Signs of Nutrient Deficiency in Trees: What Yellow Leaves Are Telling You

It’s midsummer and one tree in the yard looks wrong. The leaves are pale, or yellow between green veins, and it isn’t fall. The natural thought is that the tree is hungry and needs fertilizer. Sometimes that’s true. But yellow leaves are a symptom with several possible causes, and the pattern of the yellowing says a lot about which one you’re looking at. This guide is for homeowners trying to read that pattern before anything gets applied to the tree.

The short answer: a tree short of a nutrient often shows it as chlorosis, leaves that turn pale or yellow because they lack chlorophyll, or as weaker growth: shorter new shoots and smaller leaves. Where the yellowing starts is the first clue. Yellowing that starts on the newest leaves points toward iron, while yellowing that starts on older leaves points toward nitrogen, magnesium or potassium. But compaction, root damage, poor drainage, watering problems, insects and disease can all look the same, and high soil pH can lock nutrients away, so a soil test should come before any fertilizer.

What chlorosis looks like

Chlorosis is a yellowing of leaves that should be green, caused by a lack of chlorophyll (The Morton Arboretum). It tends to progress in stages:

• Early: leaves look lighter green than they should in summer.
• Then: the leaf yellows, often with the veins staying green while the tissue between them pales. That pattern is called interveinal chlorosis.
• Severe: leaves turn nearly white, edges scorch brown, brown spots appear between the veins, leaves come in smaller, and they wither and drop early.

In conifers, the needles yellow overall, and in severe cases turn brown and drop, and twigs and branches may die back. Those stages come from the Morton Arboretum’s guidance on chlorosis.

Yellowing isn’t the only sign, and it isn’t always the first. The University of Maryland Extension notes that visible deficiency symptoms are seldom seen in woody landscape plants, and that below-normal shoot growth and leaf size can be a more accurate indicator. New shoots that are shorter, and leaves that are smaller than the tree used to produce, are worth noticing.

Where the yellowing starts is the first clue

Where on the tree the symptoms appear is useful. According to the University of Maryland Extension:

• Newest leaves first: iron deficiency shows up first in young leaves.
• Older leaves first: nitrogen, magnesium and potassium deficiencies show up first on older leaves.

So stand back and look at where the pale leaves are. Are they at the tips of the branches, on this year’s growth? Or lower and further inside the canopy, on older leaves? Is it the whole tree, or one side or one limb?

Treat this as a pointer, not a diagnosis. The same Extension guidance says nutrient symptoms are very difficult to tell apart from symptoms caused by insects, disease, poor soil conditions and improper watering.

When the nutrient is in the soil but the tree can’t use it

Here’s the part that surprises most people: a tree can be short of iron while standing in soil that contains plenty of it.

Soil pH controls how available some nutrients are. The University of Maryland Extension notes that iron is most available in acidic soils, between about pH 4.5 and 6.0, and that above pH 6.5 iron may be present in adequate amounts but in a form the tree can’t use. The Morton Arboretum describes the same effect for both iron and manganese, which are most available between about pH 5.0 and 6.5.

Some trees are much more prone to this than others. The Morton Arboretum lists pin oak and red maple among the trees commonly affected, along with white oak, river birch, tulip tree, sweet gum, bald cypress and white pine.

If pH is the cause, a general fertilizer won’t fix it, because the problem is that the tree can’t take up the iron that’s already there. That’s one reason testing comes before treating.

Problems that look exactly like a deficiency

Plenty of yellow trees aren’t short of anything. The University of Maryland Extension lists poor root growth, root rot, root damage (including damage from over-fertilizing or digging deep around the roots), soil nematodes and poor drainage as causes of chlorosis. The Morton Arboretum adds drought, soil compaction and flooding.

In practice, that means it’s worth asking:

• Has anything changed around the roots? Construction, a new patio or driveway, trenching, grading or heavy equipment over the root zone.
• Is the ground compacted or wet? Soil that stays soggy or has been packed down can starve roots of air.
• How has the tree been watered? Both drought and too much water can show up as yellow leaves.
• Are there signs of insects or disease? Stressed trees and pests tend to go together, and it’s worth understanding how stress opens a tree up to pest problems.

Why a soil test comes before fertilizer

It’s tempting to go straight to a bag of fertilizer. The trouble is that fertilizer answers only one of the possible causes, and it can make things worse. The Morton Arboretum notes that excess potassium, magnesium and phosphorus in the soil can contribute to chlorosis, and advises against fertilizing a chlorotic tree with potassium and phosphorus unless a soil test shows a deficiency. The University of Maryland Extension includes root damage from over-fertilizing among the causes.

A soil test shows the pH and which nutrients are present, which tells you whether the tree is short of a nutrient, can’t reach one, or has a problem that fertilizer won’t touch. The University of Maryland Extension calls soil tests an important tool for diagnosing a deficiency. The Morton Arboretum’s advice is to rule out weather and care problems such as watering and drainage first, and if those check out, to consider a soil test. There’s more on why store-bought fertilizer is a gamble on a mature tree in a separate article.

American Tree Experts lists soil testing among its services on its Google Business Profile, and at least one long-time customer’s Google review mentions using Brian Rombough “for many years to feed and trim all of our trees.” When testing does point to a nutrient problem, the answer is a plan built around that tree and that soil, which is what the company’s nutrient management for trees in Montclair is for.

What to note before an arborist looks at the tree

A few observations make the first visit much more useful. (For what happens during that visit, see how arborists diagnose tree nutrient problems.)

• Which leaves: new growth at the tips, or older leaves further in?
• The pattern: yellow with green veins, evenly pale, or spotted and scorched at the edges?
• How much of the tree: all of it, one side, or one limb?
• When it started: this season, or getting worse over several years?
• What’s changed: construction, grading, a new lawn or bed over the roots, a change in watering.
• What’s been applied: any fertilizer, weed-and-feed on the lawn beneath the tree, or other treatments.

If a tree is pale this year, it’s worth having the cause confirmed before the season ends, rather than guessing at a treatment and waiting another year to see whether it worked.