Magnesium deficiency

Shows as yellowing between the veins of older leaves while the veins stay green, common on tomatoes and citrus in pots or sandy soils.

Interveinal yellowing (a green "herringbone" of veins on a yellow leaf), oldest leaves first.

A foliar spray of Epsom salts (magnesium sulfate, ~1 tablespoon per litre) gives a quick green-up; repeat if needed.

Maintain balanced soil with compost, avoid excess potassium, and check pot mixes which leach magnesium.