"The leaf is yellowing — must be nitrogen." But yellow leaves are a symptom, not a diagnosis. Iron chlorosis, magnesium chlorosis, nitrogen chlorosis, and even viral mosaic all look similar at first glance — but they differ in where on the plant the symptom appears, which leaves are affected, what pattern it follows, and in what sequence. Nitrogen and magnesium are mobile elements: deficiency shows on old leaves first. Iron and calcium are immobile: deficiency shows on new growth. This distinction is the first and most important step in diagnosis.
Quick glossary: Chlorosis — yellowing of leaf tissue due to chlorophyll deficiency; can be interveinal (tissue yellows between green veins) or general (whole leaf yellows uniformly). Necrosis — tissue death; brown or black spots, dried margins; follows chlorosis or occurs independently. Element mobility — the plant's ability to redistribute an element from old tissue to new when it is deficient; mobile elements (N, P, K, Mg, S) move from old leaves, so deficiency appears at the lower canopy first; immobile elements (Ca, Fe, Mn, Zn, Cu, B) stay where they are, so deficiency appears on new leaves and growing tips.
Step One: Where on the Plant
Before looking at colour or pattern — determine where the symptom is:
Symptom on old lower leaves → deficiency of a mobile element. The plant has drawn the element from old tissue for new growth. Candidates: N, P, K, Mg, S.
Symptom on new upper leaves or growing tips → deficiency of an immobile element, or pH lock-out. The plant cannot redistribute this element. Candidates: Ca, Fe, Mn, Zn, Cu, B.
Symptom uniformly across the whole plant simultaneously → general osmotic stress, extreme temperature, systemic toxicity (excess Na, Cl, or EC that is too high overall).
This rule is not absolute — in severe acute deficiency, symptoms can appear across the entire plant. But for early or moderate symptoms, leaf position gives 80% of the answer before any colour analysis is needed.
Symptom Map: What Looks Like What
Uniform yellowing from the bottom up (general chlorosis of old leaves):
Nitrogen deficiency — the most common. Old leaves yellow uniformly from the bottom tier upward. EC is often low, or the plant is growing actively with insufficient N. Thin stems, overall growth slowed.
Interveinal chlorosis from the bottom (tissue between green veins yellows):
Magnesium deficiency — veins stay green, tissue between them yellows. Characteristic "herringbone" or "window pane" pattern. Often caused by excess K or Ca (antagonism) rather than a genuine shortage of Mg in the solution.
Interveinal chlorosis from the top (new leaves):
Iron deficiency — veins green, between them pale yellow or nearly white. New leaves affected first. Most common cause: pH above 6.5 and the wrong chelate form (EDTA at pH 6.5+), not a shortage of iron in the recipe.
Manganese deficiency — similar to Fe but appears at a slightly lower pH than Fe, and often when Fe in the recipe is excessive (Fe→Mn antagonism).
Distortion of new leaves, necrosis of margins or growing tips:
Calcium deficiency — new leaves curled, deformed, margins necrotic, the growing tip may die back. Ca is immobile and cannot be redistributed. Check transpiration (low transpiration → low Ca transport) and EC.
Boron deficiency — similar to Ca but leaves are thicker and more brittle, growing tips deformed in a characteristic way.
Marginal chlorosis (leaf edges yellow or brown):
Potassium deficiency at moderate severity — margins yellow, then brown. But the same symptom appears with excess NH₄⁺, Na, or Cl — toxicity. Check the recipe composition and Na/Cl content in the water.
Bronze or purple tints:
Phosphorus deficiency — lower leaf surface takes on a purple or violet hue. Often at low solution temperature (<16°C) where P becomes less available.
Sulphur deficiency — young leaves yellow uniformly (similar to N but top-down rather than bottom-up).
Three Questions Before Any Diagnosis
Before adding any element:
What is the pH? At pH > 6.5 — Fe, Mn, Zn, Cu become less available regardless of the recipe. Interveinal chlorosis on new leaves at pH 6.8 = pH lock-out, not a deficiency.
What are the element ratios in the recipe and source water? Mg deficiency with normal Mg in the recipe — check Ca:Mg ratio (antagonism). K deficiency with normal K — check NH₄⁺ and Mg.
When did the symptom appear? Suddenly after a recipe change or pH adjustment — most likely a chemical upset. Gradually over 1–2 weeks — a slow-building deficiency or accumulating antagonist.
Three Mistakes That Cost the Most
Adding N for any yellowing. Yellow leaves are not a nitrogen deficiency diagnosis. Interveinal chlorosis on new leaves with yellowing — that is Fe or Mn, not N. Adding N makes the situation worse by raising EC and shifting the NH₄⁺ ratio.
Ignoring pH as the first check. 80% of visible micronutrient deficiencies with a normal recipe come from pH being above the correct range. The first check for any chlorosis on new leaves is to measure pH. Not buy new fertilisers, not change the recipe — measure pH.
Diagnosing from a single photo without context. "What deficiency is this?" from a photo with no information about pH, EC, water composition, substrate, and recipe is unreliable. Context matters more than the photo. The symptom is a clue; the system parameters are the diagnosis.
How to Know the Diagnosis Is Correct
After correcting the cause (adjusting pH, changing the recipe, resolving antagonism) — new leaves that grow after the intervention are healthy. Already-affected leaves do not recover — chlorosis and necrosis remain on them, but new growth appears without symptoms. If new leaves grow with the same symptoms, the cause has not been corrected.
For deeper understanding: Ion Antagonism: Why You Can't Just "Add More Potassium" — explains why most "deficiencies" on a normal recipe are actually antagonisms from incorrect element ratios.