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Quick Summary
- Read the leaf position first: Mobile nutrients (N, P, K, Mg) show deficiency on old lower leaves; immobile nutrients (Fe, Ca, S, Mn) show it on new upper growth.
- Check vein colour: Interveinal chlorosis (yellow tissue, green veins) points to magnesium or iron; uniform yellowing veins included points to nitrogen or sulfur.
- pH before nutrients: Most “deficiencies” are lockouts caused by pH drift above 6.5, confirm pH and EC are in range before adding any supplement.
- The big four: Nitrogen (whole-leaf fade, bottom up), magnesium (green veins, lower leaves), iron (near-white interveinal, top growth), potassium (edge burn, older leaves).
- One change at a time: Make a single adjustment and allow a few days, new growth coming in healthy within a week confirms the correct fix.
Your pH is dialed in. Your reservoir temperature is fine. And your leaves are still turning yellow.
That’s the moment most hydroponic growers start guessing, more nitrogen, less nitrogen, a full flush, a new brand of nutrients. Guessing is expensive and slow, because most deficiencies take a week or two to show up after the plant actually runs short. By the time you see yellow, you’re already behind.
The leaf tells you exactly what’s wrong if you know where to look. Position on the plant, pattern of the yellowing and whether the veins stay green all point to a specific nutrient. This chart walks through the most common culprits so you can go from “something’s wrong” to “here’s the fix” in one look.
Read the leaf before you read the chart
Two questions matter more than anything else:
Where on the plant is it happening? Mobile nutrients nitrogen, phosphorus, potassium, magnesium, get pulled from old leaves and shipped to new growth when supply runs short. Deficiencies in these show up on the lower, older leaves first. Immobile nutrients, iron, calcium, sulfur, can’t be relocated, so deficiencies show up on the new growth at the top.
Are the veins green or yellow? Interveinal chlorosis (yellow between green veins) points to magnesium or iron. Uniform yellowing across the whole leaf, veins included, usually points to nitrogen or sulfur.
Those two data points narrow the list from a dozen possible nutrients down to two or three before you’ve even opened the chart.
Note
Understanding Nutrient Mobility:
- Mobile nutrients (like Nitrogen, Potassium, Magnesium, and Phosphorus) can be relocated by the plant from older leaves to support new growth. Deficiencies in these elements always appear on older, lower leaves first.
- Immobile nutrients (like Iron, Calcium, Sulfur, Boron, and Copper) get locked into cell walls and cannot be reused. Deficiencies in these elements always show up on new growth, growing tips, and roots first.
Quick reference chart
| Nutrient | Leaf position | Pattern | Fix |
|---|---|---|---|
| Nitrogen (N) | Older, lower leaves | Uniform pale yellow, veins included; whole leaf fades | Increase N concentration or switch to a higher-N grow-phase formula |
| Magnesium (Mg) | Older, lower leaves | Interveinal yellowing, veins stay green; can progress to purple/red tinge | Add Cal-Mag or Epsom salt supplement |
| Potassium (K) | Older, lower leaves | Yellowing starts at leaf edges/tips, browning follows; leaf may curl | Increase K in bloom-phase nutrients |
| Iron (Fe) | New, upper growth | Interveinal yellowing, fine veins stay green, can appear almost white | Add chelated iron; check pH isn’t locking it out above 6.5 |
| Sulfur (S) | New, upper growth | Uniform pale yellow similar to nitrogen but starts at the top | Add sulfate-based supplement (many Cal-Mag products include S) |
| Calcium (Ca) | New growth, especially leaf tips and margins | Distorted or curled new leaves, brown spotting, not primarily yellow | Add Cal-Mag; check pH swings, which disrupt Ca uptake even when levels are adequate |
| Manganese (Mn) | New, upper growth | Interveinal yellowing similar to iron but more mottled/speckled | Add micronutrient supplement; verify pH is below 6.5 |
Bookmark this table. It’s the fastest way to go from symptom to cause without a lab test.
Note that this chart is primarily focused on deficiencies that cause yellowing (chlorosis). For symptoms like purple leaves, brittle stems or bluish discolouration, see our guide on non-yellow hydroponic deficiencies.
The four deficiencies that show up most
Nitrogen is the one nearly every grower runs into. Because it’s mobile, it always starts on the oldest leaves at the bottom of the plant, fading from deep green to pale yellow-green across the entire leaf, veins included. Vegetative-stage plants need more nitrogen than bloom-stage plants, so this is common right after a switch to a bloom nutrient line that hasn’t accounted for the plant still finishing its vegetative growth. For guidance on matching your feed to the growth stage, see our guide to choosing the best hydroponic nutrients for your garden.
Magnesium deficiency looks almost like nitrogen at a glance, which is why growers dose more N and watch the problem get worse. The tell is the veins: magnesium-deficient leaves keep their veins green while the tissue between them fades, sometimes moving into a rust or purple tinge at the edges before the leaf drops. Reverse osmosis systems and soft water are especially prone to this, since they strip out the magnesium that tap water would otherwise supply.
Iron deficiency confuses people because it looks like the opposite of a “leaves are dying at the bottom” problem, it shows up at the top, on the newest growth, because iron can’t move from old tissue to new. The interveinal yellowing can get severe enough that leaves look almost bleached white while the veins stay a sharp green. Nine times out of ten this isn’t a supply problem, it’s a pH problem. Iron becomes unavailable to the roots above roughly 6.5 pH, so check your reservoir before you add more iron to a tank that already has plenty it can’t use.
Potassium deficiency is the edge-burn one. Older leaves develop yellowing that starts at the margins and tips, often followed by brown, crispy edges as the tissue dies off. Heavy fruiting or flowering plants pull potassium fast, so this tends to hit mid-to-late bloom if the feed schedule doesn’t ramp K up to match.
When it isn’t a nutrient at all
Yellow leaves have other causes and treating a non-nutrient problem with more nutrients usually makes things worse.
If the yellowing is patchy, appears alongside stunted growth or shows up right after a pH swing rather than gradually over days, check pH and EC first, our guide to managing hydroponic pH levels covers target ranges by crop and how to correct drift. A lockout from pH drift can mimic almost any deficiency on this chart, because the roots physically can’t absorb nutrients that are sitting right in the reservoir.
If you’re seeing yellowing combined with a slimy film in the reservoir, cloudy water or a drop in dissolved oxygen, the cause might be algae competing with your plants for nutrients rather than a shortage in the feed itself, our algae prevention guide covers how to tell the difference and clear it out.
And if the yellowing comes with stippling, tiny holes, webbing or visible insects on the underside of the leaf, you’re looking at pest damage, not deficiency, our breakdown of common hydroponic pests and how to manage them walks through the most common hydroponic pests and how to treat each one.
Confirm before you correct
Before adding a supplement, check pH and EC. A huge share of “deficiencies” are actually lockouts caused by pH drift and dumping more of a nutrient that’s already present just pushes EC higher without fixing anything. If pH and EC are both in range and the pattern on the chart still matches, then it’s safe to adjust the feed. Make one change at a time and give it a few days, plants don’t green back up overnight, but you should see new growth coming in healthy within a week if you’ve got the right fix.
Frequently Asked Questions
Why are my hydroponic leaves turning yellow?
Yellow leaves in hydroponics are most commonly caused by nitrogen deficiency, pH-induced nutrient lockout, magnesium deficiency or iron deficiency. Check pH first, if it has drifted outside 5.5–6.5, nutrients become chemically unavailable even when they're present in the reservoir. Then examine which leaves are affected: old leaves yellowing uniformly points to nitrogen or magnesium; new growth yellowing with green veins points to iron or manganese.
How do I tell the difference between nitrogen and magnesium deficiency?
Both start on older, lower leaves, but the vein colour gives it away. Nitrogen deficiency causes the entire leaf, veins included, to fade to pale yellow-green. Magnesium deficiency causes interveinal chlorosis, the tissue between the veins yellows while the veins themselves stay green, sometimes progressing to a rust or purple tinge at the leaf edges.
Why does iron deficiency show up on new leaves, not old ones?
Iron is an immobile nutrient, once it is fixed in older leaf tissue, the plant cannot relocate it to new growth. So when iron supply runs short, new leaves at the top of the plant show the deficiency first, with interveinal yellowing so severe the leaves can look almost white. In most cases the problem is not an iron shortage but pH above 6.5 locking iron out at the root.
Should I add more nutrients when I see yellow leaves?
Not immediately. First check pH and EC. A large share of apparent deficiencies are actually lockouts caused by pH drift, adding more nutrients to a reservoir that already has enough simply raises EC without fixing the absorption problem. Confirm pH is in range and EC matches your growth stage, then make one targeted adjustment and allow a few days before evaluating.
What causes yellowing at leaf edges and tips in hydroponics?
Edge and tip yellowing on older leaves is the classic sign of potassium deficiency. It begins as yellowing at the margins and tips, then progresses to brown, crispy edges as the tissue dies. It is most common mid-to-late bloom when heavy-fruiting plants pull potassium faster than a standard feed schedule replenishes it.
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