Why are my tomato leaves turning yellow?
By Joe, Founder of Dr Forest · May 2026, updated October 2026 · 12 min read
It usually starts at the bottom. Somewhere between July and September a tomato in a grow bag or a greenhouse border that looked fine last week has a skirt of yellow leaves. Before you buy anything, push a finger an inch into the compost, then look at where the yellow starts and the pattern inside one leaf. That is most of the diagnosis.
I make a tomato feed in Stockport, Greater Manchester, so treat the product notes with the scepticism you would apply to anyone selling you something. The diagnostic still holds if you never buy a bag from me.
If the plant is wilting and the compost is wet, stop watering. If the yellow starts on the oldest leaves, you are in nitrogen, magnesium or potassium, or the roots are drowning. If it starts on the newest leaves, iron is the likeliest nutrient, and that is often a pH problem in chalky ground. If only one side of the plant, or one half of a leaf, is yellow, suspect fusarium or verticillium wilt. Fine pale speckling under glass is usually red spider mite, not feed.
When yellow leaves are normal
Some yellowing is the plant doing its job. The cotyledons, the first two rounded seed leaves, are meant to yellow and drop. A few yellow lower leaves in the fortnight after planting out is root disturbance: keep the watering steady, hold off liquid feed for a few days, and it settles. Shaded lower leaves under a crowded canopy yellow too, and on a cordon you would take the leaves below each picked truss off anyway.
In September the old leaves fade as the plant puts what it has into the last of the fruit. That is the season winding down, not a crisis.
Tomato leaves turning yellow from water
In UK gardens, check the water first. Saturated compost and a dry pot both yellow a tomato, and people often feed the wrong one.
Overwatering is the commoner of the two here. A grey, drizzly stretch keeps compost wet far longer than the plant wants, and roots in airless, waterlogged compost cannot take up nutrients, so the plant goes pale with plenty of feed in the pot. The clearest sign is a plant wilting while the compost is still wet. That plant is suffocating.
Underwatering yellows the leaves from the margins inward until they curl and go crisp. In a hot week a big tomato can drink a 30 cm pot dry inside a day.
The test is the same for both. Push a finger an inch into the compost: if it comes out dry, water deeply; if damp, wait. Water at the base in the morning, on the plant's schedule rather than the calendar's.
One trial is worth knowing here. Joana Machado and colleagues at Porto and Wageningen grew young Micro-Tom plants, a dwarf lab variety, in small pots of vermiculite in a growth chamber for 16 days, halving the nitrogen in their feed, letting the pots dry down to half their water-holding capacity, or both [1]. The drying made no significant difference to the chlorophyll meter reading. Halving the nitrogen cut it by 14 percent, and both together by 18 percent, before any yellowing showed to the eye. A mild dry-down is not a grow bag left bone dry in a heatwave, but the lesson holds: a plant that is a bit dry is not usually yellow because it is dry. If the compost checks out and the leaves are still pale, look at feed next.
Figure 1 · chlorophyll meter, 16 days
Halving the nitrogen lowered the meter reading on young tomato leaves. Letting the pots dry to half did not.
cv. Micro-Tom in vermiculite, growth chamber. SPAD on the youngest fully expanded leaf. Control 10.5 mM N, kept at full water-holding capacity. Nitrogen treatments 5.3 mM N. Water treatments not rewatered, drying to 50 percent of capacity by day 16.
Which nutrient is short? Read the leaf
Once watering is ruled out, a shortage is usually next, often a few weeks after the first fruits set. Ask where the yellow starts, then what pattern it makes inside the leaf. Oldest leaves first means wet roots or a mobile nutrient (nitrogen, magnesium or potassium) that the plant is stripping from old growth to feed the new. Youngest leaves first means a nutrient it cannot move, and in practice that is nearly always iron.
Nitrogen deficiency
What it looks like: an even pale-green-to-yellow fade across the whole leaf, with no green veins showing, working up from the bottom.
Why it happens: nitrogen dissolves, heavy rain washes it out of the root zone, and compost reused from last year is usually short of it by midsummer. A slow-release feed made from organic ingredients adds one wrinkle. Soil microbes release its nitrogen, and they work slowly in cold compost, so a tomato planted out in a chilly May can look pale for a few weeks before it catches up.
In Machado's trial, half the nitrogen, not none, cost 31 percent of the leaf area in 16 days (Figure 1). A patio tomato carrying fruit in July draws harder on its nitrogen than a seedling in a growth chamber, so I would expect the fade to show at least as clearly.
Quick fix: feed the roots, with a balanced or nitrogen-rich liquid watered into damp, not wet, compost. A high-potash tomato liquid is the wrong bottle here. A foliar spray gives a short lift on top: Liquid Amino Acids, which I make, from £14.99, goes on at 10 ml per litre for pale lower leaves, dropping back to 3 ml once they recover. If Tomato Feed 3-4-6 is already your base and the whole leaf is evenly pale, move towards its top rate of 3 g per litre of pot volume before stacking another feed on top.
The evidence for amino-acid products is narrower than the marketing. Athanasios Koukounaras and colleagues grew 'Belladonna' tomatoes in the soil of an unheated greenhouse in Thessaloniki, Greece, and dosed them six times up to week 12 with a protein hydrolysate (11.3 percent L-amino acids, 4 percent nitrogen), either at the roots alone or split between roots and leaves [3]. On the normal base fertiliser, the mostly-foliar split (0.3 percent at the roots, 0.9 percent on the leaves) raised the weight of graded fruit. Where the base fertiliser had been cut by 40 percent, the root-only dose at 2.7 percent raised total and marketable yield by setting more fruit. It measured fruit, not leaf colour, with a different product in Greek soil. What I take from it: on a genuinely underfed plant the gain came through the roots, so treat a spray as a short-term lift and let a slow-release base do the steady feeding.
Magnesium deficiency in tomatoes
What it looks like: yellow between the veins on the older leaves, with the veins still vivid green. Growers call it fishbone. Magnesium sits at the centre of every chlorophyll molecule, so without it the leaf cannot make its green.
Why it happens: light, sandy and acid soils run short first. So do plants hammered with high-potash tomato liquids, because potassium and magnesium compete at the root, and heavy potash can lock magnesium out even when the soil has enough.
Quick fix: Liquid Magnesium, which I make, from £12.99, is sprayed at 3 to 6 ml per litre on both sides of the leaves, early morning or late evening; the label says to expect a change in 7 to 14 days. Epsom salts (magnesium sulphate) are the cheap standard. The RHS gives 20 g per litre of water as a summer leaf feed, sprayed or from a watering can, repeated two or three times a fortnight and kept off leaves in bright sun, which can scorch them. For the longer term on light, free-draining ground, it suggests Epsom salts or kieserite worked into the soil at 30 g per square metre. Whichever you use, cut back any high-potash liquid while you correct it. Epsom salts are oversold for most garden plants (see Epsom salt for roses), but a tomato with a real magnesium shortage is where they earn their keep.
Iron deficiency
What it looks like: the same yellow-between-veins pattern, but on the youngest leaves at the top. Iron cannot move out of old tissue, so new growth goes short first.
The catch: it is rarely a lack of iron in the soil. As soil turns alkaline, iron becomes much less plant-available, so on chalk or limestone new leaves can yellow with plenty of iron in the ground. Cold, wet roots in spring do the same for a while, and in pots, hard tap water slowly raises the compost pH over a summer; a water butt is the fix. Test the pH before you buy iron.
Quick fix: a trace-element spray gets iron into the leaf when the roots can't. Liquid Micronutrients, which I make, from £15.99, carries seven trace elements: the iron, zinc, manganese and copper are chelated so they stay soluble and plant-available, and the boron, molybdenum and cobalt are in soluble form. The home-garden rate is 7.5 ml in a 9-litre can. It treats the leaf, not the pH; for that, see how to raise or lower soil pH.
Potassium deficiency
What it looks like: scorched, brown-edged margins on the older leaves, with the centres staying green for a while. It often turns up around fruit-set, when potassium demand spikes.
R.T. Besford and G.A. Maw grew young 'Amberley Cross' plants in sand, fed with a nutrient solution, at the Glasshouse Crops Research Institute in Littlehampton, West Sussex [2]. Flower development slowed once potassium in the feed fell below 0.53 meq per litre, fruit setting efficiency dropped below 2.03, and maximum yields came at 5.03 or 10.23. Those are concentrations in the feed water, not a garden rate. What they show is the order of events: flowers and fruit set suffer before the scorch is dramatic, so by the time you see brown margins the truss has probably been held back already.
Quick fix: sulphate of potash, from £6.25, is 50 percent K2O with no chloride. The label rate for an established container is 2 to 6 g per 10 litre pot every four to six weeks; a level teaspoon is about 5 g. Go easy, because extra potash on top of a tomato feed is how a lot of magnesium shortages start. Tomato Feed 3-4-6 gets its potassium from sulphate of potash and Yorkshire polyhalite, which brings calcium, magnesium and sulphur with it (more in polyhalite vs sulphate of potash).
Calcium deficiency in tomato leaves
True calcium shortage in tomato leaves is rare. Older leaves rolling upward after hot days and cold nights, heavy deleafing or a dry spell is physiological leaf roll: alarming to look at, mostly harmless, and it usually stops by itself. Twisted, cupped or fern-like new growth points to weedkiller drift, or manure or compost carrying a residue such as aminopyralid.
Where calcium does show is on the fruit. Blossom end rot comes from uneven watering, which interrupts calcium on its way to the fruit, not from a lack of it in the soil, so steady watering fixes it and adding calcium usually doesn't. Tomato Feed 3-4-6 already has calcium in the base. The full answer is in blossom end rot in tomatoes.
Soil pH, when the feed is there but out of reach
Soil pH decides how much of what is in the ground a tomato can actually reach. Above about pH 7.0, common on chalk and limestone from the Downs and the Chilterns to the Cotswolds and the White Peak, iron, manganese, zinc and boron become less plant-available, and young leaves yellow first. Below pH 5.5, common on sandy and peaty soils and across much of the rain-washed North West, calcium and magnesium run short and phosphorus gets harder to reach, which shows on the older leaves.
Tomatoes are happiest between pH 6.0 and 6.8. Fresh multipurpose compost is usually adjusted already, so test the border or allotment bed, not the bag, in early spring. A kit costs a few pounds and earns its place in one season.
Figure 2 · availability window
The gold band is pH 6.0–6.8, where a tomato can reach the major nutrient groups at once.
Schematic of textbook availability, not a trial curve. Use it to decide whether to test, not as a dosing chart.
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Pests and disease, the last suspects
If watering, feeding and pH are sorted and the yellowing is still spreading, look harder. Pest and disease yellowing rarely comes on its own: look for speckling, spots, blotches, mould or one-sided damage.
Red spider mite, under glass, leaves fine pale speckling that turns bronze-yellow, with webbing underneath, and it loves a hot, dry greenhouse. Damp down the paths and consider the predatory mite Phytoseiulus persimilis. Whitefly rise in a cloud when you brush the plant and leave sooty mould on the lower leaves; the parasitic wasp Encarsia formosa is the greenhouse control.
Leaf mould (Fulvia fulva), also under glass, shows as pale yellow blotches on top of older leaves with olive-brown velvet directly beneath. Ventilate, space the plants, bin affected leaves, and grow resistant varieties next year.
Late blight (Phytophthora infestans), outdoors in a wet summer, is the blight that matters in the UK: fast-spreading brown patches, brown streaks on the stems, fruit rotting on the truss. It browns leaves rather than yellowing them, and there is no garden cure. Strip and bin affected growth, pick sound fruit early, and watch for Hutton Criteria blight warnings. Early blight (Alternaria solani), less common here, gives brown bullseye spots with yellow halos on the lower leaves. Strip and bin those.
Fusarium and verticillium wilt yellow one side of the plant or one half of a leaf. The plant wilts by day and recovers at night, until it stops recovering, and a cut stem usually shows brown staining inside. Nothing reverses it. Bin the plant and keep tomatoes, peppers, aubergines and potatoes off that ground for three to four years, or grow in fresh compost or on a grafted, resistant rootstock.
Mosaic viruses leave mottled yellow-and-green leaves, often twisted, on a stunted plant. Cucumber mosaic virus is spread by aphids. Bin the plant, control the insects, and clean your tools between plants.
Quick reference
| What you see | Likely cause | First fix |
|---|---|---|
| Wilting while the compost is still wet | Overwatering, roots starved of air | Stop watering, let the top inch dry, check drainage |
| Uniform pale fade, lower leaves first | Nitrogen shortage | Liquid feed to the roots, then a slow-release base |
| Yellow between green veins, lower leaves | Magnesium shortage, often worsened by potash | Liquid magnesium, or Epsom salts at 20 g per litre on the leaves |
| Yellow between green veins, top leaves | Iron, usually pH above 7.0 | Check pH, then a trace-element spray |
| Scorched brown edges, older leaves | Potassium shortage at fruit-set | Sulphate of potash, 2 to 6 g per 10 litre pot every 4 to 6 weeks (label rate) |
| Older leaves rolled upward, plant otherwise healthy | Physiological leaf roll | Nothing. Steady watering; it usually stops |
| Twisted, cupped or fern-like new growth | Weedkiller drift or residue in manure | Find the source. No feed fixes it |
| Fine pale speckling, webbing, under glass | Red spider mite | Damp down, raise humidity, biological control |
| Yellow blotches on top, mould underneath | Leaf mould | Ventilate, remove and bin affected leaves |
| Sunken dark patch on the fruit base | Blossom end rot, uneven watering | Water evenly and mulch. See blossom end rot |
Stopping it next season
The cleanest answer to yellow leaves is not to get them. The full season plan lives in best fertiliser for tomatoes. The short version: test the pH of beds in spring, use fresh compost in pots, and put slow-release Tomato Feed 3-4-6 in the planting hole. From fruit-set, top it up every three weeks at 1 to 3 g per litre of pot volume, or every two weeks only at peak fruiting and only if the whole leaf is evenly pale. Add potash only for real potassium scorch, water steadily, and mulch.
Why a base made from organic ingredients? For the soil and the fruit, not the leaf colour. A 2023 meta-analysis of 124 tomato studies, pot and field, mostly outside the UK, found organic fertilisers raised soil organic matter by about 24 percent over chemical-only feeding, with yield roughly level (up 3.5 percent), and the ordinary organic-fertiliser group gave about 11 percent more fruit sugar and lycopene [4]. A larger one, covering 313 studies, found organic feeding raised sugars and aroma compounds but on its own cost about 5.5 percent in yield, while organic plus mineral raised yield by about 12 percent [5]. Neither measured yellow leaves, and fruit chemistry is not a taste test. My reading for a UK pot or border: an organic base builds the soil, and a targeted mineral fix for a deficiency you can see is not cheating. More in organic vs chemical tomato fertiliser and Tomorite vs Dr Forest; the bags sit in Tomatoes & Edibles.
A yellow leaf on a tomato is information, not an emergency. Name where it started. Name the pattern. Then open one bottle, or none.
Questions people actually search
Why are my tomato leaves turning yellow?
Usually watering first, then a shortage of nitrogen, magnesium, iron or potassium. Where the yellow sits on the plant names the family: bottom leaves first for a mobile nutrient or wet roots, top leaves first for iron, one side of the plant for a wilt. Read the leaf before you open a bottle.
Why is my tomato plant turning yellow?
Start with the compost, not the feed. A plant that wilts while the pot is still wet has suffocating roots. Pale from the bottom up is often nitrogen. Yellow only at the top, with green veins on the new leaves, points to iron, mostly on chalky ground above pH 7.0. A few yellow lower leaves just after planting out is transplant shock and settles down.
Can overwatering cause yellow tomato leaves?
Yes. In UK gardens, wet compost is the usual first cause. Waterlogged compost has no air, roots need oxygen to take up nutrients, and the plant yellows from the bottom even with feed in the pot. The tell is a wilt while the compost is still damp. Let the top inch dry, then water at the base in the morning.
What does magnesium deficiency look like on tomatoes?
Yellow between the veins on the older leaves, with the veins staying green, so the leaf looks like a fishbone. High-potash tomato liquids can make it worse, because potassium and magnesium compete at the root. A leaf spray of Epsom salts at 20 g per litre, or a liquid magnesium feed, is the quick fix. Ease off any high-potash feed while you do it.
How do I tell magnesium deficiency from iron deficiency in tomatoes?
Both yellow between green veins. Magnesium shows on the oldest leaves at the bottom, because the plant can move it. Iron shows on the youngest leaves at the top, because it cannot. If the yellowing is at the top, test soil pH before you add iron. Above about pH 7.0 the iron is usually still in the soil, just much less plant-available.
Should I use Epsom salt on yellow tomato leaves?
Yes, if the yellow is between green veins on the older leaves, which points to magnesium. The RHS gives 20 g of Epsom salts per litre of water as a summer leaf feed, repeated two or three times a fortnight, and for the longer term on light soils, Epsom salts or kieserite worked into the soil at 30 g per square metre. It will not fix yellowing from wet roots, nitrogen shortage or iron.
Should I remove yellow leaves from my tomato plant?
If they are spotted with blight or leaf mould, take them off straight away and bin them. Otherwise fix the cause first, then remove leaves that have gone fully yellow: they will not green up, they block airflow and they catch fungal spores. Take a few at a time and never strip the plant bare.
How often should I feed tomatoes if the leaves are yellowing?
Not more often until you know which yellow you have. A slow-release tomato fertiliser in the planting hole, top-ups at fruit-set, and a targeted fix only for the deficiency you can see covers most UK pots and borders. Weekly liquid feed on wet compost can make yellowing worse. The full programme is in the best fertiliser for tomatoes guide.
Keep reading
Founder of Dr Forest. Joe started the business in 2020 and formulates the blended fertilisers and mixes and packs them in small batches in Stockport.
The working, shown
- Machado, J., Vasconcelos, M.W., Soares, C., Fidalgo, F., Heuvelink, E. and Carvalho, S.M.P. (2023). Young tomato plants respond differently under single or combined mild nitrogen and water deficit: an insight into morphophysiological responses and primary metabolism. Plants 12(5):1181. doi:10.3390/plants12051181
- Besford, R.T. and Maw, G.A. (1975). Effect of potassium nutrition on tomato plant growth and fruit development. Plant and Soil 42(2):395–412. doi:10.1007/BF00010015
- Koukounaras, A., Tsouvaltzis, P. and Siomos, A.S. (2013). Effect of root and foliar application of amino acids on the growth and yield of greenhouse tomato in different fertilization levels. Journal of Food, Agriculture and Environment 11(2):644–648. No DOI issued; abstract at the publisher.
- Fan, H., Zhang, Y., Li, J., Jiang, J., Waheed, A., Wang, S., Rasheed, S.M., Zhang, L. and Zhang, R. (2023). Effects of organic fertilizer supply on soil properties, tomato yield, and fruit quality: A global meta-analysis. Sustainability 15(3):2556. doi:10.3390/su15032556
- Yang, J., Mattoo, A.K., Liu, Y., Zvomuya, F. and He, H. (2023). Trade-offs of organic and organic-inorganic fertilizer combinations in tomato quality and yield: A global meta-analysis (1992–2021). European Journal of Agronomy 151:126985. doi:10.1016/j.eja.2023.126985
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