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Yellow leaves send more people to nutrient shops than any other symptom, and most of those purchases are unnecessary. In hydroponics the nutrients are usually present. The plant just cannot reach them.
Work through this in order. Do not skip to the symptom table, because the first three checks resolve the majority of cases.
Step 1: Is it just the oldest leaves?
The bottom one or two leaves yellowing and dropping on an otherwise healthy plant is senescence. The plant is retiring shaded, unproductive leaves and moving their nitrogen into new growth. This is normal and correct.
It is only a problem if it is moving up the plant, affecting several leaves at once, or the new growth looks wrong too.
Step 2: Check pH before anything else
This is the step that saves the money. Most apparent deficiencies in hydroponics are lockout, not absence. Nutrients are only chemically available to roots within particular pH bands, and outside those bands they are present in the water but unusable.
Keep the reservoir at 5.5 to 6.5, and toward 5.8 to 6.2 if you want one number.
Check it with a VIVOSUN digital pH and TDS/EC meter set rather than test strips, which are too coarse to distinguish 6.2 from 6.8, and that is exactly the range where iron availability falls off. Correct with a Humboldts Secret pH Up and Down kit a few drops at a time.
The classic case: iron becomes progressively unavailable above about 6.5. A grower sees interveinal yellowing on new leaves, adds iron, sees no change, adds more. The problem was never iron. It was a reservoir sitting at 7.0 because it was topped up with hard tap water.
Correct the pH, then wait five to seven days before concluding anything. Existing damaged leaves will not recover. Judge it on the new growth.
Step 3: Check EC and the roots
EC too low produces genuine, uniform, whole-plant paleness. Common when someone has been topping up with plain water for weeks without rebuilding the solution.
EC too high also causes yellowing, usually with crispy leaf margins alongside it. Salt buildup makes water uptake harder and mimics several deficiencies at once.
Damaged roots cannot feed the plant no matter how good the solution is. Lift a plant and look. Brown, slimy, or foul-smelling roots mean you have a root rot problem wearing a deficiency costume. Fix that first and the yellowing resolves on its own.
Step 4: Read the pattern
If pH, EC and roots all check out, now the pattern is meaningful. The key question is where on the plant it started.
Old leaves first means a mobile nutrient. The plant can move these internally, so it cannibalises old growth to supply new growth. Nitrogen, phosphorus, potassium and magnesium are mobile.
New leaves first means an immobile nutrient. The plant cannot relocate these, so shortage shows up at the growing tip while old leaves stay fine. Iron, calcium, sulphur, manganese, zinc and boron are immobile.
That single distinction narrows the field by more than half before you look at anything else.
| What you see | Where | Likely cause |
|---|---|---|
| Uniform pale yellowing, whole plant, oldest leaves worst | Old first | Nitrogen |
| Yellow between veins, veins stay green | Old first | Magnesium |
| Yellow then brown crispy leaf margins, curling | Old first | Potassium |
| Dark or purplish leaves, stunted, slow | Old first | Phosphorus |
| Yellow between veins, veins sharply green, upper leaves | New first | Iron (usually pH lockout) |
| Uniform pale new growth, veins included | New first | Sulphur |
| Distorted new growth, brown tips, tip burn | New first | Calcium |
| Interveinal yellowing plus short internodes | New first | Zinc |
| Pale mottling on new leaves, less sharp than iron | New first | Manganese |
Things that are not deficiencies at all
- Light bleaching. Yellow or white patches on the leaves closest to the fixture, sharply confined to the top of the canopy. Raise the light.
- Transplant shock. A few days of yellowing after moving a plant is normal. Wait a week.
- Cold reservoir. Below about 60F, uptake slows, phosphorus especially, and the plant looks deficient while the solution is fine.
- Overcrowding. Shaded lower leaves yellow because they are no longer earning their keep.
- Tap water. Hard water pushes pH up over successive top-ups and drags iron availability down with it. If your pH keeps climbing on its own, this is why.
What to actually do
- Measure pH and EC and write them down.
- If pH is out of 5.5 to 6.5, correct it and change nothing else for a week.
- If EC is out of range for your crop, correct it and change nothing else for a week.
- Inspect the roots.
- Only then, if new growth is still coming in wrong, treat the pattern.
Change one variable at a time. The most common way people lose a crop to yellowing is not the deficiency itself, it is adding four supplements in three days and losing any ability to tell what worked.