Iron vs nitrogen chlorosis: how to tell

Two of the most common plant yellowing problems are iron chlorosis and nitrogen deficiency -- and they look nothing alike if you know where to look. Treating them the wrong way makes the problem worse: applying iron to a nitrogen-deficient plant does nothing, and applying nitrogen to an.

Two of the most common plant yellowing problems are iron chlorosis and nitrogen deficiency — and they look nothing alike if you know where to look. Treating them the wrong way makes the problem worse: applying iron to a nitrogen-deficient plant does nothing, and applying nitrogen to an iron-chlorotic plant in high-pH soil may actually acidify the soil slightly, which helps the iron problem — but only by accident, not by design.

Iron chlorosis

What it is

Iron chlorosis is not, in most cases, a shortage of iron in the soil. It is an unavailability of iron to the plant, caused by high soil pH. Per Penn State Extension, when soil pH rises above approximately 6.5–7.0, iron converts to insoluble forms that roots cannot absorb, even when iron is present in the soil.

Per NC State Extension, plants with high iron requirements — pin oak (Quercus palustris), blueberry, azalea, rhododendron, red maple, river birch, and others — are most commonly affected. Species that prefer acidic soil conditions develop chlorosis when planted in or near alkaline fill, concrete leachate, or lime-treated soil.

Symptoms

Location: New, young leaves — the leaves at the growing tips. Per Penn State Extension, because iron is an immobile nutrient (it cannot be relocated from old to new tissue), new growth suffers most. Old leaves may still look green while the new growth is severely yellowed.

Pattern: Interveinal chlorosis — the tissue between the veins turns yellow to near-white while the veins themselves remain distinctly green. The contrast between the green veins and yellow interveinal tissue is the defining feature. In severe cases, the entire leaf may turn pale yellow to white, and the green veins may be only slightly visible.

Distribution: Typically the entire newest flush of growth at the branch tips is affected, while lower, older leaves remain darker green.

Confirmation

Per Penn State Extension, a quick field test: obtain the soil pH near the plant's root zone. A pH at or above 6.8–7.0 strongly supports iron chlorosis as the diagnosis. In my own Long Island sandy loam (pH typically 6.2–6.5 without limestone), iron chlorosis is less common than in higher-pH soils — but any plants placed near a concrete foundation or over a calcareous rubble layer can develop it.

Plants most commonly affected

Per Penn State Extension:

• Pin oak: the classic iron chlorosis tree in alkaline soil or near concrete
• Blueberry: highly acid-demanding; develops chlorosis above pH 5.5
• Rhododendron and azalea: prefer pH 4.5–5.5; chlorosis above 6.5
• Red maple, sweet gum, bald cypress: moderate sensitivity
• River birch: moderate sensitivity

Correction

Per Penn State Extension:

1. Soil acidification (long-term): Elemental sulfur applied to the root zone lowers soil pH over 3–6 months as soil microbes oxidize it to sulfuric acid. Rate depends on current pH and soil type. Per Penn State Extension, this is the only durable long-term correction.

1. Iron chelate (short-term): Chelated iron (Fe-EDTA, Fe-DTPA, Fe-EDDHA) provides iron in a form available at higher pH. Soil drench or foliar spray provides temporary correction. Per Penn State Extension, chelates must be reapplied as the pH issue persists.

1. Trunk injection of iron: For large trees (pin oak with widespread chlorosis), direct vascular injection of iron solution provides a rapid, season-long correction. Per Penn State Extension, injections need to be repeated annually or biannually while the pH issue persists.