mix micronutrient covering zinc, iron, manganese, copper, boron and molybdenum is more sensible than guessing at individual elements. Apply as a foliar spray at about 1 g per litre.

Adding a humic and fulvic acid to the watering can improves what the roots can take up from an old, tired mix. It is not a fertiliser, but on a plant that has been in the same pot for two years it often produces more visible improvement than adding more feed.

A fortnightly seaweed extract helps a recovering plant push new roots, which is what actually ends the problem.

How to apply a correction to a houseplant

1.    Mix at the lower end of the label rate. Houseplants in small pots burn easily and the margin for error is smaller than in a garden bed.

2.    Spray in the evening or early morning, never in direct sun.

3.    Cover the underside of the leaves. That is where most absorption happens.

4.    Water the pot lightly first if the mix is bone dry, so the plant is not stressed when you feed it.

5.    Wait a full week before judging the result. Nothing works in two days.

Do not correct three things at once. If you spray iron, magnesium and a general feed together and the plant improves, you have learned nothing about what was wrong.

When it is time to repot instead

If a plant has been in the same mix for more than two years, if water runs straight through without wetting the mix, or if you can see a white salt crust on the surface, feeding will not solve it. The mix has broken down and lost structure.

Repot into fresh medium, trim any dead roots, and hold off on fertiliser for three weeks while the plant re-establishes.

Michigan State University Extension sets out a clear diagnostic sequence for this, including how to separate deficiency from root damage. It is a useful cross-check before you treat. The RHS also maintains a good visual reference for the individual deficiencies.

Take a photograph of the leaf when you first notice yellowing, and another a week after treatment. Memory is unreliable about colour, and the comparison tells you whether the diagnosis was right.