My leaves look wrong. Where do I start?
Not with a photo chart, and not with a bottle. Start with three numbers, because in a hydroponic system the odds are good that the solution contains the nutrient your plant appears to be missing. pH decides whether the roots can take it up at all; VPD decides whether the plant can move it once taken; EC tells you whether the problem is too little or too much. Reach for the diagnostic key after those, not before.
A plant under stress cannibalizes itself. Nutrients it can move — N, P, K, Mg, Mo — get stripped out of old leaves and sent to new growth, so those deficiencies appear at the bottom of the plant first. Nutrients it can't move once they're deposited — S, Ca, Fe, Mn, Zn, B, Cu — can only fail where new tissue is being built, so those appear on the newest leaves.
So before you compare anything to a picture: is it the old growth or the new growth? That single question eliminates half the table below. Magnesium and iron produce almost identical interveinal yellowing and are constantly confused — but magnesium is a bottom-of- the-plant problem and iron is a top-of-the-plant one, and that's the whole distinction.
The diagnostic key
Old growth first — the mobile nutrients:
| Nutrient | What it looks like | Commonly confused with |
|---|---|---|
| Nitrogen (N) | Whole lower leaves pale, then uniformly yellow, then drop. Plant pales from the bottom up. | Shaded lower leaves being cannibalized — check DLI before feeding. |
| Phosphorus (P) | Dark, dull, slightly blue-green leaves; purple or red petioles and stems; slow growth. | Cold roots — purpling from low root temperature is far commoner. |
| Potassium (K) | Margins of older leaves yellow then scorch brown, working inward. Edges curl. | Nutrient burn, which also scorches — but burn starts at the tips of the newest leaves. |
| Magnesium (Mg) | Interveinal yellowing on lower leaves — veins stay green, tissue between them goes pale. | Iron, which looks identical but appears on new growth instead. |
| Molybdenum (Mo) | Pale older leaves with upward-cupped, narrow new growth. Rare. | Nitrogen — molybdenum is needed to use nitrate. |
New growth first — the immobile nutrients:
| Nutrient | What it looks like | Commonly confused with |
|---|---|---|
| Sulfur (S) | Whole new leaves uniformly pale, veins included. Like nitrogen but at the top. | Nitrogen — the difference is only which end of the plant. |
| Calcium (Ca) | New leaf tips and margins brown and crisp; growing points distort or die. Splotchy, not uniform. | Almost always a transpiration problem, not a solution one — see the fact box. |
| Iron (Fe) | Sharp interveinal chlorosis on the newest leaves — bright green veins on yellow tissue. | Magnesium, but Mg shows on old leaves. Usually pH, not a missing bottle. |
| Manganese (Mn) | Interveinal mottling on new leaves with necrotic flecks; veins less sharply green than iron. | Iron. Both lock out at the same high pH. |
| Zinc (Zn) | New growth small and bunched; short internodes give a rosette look. | Light stress or a heat-stressed growing tip. |
| Boron (B) | Growing tips die back; new leaves thick, brittle and distorted. | Calcium — both wreck the growing point. |
| Copper (Cu) | New leaves dark, limp and slightly blue; tips wilt without drying. | Overwatering or low dissolved oxygen. |
Symptoms as described in extension and controlled-environment references. Two cautions worth more than the table: deficiencies rarely arrive alone (a pH problem locks out several nutrients at once, which is why "it looks like everything" usually means pH), and a plant that has been fixed keeps its damaged leaves — judge the fix on new growth, not on whether the old spots went away. They won't.
Calcium isn't a calcium problem. Calcium moves only in the transpiration stream — it rides water up the xylem and cannot be redirected once it lands. Controlled-environment research is blunt about this: tipburn in hydroponic lettuce happens with plenty of calcium in the root zone, because high humidity and still air stop the plant transpiring enough to deliver it to the leaves that are growing fastest. A 2025 trial found airflow fans controlled it as well as a calcium biostimulant did. If your VPD is under 0.4 kPa or your fans are off, adding CalMag treats the wrong thing — check the VPD calculator first, and aim for the 0.8–1.2 kPa band.
Iron isn't an iron problem. Iron precipitates out of solution as pH climbs — it's unavailable above 6.5 on the chart below and about half gone by 7.3. The bottle is fine; the pH isn't.
Nitrogen on lower leaves may just be shade. A plant pulls nitrogen out of leaves that aren't paying their way, and a leaf at the bottom of a dense canopy isn't. If the fade is confined to leaves in deep shade while the top is green and growing, that's normal senescence — check the DLI before you feed.
pH lockout: what's available when
This is why "check pH first" is not a brush-off. Each nutrient has a band it stays available in, and outside it the nutrient is in the reservoir but not in the plant. Counting how many are simultaneously available across the range:
| pH | Available | Locked out |
|---|---|---|
| 5.00 | 5 of 12 | N, P, K, Mg, S, Ca, Mo |
| 5.50 | 10 of 12 | Mg, Ca |
| 6.00 | 12 of 12 | — none — |
| 6.25 | 12 of 12 | — none — |
| 6.50 | 12 of 12 | — none — |
| 6.75 | 8 of 12 | Fe, Mn, Zn, Cu |
| 7.00 | 8 of 12 | Fe, Mn, Zn, Cu |
| 7.50 | 6 of 12 | P, Fe, Mn, Zn, B, Cu |
Computed from the availability bands in the classic chart. 6.0–6.5 is the only band where all 12 are available at once — which is where the standard "keep it between 5.5 and 6.5" advice comes from, and it's a genuine result rather than folklore. At 5.0 you are down to 5.
One honest caveat about that chart, which almost nobody who reproduces it mentions: it descends from soil agronomy work of the 1930s and 40s, where the mechanism is adsorption onto soil particles. In solution culture the mechanism is different — precipitation and the stability of the chelate the iron is wrapped in — so treat the bands as the right shape rather than as physics. The practical conclusions survive either way: iron and manganese go first when pH climbs, calcium and magnesium suffer when it falls, and the middle is the place to be.
Deficiency, or too much?
Scorched leaf edges are the symptom growers most often read backwards, because both ends of the scale produce them. The tells:
| Deficiency | Nutrient burn | |
|---|---|---|
| Where it starts | Depends on the nutrient — bottom for mobile, top for immobile | Tips of the newest, fastest-growing leaves, always |
| How it spreads | Slowly, over days, following the leaf’s veins or margins | Tips brown and curl up, then the margins go, then whole leaves |
| What the meter says | EC at or below the band for the stage (flower wants 1.6–2.4) | EC above it — over 3 is stress for anything but tomatoes |
| What fixes it | More of the missing thing, or the pH that lets it in | Dilution — and the top-off calculator sizes it |
The meter settles it in a way the leaf never will, which is the argument for reading EC before diagnosing anything. The EC ↔ PPM converter also subtracts your tap water, without which a hard-water reading looks like a well-fed reservoir.
The other things that imitate a deficiency
Root rot and low dissolved oxygen. Brown, slimy, sour-smelling roots produce yellowing, wilting and several apparent deficiencies at once, because damaged roots can't take up anything. Nothing you add to the reservoir fixes it. Check the roots before the chart — and if they're brown, the problem is oxygen or temperature.
Solution temperature. Above about 75 °F a reservoir holds less dissolved oxygen and grows more pathogens; cold roots slow phosphorus uptake and turn stems purple in a convincing imitation of a phosphorus deficiency.
Light. Too little starves the lower canopy into yellowing; too much bleaches the tops white and cups the leaves, which reads as almost anything. The DLI calculator tells you which side of that you're on.
Drift you didn't notice. A reservoir that hasn't been topped off concentrates as the plants drink, and both EC and pH move while you're not looking — which is the same mechanism that decides how forgiving your system is in the first place.
What to actually do
In order, and stopping as soon as something explains it:
- Look at the roots. White and firm, or brown and slimy? If it's the second, stop reading charts.
- Read pH. Outside 6.0–6.5 and you have found a plausible cause of any symptom on this page. Correct it before adding anything.
- Read EC, net of your tap water. Below the band, feed; above it, dilute; inside it, the problem is not the strength of the solution.
- Check VPD and airflow — especially for anything happening on new growth, and doubly for crisp tips.
- Only now use the key above, and treat the single most likely candidate. One change at a time, or you won't know which one worked.
- Judge it on new growth, seven to ten days later. Damaged leaves never recover, and waiting for them to is how people conclude a correct fix failed.
The unglamorous truth behind almost all of this: a fresh reservoir of correctly mixed solution at the right pH fixes more "deficiencies" than any supplement, because it resets every variable at once. If you're two weeks into a drifting reservoir and something looks wrong, change it out and see what's left — the dosing calculator will mix it and the top-off calculator will keep it there.
Sources: Ohio State CEA Center and e-GRO alerts on tipburn in hydroponic lettuce, and Frontiers in Plant Science (2025) on airflow versus calcium biostimulants — the basis for treating tipburn as a transpiration problem; extension and controlled-environment references for symptom descriptions and nutrient mobility; the pH availability bands are the classic chart, whose soil-agronomy origins are noted in the text. Stage VPD and EC bands as on the VPD and EC ↔ PPM pages.