Tomato leaves turning yellow? 6 signs to check

Tomato leaves turning yellow? It comes down to water or feed, and in pots it's usually nitrogen. Push a finger an inch into the compost first: wet with a wilting plant means stop watering, bone dry means water. If it's just damp, look where the yellow starts. An even pale fade from the bottom up is nitrogen, so feed the roots. Yellow between green veins is magnesium on the older leaves and iron on the newest. Brown, crispy edges mean potassium.

When customers send me photos of yellow tomato leaves, it's nearly always nitrogen deficiency. The plants are growing in small pots and they are not being fed regularly enough. If you grow in small pots, you need to be on top of the fertiliser. No nitrogen, and the leaves go yellow. It's really that simple. There are other possibilities, but nine times out of ten it's nitrogen.

Full disclosure: I formulate and blend the Dr Forest tomato feed myself, and I sell the other Dr Forest products named here.

When yellow leaves are normal

Some yellow leaves on tomato plants need no fixing. The two narrow seed leaves are meant to yellow and drop. A few yellow lower leaves in the fortnight after planting out is root disturbance: water steadily, hold off liquid feed for a few days, and it settles. Young plants put out into cold, wet soil can go purple on the older leaves, then dull yellow, because cold roots struggle to take up phosphorus. Go easy on the water until the weather warms, and plant out in late May or early June next time [1]. Shaded lower leaves fade too, as do the oldest leaves in September.

Tomato leaves turning yellow from water

Overwatering and underwatering both leave tomato plant leaves turning yellow, and overwatering is the more common of the two. Roots in soggy compost can't take up nutrients, so the plant goes pale with plenty of feed in the pot. If it wilts while the compost is wet, stop watering and let the top inch dry. A dry plant yellows from the leaf edges inward until they curl and go crisp: water deeply at the base, in the morning.

Which nutrient is short? Read the leaf

If the watering checks out, look where the yellow starts. Bottom leaves yellowing first means nitrogen, magnesium or potassium, which the plant moves out of old growth to feed the new. Youngest leaves first means a nutrient it can't move, nearly always iron [1].

Nitrogen deficiency

The whole leaf fades evenly from pale green to yellow, starting at the bottom of the plant and working up [1]. Compost reused from last year is usually short of it by midsummer.

Feed the roots with a balanced or nitrogen-rich liquid, watered into damp compost. A high-potash tomato liquid won't sort this one. Liquid Amino Acids on the leaves gives a short lift on top. If Tomato Feed 3-4-6 is already your base, move towards its top rate of 3 g per litre of pot volume before adding another feed. Other plants go short in the same way, and my guide to when plants need a high nitrogen feed covers the rest of the garden.

Magnesium deficiency in tomatoes

The leaf yellows between the veins while the veins stay green, like a fishbone [1]. Check the middle of the plant as well as the bottom: in a 2024 trial on tomatoes grown in a nutrient solution, the yellowing started on the middle leaves [2]. Sandy and acid soils run short first, and so do tomatoes given lots of high-potash feed [1].

Liquid Magnesium goes on at 3 to 6 ml per litre, sprayed on both sides of the leaves early or late in the day, every 2 to 4 weeks, or at 6 to 10 ml per litre as a drench. Epsom salts are the cheap option, and a real magnesium shortage is where they earn their keep (see Epsom salt on roses). The RHS gives 20 g per litre as a leaf spray, repeated two or three times a fortnight, in dull weather only as it scorches in sun, or 30 g per square metre into light soil for the longer term [1].

Iron deficiency

The same yellow between green veins, but on the youngest leaves at the top [1]. There's usually plenty of iron in the ground; on alkaline chalk or limestone the roots just can't get at it. In pots, hard tap water slowly raises the pH of the compost over a summer, so use a water butt if you can. Test the pH before you buy iron.

Meanwhile, a chelated trace-element spray such as Liquid Micronutrients gets iron into the leaf, at 7.5 ml in a 9 litre can. For the pH itself, see soil pH below.

Potassium deficiency

The older leaves scorch brown from the edges inward [1] while the centres stay green, often around fruit-set. Use sulphate of potash, 50% K₂O with no chloride, at the label rate of 2 to 6 g per 10 litre pot every four to six weeks. A level teaspoon is about 5 g. If you'd rather water it in, there's also a liquid potassium fertiliser, used at its label rate.

Go easy, though. If the yellowing is between the veins on the older leaves, that is usually magnesium being crowded out by too much potash. Hold off the extra potash until the magnesium has taken effect. Tomato Feed 3-4-6 also uses Yorkshire polyhalite, which brings magnesium with the potash.

Calcium deficiency in tomato leaves

Calcium shortage rarely shows in tomato leaves. It shows on the fruit as blossom end rot, when the compost dries out and calcium stops reaching the fruit [1]. Older leaves rolling upward after hot days and cold nights is physiological leaf roll, and it usually stops by itself. Twisted, fern-like new growth points to weedkiller drift, or manure carrying aminopyralid.

Soil pH, when the feed is there but out of reach

Tomatoes are happiest between pH 6.0 and 6.8. Above about 7.0, iron, manganese and zinc get harder to reach and the young leaves yellow first. Below 5.5, on sandy and peaty soils, magnesium runs short on the older leaves, and calcium shortage shows on the fruit as blossom end rot. Test beds and borders in early spring, and if the reading needs shifting, see how to make soil more acidic or raise pH.

Pests and disease, the last suspects

If the water, feed and pH all check out, look closer.

• Red spider mite, under glass: fine pale speckling that turns bronze, with webbing underneath. Damp down, and consider the predatory mite Phytoseiulus persimilis.
• Leaf mould, under glass: pale yellow spots and blotches on top of the older leaves, with olive-brown velvet beneath. Ventilate, space the plants out and bin affected leaves.
• Fusarium or verticillium wilt: one side of the plant, or half a leaf, yellows. The plant wilts by day and picks up at night, and a cut stem shows brown staining. Bin it. The fungus lives in the soil for years, so keep tomatoes, peppers, aubergines and potatoes off that ground and grow next year's tomatoes in fresh compost in pots or bags.

Quick reference

What you see	Likely cause	First fix
Wilting, compost wet	Overwatering	Stop watering
Even pale fade, lower leaves first	Nitrogen	Liquid feed to the roots
Yellow between green veins, older or middle leaves	Magnesium	Liquid magnesium or an Epsom salt leaf spray
Yellow between green veins, top leaves	Iron, usually pH above 7.0	Check pH, then a trace-element spray
Brown, scorched edges, older leaves	Potassium	Sulphate of potash
One side of the plant yellow and wilting	Fusarium or verticillium wilt	Bin the plant

Stopping it next season

To stop tomato leaves turning yellow next season, plant into fresh compost, put slow-release Tomato Feed 3-4-6 in the planting hole, keep the watering steady and put down an organic mulch. When to top it up, and by how much, is in my guide to feeding tomatoes.

If you're weighing up organic vs chemical tomato feed, start with Tomorite's NPK and ingredients, or browse the tomato, chilli and pepper feeds.

Questions people ask

Q: Why are my tomato leaves turning yellow? Most often it's nitrogen running out in small pots that aren't fed often, so check the compost isn't waterlogged, then feed. Where the yellow starts tells you the rest: bottom leaves for nitrogen, magnesium, potassium or wet roots, top leaves for iron, one side of the plant for a wilt.

Q: Why is my tomato plant turning yellow? If the whole plant is going pale from the bottom up, it's most likely short of nitrogen or sitting in wet compost. A few yellow lower leaves in the fortnight after planting out is just transplant shock and settles down.

Q: What does magnesium deficiency look like on tomatoes? Yellow between the veins on the older leaves, with the veins staying green like a fishbone, often starting on the middle leaves. An Epsom salt leaf spray at 20 g per litre or a liquid magnesium feed is the quick fix.

Q: Should I remove yellow leaves from my tomato plant? Yes, straight away if they're spotted with blight or leaf mould, and bin them. Otherwise fix the cause first, then take off fully yellow leaves a few at a time, as they won't green up.

Q: Can I feed tomatoes if the leaves are yellowing? Yes, as long as the compost isn't waterlogged, because roots sitting in wet compost can't take the feed up. For pale, evenly yellowing leaves use a balanced or nitrogen-rich feed rather than a high-potash one, then go back to your usual tomato feeding once the new leaves come through green.

Sources

1. Royal Horticultural Society. Nutrient deficiencies. rhs.org.uk/prevention-protection/nutrient-deficiencies
2. Li, H., Liu, F., Zhang, X., Gao, J. and Chen, P. (2024). Magnesium deficiency or excess hinders tomato growth, potassium and calcium uptake. Plant, Soil and Environment 70(11): 719 to 730. doi.org/10.17221/473/2023-PSE