Why do most deficiency diagnoses get it wrong?
The most common mistake is looking at the symptom without asking where it is. Interveinal chlorosis on an old leaf and the same pattern on a new leaf can look identical — but one is a Mg deficiency (mobile) and the other is Fe or Mn (immobile). Treating them the same usually makes things worse. The second mistake is adding more nutrients without checking pH first: outside the 5.5–6.5 range in solution, Ca, Fe, and Mn become practically unavailable even when they're physically present (Veazie et al., 2025, DOI 10.1002/agg2.70044).
What's the difference between a mobile and an immobile nutrient?
The plant can shuttle certain nutrients from old tissue to the growing tip. Those are the mobile ones: nitrogen, phosphorus, potassium, magnesium. When they run short, the plant pulls them from its own older leaves, so symptoms show up at the bottom first. Immobile nutrients — calcium, iron, manganese — can't be relocated once they're built into tissue. When they're lacking, damage shows up on the newest leaves or growing tips.
- Symptoms on old leaves (progressing bottom-up): suspect N, P, K, or Mg first.
- Symptoms on new leaves or growing tips: suspect Ca, Fe, or Mn first.
- Symptoms across the whole plant simultaneously: think toxicity, severely off pH, or excessive EC — not a simple deficiency.
- Never diagnose from a single symptom: always confirm pH and EC before making any correction.
What does a Calcium deficiency look like?
Calcium is immobile: damage appears first on young leaves and growing tips. Llewellyn et al. (2023, DOI 10.3390/plants12030422) documented the progression in hydroponic cannabis: yellow rings around brown necrotic centers, followed by irregular necrosis along the margins, and upward leaf curl. The working optimal range is 100–160 ppm Ca during veg and early bloom; in the second half of flower the demand drops (Saloner & Bernstein, 2023, DOI 10.3390/agronomy13122865).
What does a Magnesium deficiency look like?
Magnesium is mobile and sits at the center of every chlorophyll molecule. Deficiency starts on old leaves as interveinal chlorosis: veins stay green while the tissue between them turns yellow. Morad & Bernstein (2023, DOI 10.3390/plants12142676) documented drops in photosynthesis, transpiration, and stomatal conductance under very low Mg. The confirmed optimal range is 35–70 ppm Mg in solution. Don't confuse this old-leaf chlorosis with Fe or Mn deficiency, which attacks new growth.
What do Iron and Manganese deficiencies look like?
Both are immobile and share the IRT1 transporter, so they compete with each other. The recommended working Fe:Mn ratio is roughly 3:1 to 5:1 to prevent excess of one from inducing a deficiency of the other. Fe deficiency causes interveinal chlorosis on young leaves (green veins, pale yellow tissue). Mn deficiency shows a similar pattern with finer 'netting'; Cockson et al. (2019, DOI 10.3390/app9204432) documented that Mn chlorosis is interveinal yellow, without the reddish tint it's sometimes attributed.
What should you check before concluding it's a deficiency?
- Solution pH between 5.5 and 6.5 (Veazie et al. 2025). Outside that range, Ca, Fe, and Mn are locked out even if your solution contains them.
- EC: excessive EC can cause nutrient burn symptoms that mimic deficiencies.
- High K antagonizes Ca and Mg: excess potassium displaces divalent cations (Bevan et al. 2021). If K exceeds ~175 ppm, check your Ca and Mg levels.
- Feeding history: the same Ca deficiency hits harder during veg and early flower (high demand) than in late bloom, when uptake naturally drops.
- Check at least 3 leaves before deciding: an isolated symptom can be physical damage, light burn, or nutrient splash.
