Leaves are the plant's dashboard. When something is wrong nutritionally, the plant tells you — through color changes, spots, curling, and where on the plant the symptoms appear.
The single most important diagnostic question: where did it start — old growth at the bottom, or new growth at the top? That one answer eliminates half the possibilities.
The mobile vs. immobile rule
This is the master key to nutrient diagnosis.
Mobile nutrients (nitrogen, phosphorus, potassium, magnesium) can be moved by the plant from old leaves to where they're needed. When a mobile nutrient runs low, the plant pulls it from the bottom first. Symptoms appear on old growth and move upward.
Immobile nutrients (calcium, iron, manganese, boron, zinc, copper, sulfur) cannot be relocated once deposited in a leaf. When an immobile nutrient runs low, only the newest growth is affected. Symptoms appear at the top of the plant.
This rule alone tells you whether you're dealing with N/P/K/Mg or Ca/Fe/Mn and friends — before you read a single chart.
Before you diagnose: check pH first
Most "deficiencies" are actually pH lockout. The nutrients are present in the medium but chemically unavailable because the pH is outside the absorption window.
The correct ranges:
| Medium | pH range |
|---|---|
| Soil | 6.0–6.8 |
| Coco coir | 5.5–6.3 |
| Hydro (DWC/NFT) | 5.5–6.1 |
| Living soil / no-till | 6.2–6.8 |
How to check: test runoff pH. Water with pH'd water (target the middle of your range), collect the runoff, measure it. If runoff is more than 0.5 below your input, the root zone is acidifying — likely salt buildup. If it's more than 0.5 above, something is buffering up (limestone in the mix, hard water, or degrading organics).
If pH is out of range, correct it first and wait 3–4 days before treating any specific deficiency. Adding more nutrients to a locked-out plant makes it worse.
For a complete pH guide, see pH for beginners. For lockout recovery, see nutrient lockout recovery.
The deficiency chart
Mobile nutrients (symptoms start at the bottom)
Nitrogen (N) — the most common deficiency
| What you see | Where | Details |
|---|---|---|
| Even yellowing of entire leaves | Bottom leaves first, moves up | Starts as light green, progresses to full yellow |
| Leaves drop off after yellowing | Bottom | Plant is cannibalizing stored N |
| Small, pale new growth | Top (severe cases) | The plant has exhausted its reserves |
The fix: increase nitrogen in your feed. In veg, this means upping your base nutrient. In flower, some yellowing of bottom leaves is normal (the "fade") — don't chase it unless it's climbing rapidly past the lower third.
Don't confuse with: overwatering (drooping but green), light deprivation (even yellowing but all levels at once), or the natural fade in weeks 7–8 of flower.
Phosphorus (P)
| What you see | Where | Details |
|---|---|---|
| Dark, dull leaves with blue-green tint | Bottom/mid | First sign before obvious damage |
| Purple stems and petioles | Throughout | Not always P — some strains are naturally purple |
| Red, bronze, or rust-brown patches | Bottom leaves | The classic P deficiency pattern |
| Slow bud development | Flowers | In bloom, P is critical for flower formation |
The fix: increase bloom nutrients. Check that pH allows P absorption (it locks out below 5.5 and above 7.0). Cold root zones (below 65°F / 18°C) dramatically reduce P uptake — warm your reservoir or medium first.
Don't confuse with: cold temperatures (which cause purple stems via anthocyanins, not P deficiency), or magnesium deficiency (which shows as interveinal yellowing, not dark dullness).
Potassium (K)
| What you see | Where | Details |
|---|---|---|
| Yellow-brown scorched leaf edges | Bottom-mid | The "burnt edge" look — very distinctive |
| Brown, crispy leaf margins | Progresses upward | Edges die while center stays green |
| Weak stems, slow growth | Whole plant | K is critical for water regulation and transport |
The fix: increase K in your feed — bloom nutrients are K-heavy. Flush first if you suspect salt buildup (burnt tips plus burnt edges usually means lockout, not deficiency).
Don't confuse with: nutrient burn (which starts at the very tips, not the edges), heat stress (which causes upward leaf curl before edge damage), or wind burn (which is localized to fan-facing leaves).
Magnesium (Mg)
| What you see | Where | Details |
|---|---|---|
| Yellow between veins, veins stay green | Bottom-mid leaves | The "interveinal chlorosis" pattern |
| Rust-colored spots in the yellow areas | Bottom leaves (advanced) | Spots develop as yellowing worsens |
| Leaves curl upward at edges | Bottom-mid | Less common but diagnostic |
The fix: Cal-Mag supplement, or Epsom salt (magnesium sulfate) at 1 teaspoon per gallon. Very common in coco coir, which has a natural calcium/magnesium affinity that pulls these from the nutrient solution.
Don't confuse with: iron deficiency (which shows on NEW growth, not old), or pH lockout (which can mimic multiple deficiencies simultaneously — check pH first).
Immobile nutrients (symptoms start at the top)
Calcium (Ca)
| What you see | Where | Details |
|---|---|---|
| Rusty brown spots on new growth | Upper leaves | Random spots, not edge-following |
| Twisted, distorted new leaves | Growing tips | Ca is needed for cell wall formation |
| Hollow stems when split open | Stems | Advanced calcium deficiency |
| Blossom end issues | Buds | Rare but possible |
The fix: Cal-Mag supplement (standard dose: 2–5 mL/gal). This is the #1 deficiency under modern LED lights — cooler canopies mean less transpiration, which means less calcium transport from roots to leaves. If you grow in coco under LED, run Cal-Mag as a default, not a treatment.
Don't confuse with: pest damage (which has irregular patterns and may include webbing or insects), or pH-caused nutrient burn (which scorches tips, not random spots).
Iron (Fe)
| What you see | Where | Details |
|---|---|---|
| New leaves emerge yellow-white | Top/newest growth | Veins eventually stay green but leaf tissue is pale |
| Interveinal chlorosis (like Mg, but on NEW growth) | Growing tips | This is the key distinction from magnesium |
The fix: almost always a pH problem, not a true iron shortage. Iron locks out above pH 6.5 in soil and above 6.0 in hydro/coco. Fix pH and the symptoms resolve within a week. Only supplement iron directly if pH is confirmed correct.
Manganese (Mn), Zinc (Zn), and other micros
These are rarer and harder to diagnose individually:
| Nutrient | Key symptom | Where |
|---|---|---|
| Manganese | Tan/gray spots between veins | New growth |
| Zinc | Extremely tight internode spacing, small new leaves | Growing tips |
| Boron | Thick, brittle stems; hollow core | Stems/new growth |
| Copper | Wilting new leaves, dark blue-green | New growth |
| Sulfur | Uniform yellowing of new leaves (like N, but top-first) | New growth |
The fix for all micros: check pH first (almost always the cause). If pH is correct, a quality base nutrient at proper strength provides adequate micros. Micro deficiencies in a well-fed plant at correct pH are genuinely rare.
Overfeeding vs. underfeeding
| Sign | Overfed | Underfed |
|---|---|---|
| Leaf tips | Burnt brown, curling down | Pale, not burnt |
| Color | Very dark green (N toxicity) | Light green to yellow |
| New growth | Clawing downward (N toxicity) | Small, pale, slow |
| Leaf texture | Shiny, waxy | Dull, thin |
Burnt brown leaf tips are the universal sign of overfeeding. If you see them, reduce feed strength to 50% and give plain pH'd water for the next 1–2 waterings before resuming at reduced strength.
For a nitrogen toxicity deep-dive, see nitrogen toxicity.
The three fixes that solve 90% of cases
1. Fix pH. Check runoff. If it's out of range, flush with 2–3x pot volume of pH'd water and resume feeding at your target pH. This alone resolves most "deficiency" symptoms because the nutrients were present but locked out.
2. Add Cal-Mag. Under LED in coco or light mixes, calcium and magnesium are the two most common genuine deficiencies. A Cal-Mag supplement at 2–5 mL/gal solves them both.
3. Reduce feed strength. If you see burnt tips, dark green leaves, or clawing — you're overfeeding, not underfeeding. Cut to 50%, flush, and rebuild.
How to tell deficiency from other problems
- Spots that wipe off → pests or mildew, not nutrition. See pest identification
- Symmetrical patterns across the whole plant → nutrition or pH
- One branch only → physical damage or localized root issue
- Overnight droop with no color change → watering or root zone, not feed. See overwatering/underwatering
- Uniform browning and drying of ONLY the top canopy → light burn, not nutrition
The recovery timeline
Fix one variable at a time and give it 3–4 days.
Damaged leaves do not heal. Judge recovery by new growth only. If new leaves come in green and healthy, your fix worked — even if the old leaves still look terrible. Never remove damaged leaves to "clean up" during diagnosis; they're still photosynthesizing, and removing them adds stress on top of the deficiency stress.