Why are my tomato leaves turning yellow?
Tomato troubleshooting · retrofit
Why are my tomato leaves turning yellow?
By Joe, Founder of Dr Forest · May 2026, updated September 2026
Push a finger an inch into the compost. Then look at where the yellow starts, and the pattern inside the leaf. That is the test.
Tomato leaves turning yellow usually name a cause once you know where the colour starts. I make a tomato fertiliser 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.
Yellow tomato leaves are most often watering, then a nutrient signal: nitrogen, magnesium, iron, potassium or calcium. Sometimes soil pH is locking those nutrients out. Rarely it is disease. The position of the yellow on the plant tells you which family you are in. The pattern within a single leaf narrows it further.
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, you are in iron or calcium, and iron is almost always pH. One-sided yellowing is a wilt until proven otherwise.
In short · the test, then the cause
Do this first
Finger an inch into the compost. Note whether the yellow starts at the bottom, the top, or on one side. Then look at the pattern inside one leaf: even fade, yellow between veins, or scorched margins.
Most likely causes
Inconsistent or excess watering. Nitrogen on lower leaves. Magnesium in a fishbone pattern. pH locking iron out of young growth. Potassium scorch around fruit-set. Calcium on new leaves and on the fruit.
Then feed, if you still need to
Foliar for a fast colour response. Soil drench or a slow-release tomato 3-4-6 for staying power. Do not stack a high-potash liquid on a magnesium problem. I sell the bags I name below.
When yellow leaves are normal
Some yellowing is the plant doing its job. Rule these out before you reach for a remedy.
The first two rounded leaves on a young seedling, the cotyledons, are supposed to yellow and drop once the true leaves take over. They fed the seedling. Pinch them off if they bother you, or leave them.
A few yellow lower leaves in the week or two after planting out is the tomato adjusting to disturbed roots. Keep watering steady, hold off on liquid feeds for a few days, and the plant usually sorts itself out.
September yellowing on a plant that is still carrying fruit is energy going into ripening what is left, rather than building new foliage. That is the season winding down, not a crisis.
Tomato leaves turning yellow from water
In UK gardens, water is the first thing to check. Saturated compost and a dry pot both yellow a tomato, and they can look similar enough that people feed the wrong problem.
Overwatering is the bigger of the two here. Cool, grey, drizzly stretches keep compost saturated for far longer than the plant wants. Saturated soil has no air. Roots take in nutrients through active transport, which costs energy, which needs aerobic respiration. Drown the roots and you get a plant that cannot feed itself even with plenty of nutrition in the pot.
The clearest sign: the plant is wilting and the compost is still wet. That plant is suffocating.
Underwatering looks different. Leaves yellow from the margins inward, often curling and going crisp before they drop. In a hot week a large tomato in a 30 cm pot can drink the pot dry inside a day, particularly once it is flowering.
The fix for both is the same test. Finger an inch into the compost before you water. If it comes out dry, water deeply. If it comes out damp, wait. Water in the morning at the base, never overhead, and never on a fixed calendar. The schedule should be the plant's.
Joana Machado's group ran young Micro-Tom plants for 16 days with nitrogen cut to 50 percent, water cut to 50 percent, or both. Water deficit alone did not drop the chlorophyll meter reading. Nitrogen deficit did. Combined nitrogen and water stress dropped it further. In a glasshouse on a mild 50 percent cut, water was not the yellowing agent. In a British patio pot in June, wet compost still comes first, because the roots run out of air long before they run out of a published nutrient solution. Use the finger test. Then read the leaf.
Nutrient deficiencies, the diagnostic way
When watering is ruled out, deficiency is usually next. Tomatoes show it fast because demand outruns most ordinary garden soils within a few weeks of fruit-set.
Two questions. Where does the yellow start? What pattern does it take inside the leaf?
Bottom, oldest leaves
The missing nutrient is mobile. The plant strips old leaves to feed new growth. Nitrogen, magnesium or potassium. Or overwatering damaging the roots.
Top, youngest growth
The nutrient is immobile. The plant cannot shift it from old tissue, so the new growth goes short. Iron or calcium. Iron is almost always a pH lock-out.
Even fade
The whole leaf goes pale, no green veins showing through. Nitrogen, or wet roots.
Yellow between veins
Fishbone or marbled. Magnesium if it is low on the plant. Iron if it is on the new leaves at the top.
Scorched brown margins
Potassium, often around fruit-set. The centre of the leaf stays green for a while. By the time the scorch shows, fruit development has already been held back.
Figure 1 · chlorophyll meter, 16 days
Tomato leaves turning yellow on the meter after a nitrogen cut, not after a water cut of the same size.
cv. Micro-Tom. SPAD on the youngest fully expanded leaf. Control 10.5 mM N and 100 percent water. Nitrogen treatments at 5.3 mM N. Water treatments at 50 percent water.
Nitrogen deficiency
What it looks like: a uniform pale-green-to-yellow fade across the whole leaf, with no mottling and no green veins showing through. It starts on the oldest leaves at the bottom and works upward. Within a week or two, the lower third of the plant can be pale; left untreated, the middle leaves go too.
Why it happens in UK gardens: nitrogen is water-soluble, so heavy rain leaches it down the profile and out of the root zone. Reused container compost from last year is almost always nitrogen-depleted by mid-season. Plants in open ground ride shortages better than plants in pots, which have nowhere else to go.
Machado's 16-day nitrogen cut was mild by garden standards, half the control rate, not zero. The chlorophyll meter still fell 14 percent, leaf area 31 percent, and total dry weight 27 percent. That is a young plant in a glasshouse. A hungry patio tomato in July will show the same fade faster.
Quick fix: an amino-acid foliar feed gets nitrogen into the leaf without waiting on the roots. Athanasios Koukounaras, Pavlos Tsouvaltzis and Anastasios Siomos sprayed and drenched greenhouse 'Belladonna' with a protein hydrolysate (11.3 percent L-amino acids, 4 percent nitrogen). When they had already cut the base fertiliser by 40 percent, a 2.7 percent root application raised total and marketable yield by putting on more fruit [3]. That trial is not our bottle. It is evidence that amino-acid nitrogen is a real route, especially when the soil programme is light. Liquid Amino Acids is the foliar I make, from £14.99. For the slower, steady feeding that stops the fade coming back, Tomato 3-4-6 supplies nitrogen in a slow-release form alongside the rest of what a tomato actually uses.
Magnesium deficiency in tomatoes
What it looks like: yellow between the veins, veins themselves still vivid green. Growers call it fishbone or marbled. Like nitrogen, it appears on the older leaves first because magnesium is mobile. Magnesium for tomato plants is not a folklore extra. It is the central atom in every chlorophyll molecule. Without it the plant cannot make the green pigment. There is no workaround.
Why it happens: light, sandy and acidic soils run short first. So do plants hammered with high-potash liquid tomato feeds. Potassium and magnesium compete at the root surface. Aggressive potash feeding can lock magnesium out even when there is enough in the soil. The symptom and the supposed remedy then pull in opposite directions.
Quick fix: Liquid Magnesium as a foliar, from £12.99, usually turns the colour inside a week. The particles are micronised, so they stick to leaf surfaces after spraying and to soil colloids after a drench, rather than dissolving away. Epsom salt works too, and people search it (epsom salt for tomatoes is a real query). It is fully water-soluble. One decent rainfall after application and it is out of the root zone. If you want a granular magnesium source in the planting hole, kieserite is the mineral I stock. Do not throw Epsom at every yellow leaf. Confirm the fishbone pattern first.
Iron deficiency
What it looks like: the same yellow-between-veins pattern as magnesium, on the youngest leaves at the top of the plant. That is the diagnostic difference. Iron is immobile, so new growth suffers first.
The catch: iron deficiency in tomatoes is almost never about how much iron is in the soil. It is a pH problem. Above pH 7.0, iron and several other trace elements lock into forms the roots cannot take up. You could add a bucket of iron filings and the plant still would not get any. Test the pH first.
Quick fix: Liquid Micronutrients is a chelated pack of seven elements, including iron, applied as a foliar, from £15.99. Chelation carries the iron into the leaf past the pH lock-out. For a longer answer, drop the soil pH towards 6.5 before the next season. The how-to is in how to raise or lower soil pH.
Potassium deficiency
What it looks like: scorched, brown-edged margins on the older leaves, centre of the leaf staying green for a while. Leaves can curl downward in worse cases. It often turns up around fruit-set, when potassium demand spikes.
R.T. Besford and G.A. Maw grew 'Amberley Cross' in sand at Glasshouse Crops Research Institute. Flower development slowed once potassium in the feed fell below 0.53 meq per litre. Fruit setting efficiency dropped below 2.03 meq per litre. Maximum fruit yields sat at 5.03 or 10.23 meq per litre [2]. Those are hydroponic units, not a garden-bag rate. What they show is that potassium shortage hits flowers and fruit before the leaf scorch is dramatic. By the time you see the brown margins, the truss has already been held back.
Quick fix: sulphate of potash supplies 50 percent K2O in sulphate form, with no chloride load, from £6.25. Sulphate is the form tomatoes prefer. Chloride at high rates scorches leaf tips and dulls flavour. For prevention rather than a rescue, Tomato 3-4-6 uses Yorkshire polyhalite as its main potassium source, which delivers potassium with calcium, magnesium and sulphur in one mineral. The longer mineral comparison is polyhalite vs sulphate of potash.
Calcium deficiency in tomato leaves
What it looks like on leaves: cupping or curling on young leaves at the top, sometimes with yellow-green tinting and stunted growth. What it looks like on fruit: blossom end rot. Dark, sunken, leathery patches on the bottom of the tomato.
Calcium moves in the transpiration stream. Anything that disrupts that flow causes localised calcium starvation in the fastest-growing tissue, even when the soil has plenty. Irregular watering is the usual culprit. So is high humidity, which slows transpiration. So is excess nitrogen, which pushes growth faster than calcium can move.
This page is not the blossom end rot guide. That lives at blossom end rot in tomatoes. The short version for yellowing new leaves: water evenly, and if you need a calcium drench, Liquid Gypsum is micronised calcium sulphate, from £12.99. It does not swing pH the way garden lime does. For calcium and magnesium together, Cal-Mag is the dual correction, from £10.49.
Soil pH, the thing most guides skip
Soil pH controls which nutrients tomato roots can actually take up, regardless of how much fertiliser is in the soil. Get the pH wrong and most of what you add stays locked.
Above pH 7.0, common in chalky and limey ground in southern and central England, iron, manganese, zinc and boron become progressively less available. The yellow that follows turns up on young leaves first.
Below pH 5.5, common in sandy and peaty soils, and in heavily composted plots that have acidified over years, calcium and magnesium availability drops. The yellowing turns up on older leaves and the fruit gets blossom end rot.
Tomatoes are happiest between pH 6.0 and 6.8. That window is grower guidance, not a Machado number. A pH test kit costs a few pounds from any garden centre and earns its place in one season. Test in early spring.
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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Disease, the last suspect on the list
Disease is rarely the first cause of yellow tomato leaves and almost never the only one. If watering, feeding and pH are sorted and the yellowing is still spreading, look harder. Disease yellowing is rarely uniform. It comes with spots, irregular patches, asymmetric distribution, and other foliage changes alongside the yellow.
Early blight (Alternaria solani). Brown bullseye spots with concentric rings, on the lower leaves, surrounded by yellow halos. Thrives in warm, humid weather. Strip and bin the affected leaves, water at the base only, and improve airflow by staking or stringing. Do not compost the strippings. Most of the answer here is cultural practice, not a bottle.
Fusarium and Verticillium wilt. Asymmetric yellowing, often on one side of the plant or one half of a leaf. The plant wilts in the day, recovers at night for a while, then stops recovering. Cut the stem and you will usually see brown discolouration in the vascular tissue. There is no garden chemical that reverses this. Pull the plant and bin it, and do not grow tomatoes, peppers, aubergines or potatoes in that ground for three to four years.
Viral infections (mosaic, TYLCV). Mottled yellow-and-green patterns with leaf curl, distortion and stunted growth. Spread by aphids and whiteflies, or by tools moving sap. Pull and bin, manage the insects, sterilise tools between plants, and choose resistant varieties next time.
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 | Amino-acid foliar, then a slow-release base |
| Yellow between green veins, lower leaves | Magnesium shortage | Micronised magnesium foliar; skip blanket Epsom |
| Yellow between green veins, top leaves | Iron, usually pH above 7.0 | Check pH, then a chelated trace feed |
| Scorched brown edges, older leaves | Potassium shortage at fruit-set | Sulphate of potash, no chloride load |
| Cupped young leaves, sunken fruit base | Calcium, disrupted by uneven watering | Water evenly; Liquid Gypsum drench. BER guide for the fruit. |
A feeding programme that stops most of this
The cleanest answer to yellow tomato leaves is to never have any. That means feeding the way tomatoes actually take up nutrients through the season, rather than grabbing the nearest bottle.
The season at a glance
- Before planting
- Test pH. Aim for 6.0–6.8. Fresh compost in containers; last year's has usually lost its readily available nitrogen.
- At planting
- Slow-release Tomato 3-4-6 in the hole or mixed through. Made with organic ingredients, plant-based, small-batch in Stockport.
- Vegetative growth
- Chelated trace foliar fortnightly. Liquid Magnesium at the first fishbone yellow.
- Fruit-set
- Sulphate of potash as a top dressing, or Liquid Gypsum as a fortnightly drench. Pair them with the slow-release base rather than doubling up.
- Throughout
- Water consistently rather than frequently. Mulch. Strip yellow leaves once the cause is fixed.
If you want the wider comparison rather than a deficiency fix, see best fertiliser for tomatoes. Weighing a garden-centre liquid against what we put in the 3-4-6? That is Tomorite vs Dr Forest. How often to feed tomatoes as a standing question belongs on the fertiliser pillar; this page is what to do when the leaves have already changed colour. The bags for tomatoes, chillies, strawberries and potatoes sit in Tomatoes & Edibles.
Premium organic fertiliser · Stockport
Built for the way tomatoes actually feed
Multi-input slow-release blend, Yorkshire polyhalite as the potassium and sulphur backbone, Scottish seaweed in every batch. No synthetic additives. Handcrafted in small batches. Made with organic ingredients. Plant-based: no slaughterhouse by-products.
- Tomato 3-4-6, from £11.50
- Liquid Amino Acids, from £14.99
- Liquid Magnesium, from £12.99
- Liquid Gypsum, from £12.99
See the full Tomatoes & Edibles collection. Polyhalite on its own is here, and the mineral story is in what is polyhalite.
A yellow leaf on a tomato in a Greater Manchester backyard is information. Name where it started. Name the pattern. Then open one bottle, or none.
Questions people actually search
Why are my tomato leaves turning yellow?
Tomato leaves turning yellow usually point to watering, then to nitrogen, magnesium, iron, potassium or calcium. The position of the yellow on the plant names the family: bottom leaves first for a mobile nutrient or wet roots, top leaves first for iron or calcium, one-sided yellowing for a wilt. Read the leaf before you open a bottle.
Why is my tomato plant turning yellow?
If the whole tomato plant is turning yellow, start with the compost, not the feed. A plant that wilts while the pot is still wet has suffocating roots. A plant that is pale from the bottom up is often short of nitrogen. A plant that is yellow only at the crown, with green veins on the new leaves, is usually iron locked out by pH above 7.0. One yellowing seedling leaf after planting out is often transplant shock and settles in a week.
Can overwatering cause yellow tomato leaves?
Yes. In UK gardens, wet compost is the usual first cause of yellow tomato leaves. Saturated soil has no air. Roots take up nutrients by active transport, which needs oxygen. Drown the roots and the plant yellows from the bottom even when the feed is 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?
Magnesium deficiency in tomatoes is interveinal chlorosis on the older leaves: yellow between the veins, veins still green, a fishbone or marbled pattern. Magnesium sits at the centre of every chlorophyll molecule, so the pigment cannot be made without it. High-potash tomato liquids can worsen it, because potassium and magnesium compete at the root. Foliar micronised magnesium is the fast correction; Epsom salt washes through after rain.
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 pH 7.0 the iron is usually in the soil and locked.
Should I use Epsom salt on yellow tomato leaves?
Only if the yellowing is the magnesium pattern: fishbone yellow on older leaves, veins still green. Epsom salt is magnesium sulphate. It is not a tomato fertiliser, and it does not fix nitrogen, iron, potassium or watering. It is fully soluble, so one decent rainfall moves it out of the root zone. For a confirmed magnesium shortage I use a micronised liquid magnesium, which sticks to leaf and soil rather than dissolving away.
Should I remove yellow leaves from my tomato plant?
Once you have named and fixed the cause, yes, take off leaves that have gone fully yellow. They will not green up again, they block airflow, and they are a landing strip for fungal spores. Snap or cut them at the stem. Do not strip a plant bare in one go, and do not pull yellow leaves while the compost is still a swamp: fix the water first.
How often should I feed tomatoes if the leaves are yellowing?
Do not raise the feed schedule until you know which yellow you have. A slow-release tomato fertiliser in the planting hole, then a fortnightly trace foliar in growth, then potash and calcium at fruit-set, covers most UK pots and borders. Liquid tomato feed every week on already-wet compost makes the yellowing worse. The full programme sits in the best fertiliser for tomatoes guide; this page is the diagnostic.
Keep reading
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. 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.
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