Nutrient Deficiencies: How to Read the Leaves and Fix the Cause

Yellow leaves are the most common thing a grower worries about, and the most commonly misread. The plant is telling you something useful, but rarely "add more feed". Read where the symptom started and what pattern it makes, and you can narrow it down to one or two nutrients in a minute. Then check pH, EC, water and roots before you add anything, because most deficiencies indoors are not a shortage in the feed at all: the nutrient is there, and the roots cannot get at it.

First: is it really a deficiency?

Plenty of things make leaves yellow, curl or spot. Rule these out first, because each has a different fix and a deficiency treatment will make some of them worse.

• Nutrient burn. Crispy brown tips on otherwise dark green leaves mean too much feed, not too little. See our nutrient burn guide.
• Overwatering. Drooping, heavy leaves that yellow all over, on a pot that never dries out, point to starved roots rather than a missing nutrient.
• Light stress. Bleaching or yellowing only on the leaves nearest the lamp, with the lower leaves fine, means the light is too close or too strong. Our light height guide has the distances.
• Pests. Fine pale speckling, silvery patches or sticky leaves are insects, not nutrition. Turn the leaves over and check against our pest prevention guide.
• Normal ageing. The oldest, most shaded leaves at the bottom of a mature plant naturally yellow and drop. One or two at a time is normal; a wave moving up the plant is not.

Where it shows first: old leaves or new growth

This is the single most useful clue. Some nutrients are mobile: when supply runs short, the plant pulls them out of its older leaves and sends them to the new growth. So a shortage of nitrogen, phosphorus, potassium or magnesium shows on the older, lower leaves first, while the top of the plant can look fine.

Others are immobile: once they are built into a leaf they stay there. A shortage of calcium, iron, sulphur, manganese, zinc or boron therefore shows on the newest growth first, at the top and the growing tips, while the old leaves stay green.

The deficiency chart

Use this as a shortlist rather than a diagnosis. Two shortages can look alike, and the real cause is usually in the last column.

Why most deficiencies are really lockout

A complete base feed, mixed at the right strength, contains everything a plant needs. So when a grower who is feeding properly sees a deficiency, the shortage is almost never in the bottle. It is at the root, and there are four usual reasons.

• pH out of range. pH decides which nutrients are soluble enough for the roots to take up. Too high and iron, manganese and zinc drop out first; too low and calcium and magnesium suffer. Aim for 6.0-7.0 in soil, 5.8-6.2 in coco and 5.5-6.0 in hydro, as set out in our pH guide.
• Cold, wet or damaged roots. Roots need warmth and oxygen to take anything up. A cold tank, a waterlogged pot or brown, slimy roots will produce deficiency symptoms no amount of feed can fix. See our root zone temperature guide.
• Soft or RO water. Rainwater, reverse osmosis water and very soft tap water carry almost no calcium or magnesium. Base feeds are often formulated expecting some to come from the tap, so on soft water you need a cal-mag supplement. Our water quality guide explains how to read your water.
• One nutrient blocking another. Nutrients compete at the root. Too much potassium makes magnesium and calcium harder to take up, and very high phosphorus can tie up zinc and iron. Stacking boosters on a full base feed is a common way to create a shortage of something else.

The five checks, in order

Work through these before you buy or add anything. Most problems are solved by step one or two.

1. Check pH of the feed going in and, in coco or soil, of the run-off coming out. If the run-off is well away from the feed, the medium has drifted and is the likely cause. Calibrate your pen first: a pen that has not seen buffer solution for a month can be half a point out.
2. Check EC. If the feed is weak for the stage, the plant may simply be hungry; raise it gradually. If the run-off EC is much higher than the feed, salts are building up and you are looking at burn or lockout, not a shortage.
3. Check the water. Measure the EC of your plain water. Very low readings mean soft or RO water that needs cal-mag; high readings mean hard water that already carries calcium and magnesium.
4. Check the roots and the root zone. In hydro, look at the roots and take the tank temperature. In pots, check the pot is drying between waterings and that the roots you can see are white.
5. Then dose one thing. Correct the cause, and if a supplement is still needed, add one product at a time so you know what worked.

Fixing the common ones

Magnesium

The most common true deficiency indoors, and the easiest to recognise: yellow between the veins of the older leaves while the veins stay green. Check pH first, because low pH makes it worse, and check that you are not overdoing potassium boosters. On soft or RO water, or in coco, add a cal-mag product as part of the normal mix. Where magnesium alone is the problem, a dedicated magnesium additive corrects it without pushing calcium higher.

Calcium

Calcium only moves with the water the plant draws up, so it reaches fast-growing tips and fruit last. Crinkled, distorted new leaves and dead tips are the signs on the plant; on tomatoes and peppers the classic sign is blossom end rot, a dark sunken patch on the base of the fruit. That is usually caused by uneven watering, or by humidity so high that the plant transpires too little, rather than by a lack of calcium in the feed. Water evenly, keep air moving, and add cal-mag if your water is soft.

Iron

Pale new leaves with crisp green veins are almost always a pH problem rather than an iron shortage. Bring pH back into range and the new growth normally greens up within a week. A chelated iron product, which stays available over a wider pH range, helps the plant through while the root zone recovers; an iron foliar spray gets iron into the leaf faster still.

Nitrogen

Even yellowing that starts with the bottom leaves and climbs, with pale, slow growth overall, means the plant needs more nitrogen than it is getting. Usually that is simply a feed too weak for the size of the plant, or a plant in a small pot that has used up what the medium held. Raise the base feed gradually, or use a single-nutrient nitrogen product for a targeted top-up.

Phosphorus and potassium

True shortages of either are rare on a complete base feed. Purple stems and dull leaves very often mean a cold root zone, which slows phosphorus uptake, so warm the roots before you add anything. Scorched edges on older leaves can be potassium, but check EC and run-off first, because burn can look similar. Reaching for a booster is the wrong first move for both.

Trace elements

Manganese, zinc and other trace elements are needed in tiny amounts and are almost always present in a complete feed. When they show, it is usually because the pH is too high. Correct that first. If you mix from a basic feed or use soft water, a cal-mag product that also carries trace elements covers the gaps.

Foliar sprays: fast, but not the fix

Spraying a nutrient onto the leaves gets it into the plant within days, which makes it useful for iron and magnesium while you correct the cause at the root. But it is a plaster, not a cure: if the pH or the roots are wrong, the symptoms come back as soon as you stop.

• Follow the label rate exactly. Foliar products are weaker than root feeds for a reason; too strong a spray scorches leaves.
• Test on a few leaves first and check them a day later before spraying the whole plant.
• Spray when the light is off or dimmed, so droplets do not act as lenses and the leaves have time to absorb it, and keep air moving so the leaves dry.
• Do not spray into high humidity or onto a plant that already has mould or mildew; wet leaves in still air invite it.

What recovery looks like

Leaves that have yellowed badly or scorched rarely turn fully green again, so do not judge the fix by them. Watch the new growth instead. If the leaves that unfold over the next five to seven days come through a healthy green and the yellowing stops spreading, you have solved it. Remove leaves that are mostly dead so they do not harbour mould, but leave partly yellow ones in place while the plant recovers; they are still feeding it.

If nothing improves after a week, go back to the five checks. The most common reason a fix does not work is that the real cause, usually pH or roots, was never corrected.

Mistakes that make it worse

• Adding more of everything. Raising the whole feed rarely fixes a single shortage and often causes burn on top of it.
• Adding three products at once. If it recovers you will not know which one worked; if it gets worse you will not know which one did it.
• Trusting an uncalibrated pen. A pH reading half a point out is enough to cause the very lockout you are trying to cure.
• Adding cal-mag to hard water. If your tap already runs high in calcium, more can push it too far and block magnesium and potassium.
• Treating the old leaves. They are the record of the problem, not the measure of the fix.
• Ignoring the roots. Brown, slimy roots or a pot that never dries out will look like a deficiency no matter what you feed.

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Quick answers

How do I tell which nutrient my plant is short of?

Start with where the symptom appears. Problems that begin on the older, lower leaves are usually nitrogen, phosphorus, potassium or magnesium, because the plant moves those up to new growth. Problems that begin at the top are usually calcium, iron, sulphur, manganese or zinc, which cannot be moved once placed. Then look at the pattern: even yellowing, yellow between green veins, scorched edges or purple tints.

Why do my plants look deficient when I am feeding them properly?

Usually because the nutrient is in the pot but the roots cannot take it up. A pH outside the right range, a cold or waterlogged root zone, soft or RO water with no calcium and magnesium added, or too much of one nutrient blocking another will all cause it. Check pH, EC, water and roots before you add anything.

What does magnesium deficiency look like?

Yellowing between the veins of the older, lower leaves while the veins themselves stay green, sometimes with rusty spots as it progresses. It is common in coco, in soft or RO water, and where potassium is high. A cal-mag product or a dedicated magnesium additive usually corrects it.

Is yellowing between green veins on new leaves iron deficiency?

Very often, yes. Iron cannot move to new growth once it is placed, so a shortage shows as pale new leaves with green veins. The most common cause is a pH that is too high, so check and correct pH first. A chelated iron product or an iron foliar spray helps while the root zone settles.

Will yellow leaves turn green again?

Badly yellowed or scorched leaves rarely recover fully. Judge your fix by the new growth: if the leaves that unfold over the next five to seven days come through healthy and green, the problem is solved. Remove leaves that are mostly dead so they do not harbour mould.

Should I just add more feed?

No. Raising the whole feed rarely fixes a single shortage and often causes nutrient burn on top of it. Measure pH and EC first. If the EC is genuinely low for the stage, raise it gradually; if it is already right, the problem is uptake, not supply.

Do I need cal-mag?

If you use RO water, rainwater or very soft tap water, almost certainly, because those waters carry little calcium or magnesium. Many coco growers also add it. If your tap water is hard, you may not need it at all, and adding it can push calcium too high. Read your water EC before deciding.

Is this the same for tomatoes, chillies and houseplants?

Yes. Magnesium yellowing on lower tomato leaves, iron chlorosis on new growth in alkaline conditions and calcium problems on fruiting crops are textbook issues for vegetable and houseplant growers. The same rules of where it shows, what it looks like and what to check apply to any plant.