Is It Really a Nutrient Deficiency? Usually Not

Pale, yellow or oddly-coloured leaves get treated with fertiliser more often than any other symptom, and most of the time the soil already contains the nutrient.

The plant cannot get at it. That is a different problem, and adding more of what is already there does not solve it — often it makes it worse, because fertiliser salts damage the roots doing the absorbing.

Before reaching for a bottle, there are two questions worth asking.

Question one: old leaves or new leaves?

This is the whole diagnostic framework, and it comes from a single fact: some nutrients move around inside a plant and some do not.

Mobile nutrients — nitrogen, phosphorus, potassium, magnesium. When supply runs short, the plant strips them out of old leaves and sends them to new growth. Symptoms appear on the oldest leaves first.

Immobile nutrients — iron, calcium, sulphur, manganese, boron. Once placed, they stay. A shortage shows up wherever tissue is being built. Symptoms appear on the newest leaves.

So:

The magnesium and iron patterns look alike — both are interveinal — and the position on the plant is what separates them. Old leaves: magnesium. New leaves: iron.

Why leaves turn yellow takes the same "start with where" approach and covers the non-nutritional causes in more depth.

Question two: is anything else wrong?

Because a real shortage in the soil is uncommon in a garden that has ever had compost added, and the following produce identical symptoms:

• Soil pH. The most frequent cause of iron chlorosis by a wide margin. Iron is abundant in almost all soils and becomes unavailable above about pH 7. Pin oaks, blueberries, azaleas, rhododendrons and citrus in alkaline soil go yellow between the veins while surrounded by iron. Adding iron to alkaline soil largely fails; the pH is the variable.
• Waterlogged roots. Roots that cannot breathe cannot absorb, so a drowned plant shows nitrogen deficiency symptoms in wet, nitrogen-rich soil. This is the single most common false positive.
• Root damage or restriction. Pot-bound, girdled, voles, construction, compaction.
• Cold soil. Uptake slows sharply below about 12 °C. Spring purpling on maize and tomatoes looks exactly like phosphorus deficiency and disappears as the soil warms, without intervention.
• Drought. Nutrients move to roots dissolved in water. Dry soil is functionally deficient soil.

The order to work in

1. Check the soil moisture and the roots. Most of the false positives above are visible here.
2. Check pH if the pattern is interveinal on new growth. A cheap test is enough to rule it in or out.
3. Consider the season. Cold soil in spring, and old leaves yellowing in autumn, are both normal.
4. Look at the pattern and the position using the table above.
5. Then, if it still looks like a deficiency, do a soil test rather than guessing. A test costs little and tells you which of fourteen nutrients is actually short.

Feeding without a diagnosis

The main risks of guessing:

• Excess nitrogen produces soft growth, delayed flowering and fruiting, more aphids, and blossom end rot in tomatoes.
• Excess phosphorus interferes with iron and zinc uptake and runs off into waterways. Most garden soils that have been composted for years are already high in it.
• Epsom salts, the universal internet remedy, supply magnesium that competes with calcium and potassium uptake. Useful only for a confirmed magnesium deficiency.
• Salt build-up in containers from repeated feeding, which produces brown leaf tips — a symptom people then treat with more feeding.

The one that usually is real

Nitrogen, on a lawn, on a heavily-cropped vegetable bed, or on anything in a container that has been in the same soil for a season. Nitrogen leaches, plants use a lot, and container mix has almost no reserve.

Uniform pale green across older growth, on a plant in well-drained soil that has not been fed — that one, feed. Almost everything else deserves the checks first.