A yellowing leaf is the most common thing gardeners write to us about, and the most commonly misread. The instinct is to water. The instinct is usually wrong.
Start at the bottom of the plant
Old leaves at the base yellowing slowly, one at a time, while new growth looks healthy: this is normal ageing. Remove them and stop worrying.
Yellow, soft and starting from the lower leaves upward
Almost always overwatering, or a pot that cannot drain. Lift the pot — if it is heavy and the soil smells sour, you have found it. Let the top two inches dry completely before watering again, and check that the drainage hole is not blocked by a root or standing in a saucer of water.
Yellow between green veins
A nutrient problem, usually iron or magnesium, and common in pots that have not been fed for a couple of months. Work compost into the surface and feed again in three weeks.
Pale, stretched and leaning
Not enough light. Move the pot, or change the plant for one that suits the corner — no amount of feeding fixes a shady spot.
Yellow with fine webbing or sticky patches
Pests. Spray with plain water first, then neem oil weekly in the evening — five millilitres of neem oil and two of mild soap in a litre of water.
One thing worth remembering: a yellow leaf never turns green again. Remove it, fix the cause, and judge your success by the new growth.