Why do tomato leaves turn yellow?

By region

The 12 °C (54 °F) figure is from the RHS: below it, a tomato will not thrive. Purple from cold at planting is in its nutrient-deficiency guide, which also says that cultivated soils rarely run short of phosphorus. When to plant out, region by region, is in when to plant tomatoes.

Where the yellow starts

On older leaves the RHS often sees magnesium: yellow between the veins, sometimes with reddish-brown tints, then dead patches and the leaf dropping. If the pattern is only on the older leaves and the plant is otherwise pushing on, it says there is no cause for concern. The same mottling of older leaves, on a vigorous plant, it treats as a lesser problem.

Nitrogen, again from the RHS, makes a spindly, stalled plant with pale yellow leaves, sometimes pink-tinted. It starts on the older leaves and moves up. Cornell, on tomato, describes the same order: the oldest leaves turn chlorotic, the younger ones yellowish-green.

Iron shows on the young leaves, with the veins staying green. Cornell describes it that way on tomato. The RHS says the same about leaf age: iron on the youngest, magnesium on the older ones. On a very chalky soil the iron is there and the roots cannot take it up, and lowering the pH of a soil with free lime, the RHS writes, is not practical.

Water, cold, pots

The RHS warns that drought, waterlogging, low light and a poor planting look like a deficiency. In dry or sodden soil the roots do not take nutrients up, even when the element is in the ground. If the plant has dried out, do not flood it: bring the moisture back gradually. The rhythm is in watering. A black patch on the tip of the fruit, with the leaves otherwise sound, is blossom end rot: uneven water, not a yellowing you feed away.

In a pot, the RHS writes, the nutrients in the compost last 6–8 weeks, and without steady water the roots do not take them up. Sodden compost blocks them too. Pots and watering are in the container garden.

Leaves that curl, with cold nights and hot days, are a separate disorder: the RHS says it usually passes on its own once the nights warm up, and that the crop suffers little. Stripping leaves at random leaves the fruit in the sun: that point is in side shoots.

What to do, and what to leave

Water first, steadily, on the soil. Then read the leaf. A fertiliser poured on at random moves the problem: too much nitrogen, the RHS and University of Minnesota Extension write, makes dark leaves, a lot of growth and fruit that is late. Too much potassium, for the RHS, reduces magnesium uptake, and that is a typical result of tomato feed used to excess.

If the pattern is magnesium and the plant is suffering, on a light, free-draining soil, the RHS gives magnesium sulphate (Epsom salts) on the soil at 30 g per square metre. In summer, as a short-term step, 20 g of Epsom salts per litre of water on the leaves, two or three times over a fortnight, and only under dull cloud: in sun the leaf scorches. If the yellow is only on a few old leaves and the top is green, skip this.

For nitrogen in the open ground, the long fix is a mulch of compost or well-rotted manure. The short one is a nitrogen fertiliser at the rate on the label. How to make the compost is in garden compost, and the layer is in mulching.

Brown blotches that are not stopped by the veins, on leaves, stems and fruit, in cool wet weather, are late blight. Do not feed that: remove the affected parts. The picture is under pests and diseases.