Why are my tomato leaves turning yellow?

By Joe, Founder of Dr Forest · May 2026

Yellow leaves on a tomato plant aren't one problem. They're a symptom of half a dozen different things, and the right fix depends on which one. Most cases sort themselves out within a week once you've named the cause.

Yellow tomato leaves are most often a nutrient signal: nitrogen, magnesium, iron, potassium or calcium running short. Sometimes it's watering. Occasionally it's a soil pH problem locking nutrients out. Rarely it's disease. The position of the yellow on the plant tells you which family of cause you're looking at, and the pattern within a leaf narrows it further.

Tomatoes are heavy feeders. Roughly twice the nutrient demand of cucumbers, four times that of beans. Run a tomato slightly short on any one element and the deficiency shows in the leaf colour before it shows in the harvest. The same sensitivity that makes them needy also makes them easy to read.

This guide covers the diagnostic logic, the five most common deficiencies, the watering and pH issues that look like nutrient problems, and a feeding programme that prevents most of them happening at all.

When yellow leaves are normal

Some yellowing isn't a problem at all. Three cases worth ruling 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've done their job feeding the seedling and the plant has no further use for them. 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 will sort itself out without intervention.

September yellowing on a plant that's still carrying fruit is the plant winding down. Energy is going into ripening what's left rather than building new foliage. That's biology working as intended.

If none of those describe what you're seeing, the rest of this guide will.

Watering, before you blame anything else

The single most common reason for yellow tomato leaves in UK gardens isn't a nutrient problem. It's water. Both too much and too little produce similar-looking symptoms, which is why so many growers fix the wrong thing.

Overwatering is the bigger of the two issues here. UK weather delivers stretches of cool, grey, drizzly days that keep compost saturated for far longer than the plant wants. Saturated soil has no air in it. Roots take in nutrients through active transport, which costs energy, which needs aerobic respiration. Drown the roots and you get a plant that can't feed itself even with plenty of nutrition in the soil.

The clearest sign: plant is wilting and the compost is still wet. That isn't a thirsty plant. That's a suffocating one.

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 30cm pot can drink the pot dry inside a day, particularly once it's flowering.

The fix for both is the same: get your 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 of the plant, never overhead, and never on a fixed schedule. The schedule should be the plant's, not the calendar's.

Nutrient deficiencies, the diagnostic way

When watering and weather are ruled out, deficiency is usually the next stop. Tomatoes show deficiencies fast because their nutrient demand outruns most ordinary garden soils within a few weeks of fruit-set.

The leaf tells you which nutrient. Two questions to ask.

Where does the yellow start? Old leaves at the bottom, or young leaves at the top? Bottom-up means the missing nutrient is mobile in the plant, which is to say the tomato can move it from where it isn't useful to where it is. The plant strips the old leaves to feed the new growth. That points at nitrogen, magnesium or potassium. Top-down means the nutrient is immobile. The plant can't shift it from old to new tissue, so the new growth goes without. That points at iron or calcium.

Where the yellow starts tells you whether the missing nutrient can move within the plant.

What pattern does the yellow take inside the leaf? Uniform fade across the whole leaf, yellow between still-green veins, or scorched along the margins? Uniform fade is nitrogen. Interveinal yellowing is magnesium if it's on the bottom leaves, iron if it's on the top. Scorched margins are potassium.

The pattern within a single leaf narrows five deficiencies down to one.

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 soil 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 tend to ride deficiencies better than plants in pots, which have nowhere to go for what they need.

Quick fix: an amino acid foliar feed gets nitrogen into the leaf within hours, and the colour usually starts to come back within a few days. Liquid Amino Acids deliver nitrogen in amino-acid form for foliar use; the amino acids carry nitrogen straight through the leaf surface without the conversion step inorganic nitrogen sources need. For the slower, steady kind of feeding that prevents deficiency in the first place, our Premium Tomato Fertiliser supplies nitrogen in a slow-release form alongside the other nutrients tomatoes need.

Magnesium deficiency

What it looks like: yellow between the veins, with the veins themselves staying vivid green. The pattern is sometimes called "fishbone" or "marbled". Like nitrogen, it appears on the older leaves first because magnesium is mobile.

Why it matters: magnesium is the central atom in every chlorophyll molecule. Without it the plant physically cannot make the green pigment that runs photosynthesis. There's no workaround.

Why it happens: light, sandy and acidic soils run short of magnesium first. So do plants that have been hammered with high-potash liquid tomato feeds. Potassium and magnesium compete at the root surface, and aggressive potash feeding can lock magnesium out even when there's enough in the soil. It's worth knowing because the symptom and the supposed remedy can pull in opposite directions.

Quick fix: Liquid Magnesium applied as a foliar spray works inside a week. The micronised particles are ultra-fine, which means they stick to leaf surfaces after spraying and to soil colloids after a drench, rather than dissolving away. Epsom salts work too, but they're fully water-soluble. One decent rainfall after application and they wash out of the root zone. Liquid Magnesium's particles resist that.

Iron deficiency

What it looks like: the same yellow-between-veins-with-green-veins pattern as magnesium, but on the youngest leaves at the top of the plant. That's the diagnostic difference. Iron is immobile in the plant, so when it runs short the new growth suffers first.

The catch: iron deficiency in tomatoes is almost never about how much iron is in the soil. It's a pH problem. Above pH 7.0, iron and several other trace elements lock into chemical forms the roots can't take up. You could add a bucket of iron filings to the soil and the plant still wouldn't get any. Test the pH first.

Quick fix: Micro 7 is a chelated trace pack containing iron, manganese, zinc, copper, boron, molybdenum and cobalt, applied as a foliar feed. Chelation gets the iron past the pH lock-out by carrying it into the leaf in a form the plant can use directly. For a longer-term answer, drop the soil pH towards 6.5 with a sulphur amendment before the next growing season.

Potassium deficiency

What it looks like: scorched, brown-edged margins on the older leaves, with the centre of the leaf staying green for a while. The leaves can curl downward in worse cases. Often turns up around fruit-set, when the plant's potassium demand spikes hard. Besford and Maw's classic 1975 study showed that potassium-deficient tomatoes suffer measurable impairment of water relations and photosynthesis at the same time as the visible scorching, which means the visible symptom is already late in the chain.

Quick fix: Sulphate of Potash supplies 41.5% K₂O in sulphate form, with no chloride load. Sulphate is the form tomatoes prefer. Chloride at high rates will scorch leaf tips and dull the fruit flavour. For prevention rather than cure, our Premium Tomato Fertiliser uses Yorkshire polyhalite as its main potassium source, which delivers potassium alongside calcium, magnesium and sulphur in one mineral and releases slowly enough to avoid the spike-and-crash pattern of liquid feeds.

Calcium deficiency

What it looks like on leaves: cupping or curling on young leaves at the top of the plant, 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, rotting from the inside out before the fruit ripens.

Why it happens: calcium moves through the plant in the transpiration stream, which is the constant flow of water from roots up to leaves. Anything that disrupts that flow causes localised calcium starvation in the fastest-growing tissue, even when the soil has plenty of calcium. Irregular watering is the usual culprit. So is high humidity, which slows transpiration. So is excess nitrogen, which pushes the plant to grow faster than it can move calcium up to the new tissue.

Quick fix: Liquid Gypsum is micronised calcium sulphate. The particle size is small enough that it's available to roots quickly, and it doesn't shift the soil pH the way garden lime does. Apply as a soil drench every two to four weeks through fruit-set. There's a longer guide on this in our liquid gypsum and blossom end rot post if you want the full mechanism. For dual calcium and magnesium correction in one product, Cal-Mag supplies both at a balanced ratio.

Soil pH, the thing most guides skip

Most "why are my leaves yellow" guides skip this section, which is a shame, because it might be the most important one.

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 in chemical forms the plant can't reach.

Above pH 7.0, common in chalky and limey areas of southern and central England, iron, manganese, zinc and boron all 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 time, calcium and magnesium availability drops. The yellowing turns up on older leaves and the fruit gets blossom end rot.

The gold band marks pH 6.0–6.8, the narrow window where tomatoes can reach every nutrient group at once.

Tomatoes are happiest somewhere between pH 6.0 and 6.8. A pH test kit costs a few pounds from any garden centre and earns its place on the shelf within one season. Testing once a year, in early spring, catches most pH-driven yellowing before it starts.

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 at the symptoms more carefully. Disease-related yellowing is rarely uniform. It comes with spots, irregular patches, asymmetric distribution, and other foliage changes alongside the yellow.

Three of the more common ones in UK gardens.

Early blight (Alternaria solani). Brown bullseye-pattern 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 around the plant by staking or stringing. Don't compost the strippings. Most of the answer here sits in cultural practice rather than chemistry.

Fusarium and Verticillium wilt. Distinctive 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'll usually see brown discolouration in the vascular tissue. There's no garden cure for either fungus. Pull the plant and bin it (don't compost), and don't grow tomatoes, peppers, aubergines or potatoes in that ground for three to four years to break the soil cycle.

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 from one plant to another. There's no garden cure for these either. Pull and bin, control the insect vectors, sterilise tools between plants, and choose resistant varieties next time. Most of the major tomato viruses now have resistant cultivars on the seed market.

Quick reference

Where on the plant does the yellow start?

What pattern does the yellow take in the leaf?

A feeding programme that prevents most of this

The cleanest answer to yellow tomato leaves is to never have any. That means a feeding programme built around how tomatoes actually take up nutrients through the season, rather than grabbing the nearest bottle off the shelf.

Before planting. Test the pH and amend if it's outside 6.0 to 6.8. Work compost into the planting site. For containers, use fresh compost rather than reusing last year's, which will have lost most of its readily available nitrogen.

At planting. Incorporate a slow-release organic fertiliser into the planting hole or into the compost. Premium Tomato Fertiliser is a multi-input blend that releases steadily from transplant through to final harvest, with Yorkshire polyhalite as the potassium and sulphur backbone. Made with organic ingredients, plant-based, no slaughterhouse waste, handcrafted in Stockport.

Through vegetative growth. A fortnightly foliar of trace elements catches small deficiencies before they become visible. Micro 7 is the standard for that. If you spot the early signs of magnesium yellowing, Liquid Magnesium as a foliar will turn it around inside a week.

At fruit-set. Potassium demand jumps and calcium demand stays high. Add Sulphate of Potash as a top dressing, or Liquid Gypsum as a fortnightly soil drench for calcium and to head off blossom end rot. Both pair well with the slow-release base feed without doubling up on anything.

Throughout. Water consistently rather than frequently. Mulch helps stabilise moisture. Strip yellow leaves once the underlying cause is fixed, since they're not contributing photosynthesis once the chlorophyll is gone, and they're a route in for fungal spores.

So, briefly

Yellow tomato leaves are usually fixable and almost always informative. The colour, the position and the pattern between them say what the plant needs. Watering first, then nutrients, then pH if neither of those landed. Disease is the last suspect, not the first one. A pH test kit, a finger in the compost, and a good look at where the yellow starts will solve the great majority of cases without anyone reaching for a sprayer.

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