Short answer. Work like a clinician, not a shopper. Note where the symptom started (oldest leaves, newest growth, or everywhere), what it looks like and how fast it is moving. Then rule out the cheap, common causes in order: root zone (water, oxygen, roots), pH, EC, environment and light, pests and disease. Only after that blame a single nutrient. Change one variable, then judge the result on growth that appears after the change.
What's happening
Most "deficiencies" growers see are not a shortage of fertiliser in the pot. They are a plant that cannot take up what is already there, because the roots are drowning, rotting, too cold, or sitting at a pH where an element is unavailable. Adding more nutrient to that plant raises EC, stresses the roots further and makes the next symptom worse. That is why every GenetixBay diagnosis tree starts below the soil line.
Two pieces of plant biology do most of the diagnostic work (Established):
- Mobility. Mobile nutrients (N, P, K, Mg) are moved out of old leaves to feed new growth, so their shortage shows first on lower, older leaves. Immobile or poorly mobile nutrients (Ca, Fe, Mn, Zn, Cu, B, S to a degree) cannot be moved, so their shortage shows first on new growth. Controlled cannabis trials match this pattern: N, P, K and Mg deficiency began on lower foliage; Ca, B, Cu, Fe, Mn and Zn began on upper or expanding foliage [1][2].
- Pattern. Uniform symptoms across the whole room point to something shared (water, pH, feed, climate, light). Symptoms on one plant, or spreading plant to plant, point to that plant's roots, a pest or a pathogen. Symptoms only under the hottest part of the light point to light or heat.
How to recognise it (the four questions)
- Where on the plant? Old/lower leaves, new/top growth, leaves nearest the light, random, or the whole plant.
- What does it look like? Uniform yellowing, interveinal yellowing (veins green), spots (with or without halos), margins and tips burnt, curling (up, down, clawing), stippling (tiny pale dots), powder, webbing, wilting.
- How is it moving? Up the plant, down from the top, outward from one plant, or static.
- What changed in the last 7–14 days? New feed, new water source, transplant, heat wave, new light or height, new clones in the room.
Write the answers down. A plant journal turns guesses into evidence.
Numbers that matter
| Parameter | Target | Notes |
|---|---|---|
| Root-zone pH, hydro/DWC | 5.5–6.0 [3] | Grower practice often allows drift to 6.2 |
| Root-zone pH, soilless (coco, rockwool) | 5.5–6.5 [3] | Below 5.5, risk of Mn toxicity; above 6.5, P, Fe and Mn availability falls [3] |
| Root-zone pH, mineral soil | 6.0–7.0 | Grower practicebeing reviewed |
| Feed EC, veg | ~1.0–1.8 mS/cm (500–900 ppm500; 700–1260 ppm700) | Grower practice; cultivar and stage dependent. Llewellyn et al. used 1.8 dS/m in veg DWC [2] |
| Runoff/root-zone EC vs input | within ~±0.5 mS/cm of input | Grower practice; rising runoff EC = salt build-up |
| Nutrient solution temperature | ~18–22 °C (65–72 °F) | Warmer water holds less oxygen and favours Pythium [5] |
| Dissolved oxygen | ≥ 6 mg/L, ideally near saturation (~8 mg/L at 25 °C) | Extension guidance for hydro crops [5]being reviewed |
| Air temperature | ~22–28 °C (72–82 °F) lights-on | Grower practice; see environment articles |
| Time for new growth to show a fix | 5–10 days | New leaves only; damaged leaves do not recover |
What to check first
Cheapest and most likely first. Do not skip ahead.
- Is it actually a problem? Lower fan leaves yellowing in late flower is normal senescence. One or two old leaves dying in veg is not an emergency. Purple stems on a single genetic line may simply be genetics.
- Water and roots. Pot weight (too heavy for days = overwatered; very light = underwatered). Drainage holes clear. In hydro: smell the reservoir, look at the roots (white and firm good; brown, slimy, smelly bad), check solution temperature and air pump.
- pH. Measure input and runoff (or reservoir). Calibrate the meter first (two-point, pH 7 and pH 4 buffers). A meter that has not been calibrated in a month is a guess.
- EC. Input and runoff/reservoir. Too high = burn, clawing, slow growth. Too low = genuine hunger.
- Environment. Air temperature, RH, VPD, night temperature, airflow, and canopy temperature under the light.
- Light. Distance or measured PPFD at the top of the canopy. Bleaching or upward taco near the light is light/heat stress, not a nutrient.
- Pests. Hand lens (10–20×) on the undersides of leaves, especially new growth. Yellow sticky cards. Shake a branch over white paper. Spider mites can complete a generation in under a week when warm [6].
- Disease. Powder, spots with halos, rotting stems, browning roots, wilting with moist media.
- Only now: a single-nutrient deficiency or toxicity. Match the pattern to the controlled-trial descriptions [1][2]. Consider a tissue test if the problem is persistent or commercial.
How to fix it
- Change one thing at a time. If you correct pH, change feed and move the light on the same day, you will never know which one worked.
- Fix the root zone before you feed. Correct watering rhythm, aeration, temperature and pH, then wait 5–7 days.
- In hydro, a fresh reservoir is a cheap reset. Mix to target EC and pH; it removes drift and imbalance in one move.
- Salt build-up or lockout: flush the medium, not the plant. If runoff EC is far above input or runoff pH is well out of range, a root-zone flush resets it: 2–3× the container volume of a light solution (coco/rockwool, EC ~0.4–0.8 mS/cm with Ca and Mg) or pH-adjusted plain water (soil), then check runoff EC and resume normal feeding. Any stage; Grower practice (see the root-zone flush). Pre-harvest flushing for taste is a different thing, and a Myth.
- In soil, slow down. Soil buffers changes; don't flush a pot that is already waterlogged, and don't flush repeatedly. Correct pH of inputs and allow drying between waterings.
- Remove nothing yet. Leave damaged leaves until you have confirmed the cause; they tell you whether the problem is still spreading.
How to confirm it worked
- New leaves that form after the fix are normal in colour and shape (5–10 days).
- The symptom stops advancing up or down the plant. Old damage stays; that is expected.
- Runoff or reservoir pH and EC stay in range for at least two checks.
- For pests: counts on sticky cards and leaf samples fall over two consecutive weekly checks.
Common mistakes
- Treating the oldest damaged leaves as the test. They will never turn green again; watch new growth.
- Adding Cal-Mag, then bloom booster, then micronutrients in a week. EC climbs, roots burn, symptoms multiply.
- Trusting an uncalibrated pH pen or pH test drops alone in hydro.
- Diagnosing from one photo under a coloured grow light. Look under white light.
- Ignoring the calendar: late-flower fade, autoflower stress and genetic colour are not problems.
Evidence
- Mobility-based diagnosis (old vs new growth) is Established plant nutrition and matches cannabis-specific trials in sand culture [1] and DWC hydroponics [2]. Note that in Llewellyn et al. Mn deficiency produced no visible symptoms, while in Cockson et al. it produced bright netted interveinal chlorosis on upper foliage [1][2]: cultivar and system matter.
- pH ranges for hydro and soilless are Supported by horticultural literature cited in cannabis research [3]; there is little cannabis-specific pH dose–response work.
- "Root zone and pH first" is Supported (consistent with plant physiology and pathogen literature [4][5]) and is strong grower consensus.
- EC targets are Grower practice; nutrient-rate trials (e.g. N rate) exist but EC optima vary by cultivar and system.
