Chlorosis Patterns: Why Iron, Magnesium, and Nitrogen Deficiencies Look Different

Chlorosis (yellowing from chlorophyll loss) appears in distinct patterns because nutrients differ in mobility and function.

Iron (Fe): Young Leaves First

• Immobile in plant—cannot translocate from old to new tissue
• Required for chlorophyll synthesis (enzyme cofactor)
• New growth has zero reserves → interveinal chlorosis on youngest leaves
• Old leaves stay green (they have stored Fe)

Magnesium (Mg): Old Leaves First

• Mobile—moves from old to new growth when scarce
• Central atom in chlorophyll (C₅₅H₇₀MgN₄O₆)
• Chlorophyll turns over rapidly; daily Mg needed for replacement
• Plant sacrifices old leaves to feed new → marginal/interveinal chlorosis on older foliage

Nitrogen (N): Old Leaves First

• Highly mobile—major component of amino acids, proteins
• Plant cannibalizes old leaves (proteolysis) to supply growing points
• Uniform yellowing of older leaves; stunted new growth

Why the Difference?

Mobility dictates pattern. Immobile nutrients (Fe, Ca, B) show deficiency in new growth. Mobile nutrients (N, Mg, K, P) show in old growth as the plant reallocates. Chlorophyll's short lifespan (days) means any disruption—Fe for synthesis, Mg for structure, N for protein backbone—quickly becomes visible.

Practical Diagnosis

1. Which leaves yellow first? (New = Fe; Old = Mg, N, K)
2. Pattern? (Interveinal = Fe, Mg; Uniform = N)
3. Soil pH? (High pH locks Fe, Mn, Zn; low pH risks Mg, Ca)
4. Test soil—don't guess.

Chlorosis is a symptom, not a disease. The pattern points to the cause.