Why are my plant's leaves turning yellow?

Yellowing is a general stress response rather than a specific diagnosis, so the pattern matters more than the colour. Which leaves yellow, and how, narrows the cause considerably.

Overwatering is the most common cause by a wide margin, and it is frequently mistaken for underwatering because both produce drooping. Waterlogged soil excludes air, roots suffocate and begin to rot, and the plant cannot take up water despite sitting in it. The signs are lower leaves yellowing and falling, soil that stays wet for days, and sometimes a sour smell. The fix is to let the soil dry substantially and to ensure the pot actually drains, which means a drainage hole and not standing in a full saucer.

Underwatering yellows leaves too, but they usually crisp at the edges first and the soil pulls away from the pot sides.

Location of the yellowing points at nutrients. Older, lower leaves yellowing while new growth stays green suggests nitrogen deficiency, because the plant moves mobile nutrients to new growth. New leaves yellowing while the veins stay green suggests iron or magnesium, often caused by unsuitable soil pH rather than absence of the nutrient.

Light matters: too little produces pale, leggy growth, while sudden strong light can scorch and bleach.

A few other causes are worth checking. Cold draughts, being pot-bound with roots circling the container, and pests such as spider mites all produce yellowing, the last usually with fine stippling visible up close.

Some yellowing is simply normal. Plants shed their oldest leaves routinely, and one or two lower leaves yellowing occasionally is not a problem to solve.